Advances in Astronomy
Volume 2019 (2019), Article ID 6325183, 12 pages
https://doi.org/10.1155/2019/6325183

Research Article

Observations of Radio Magnetars with the Deep Space Network

Aaron B. Pearlman,¹ Walid A. Majid,1,2 and Thomas A. Prince1,2

¹Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Correspondence should be addressed to Aaron B. Pearlman; aaron.b.pearlman@caltech.edu

Received 5 October 2018; Revised 11 December 2018; Accepted 27 January 2019; Published 2 June 2019

Guest Editor: Ersin Göğüş

Copyright © 2019 Aaron B. Pearlman et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The Deep Space Network (DSN) is a worldwide array of radio telescopes which supports NASA’s interplanetary spacecraft missions. When the DSN antennas are not communicating with spacecraft, they provide a valuable resource for performing observations of radio magnetars, searches for new pulsars at the Galactic Center, and additional pulsar-related studies. We describe the DSN’s capabilities for carrying out these types of observations. We also present results from observations of three radio magnetars, PSR J1745–2900, PSR J1622–4950, and XTE J1810–197, and the transitional magnetar candidate, PSR J1119–6127, using the DSN radio telescopes near Canberra, Australia.

1. Introduction

Magnetars are young neutron stars with very strong magnetic fields ( ≈ – G). They have rotational periods between ~2–12 s and larger than average spin-down rates compared to other pulsars, placing them in the top right region of the P– diagram (see Figure 1). Magnetars are primarily powered by the decay of their enormous magnetic fields, which serves as an energy source for their transient emission behavior [1–3]. It is thought that magnetars comprise at least 10% of the young neutron star population [4], and they tend to be more concentrated towards the inner part of the Galaxy [5].

Figure 1: P– diagram of pulsars in the Australia Telescope National Facility (ATNF) pulsar catalog (see https://www.atnf.csiro.au/people/pulsar/psrcat) [36]. The four radio magnetars, PSR J1745–2900, PSR J1119–6127, PSR J1622–4950, and XTE J1810–197, discussed in this paper are labeled using red stars. Magnetars (red squares), rotating radio transients (RRATs; green diamonds), and millisecond pulsars (MSPs; orange circles) are also shown, along with the population of rotation-powered radio pulsars (black circles). Radio-quiet pulsars are indicated with blue crosses and include many of the magnetars shown in red. Lines of constant magnetic field and characteristic age are derived assuming a constant braking index of . The radio pulsar death line is given by the model in Equation (4) of Zhang et al. [37].

There are currently 29 known magnetars and 2 additional magnetar candidates (normally rotation-powered pulsars), the latter exhibiting episodes of magnetar-like behavior. More than 2600 pulsars have been discovered, but only four of these are radio magnetars: PSR J1745–2900, PSR J1622–4950, XTE J1810–197, and 1E 1547.0–5408. Thus, radio magnetars are exceptionally rare and constitute 0.2% of the pulsar population. They also have large dispersion measure (DM) and Faraday rotation measure (RM) values compared to ordinary radio pulsars, which suggest that they inhabit extreme magneto-ionic environments (see Figure 2). A detailed list of properties associated with known magnetars can be found in the McGill Magnetar Catalog (see http://www.physics.mcgill.ca/~pulsar/magnetar/main.html) [5].

Figure 2: Magnitude of the Faraday rotation measure (RM) versus dispersion measure (DM) for all known pulsars (blue circles) in the Australia Telescope National Facility (ATNF) pulsar catalog (see https://www.atnf.csiro.au/people/pulsar/psrcat) [36]. We label the four radio magnetars considered in this paper, PSR J1745–2900, PSR J1119–6127, PSR J1622–4950, and XTE J1810–197, using red stars. The one other radio magnetar, 1E 1547.0–5408, is indicated by a red square. We also show the Galactic Center (GC) pulsars using black diamonds and the four fast radio bursts (FRBs) with RM measurements in the FRB catalog (see http://frbcat.org) [38] using green crosses.

Radio magnetars often have flat or inverted radio spectra, and their radio emission is highly linearly polarized (e.g., [6–10]). As a result, they are capable of being detected at very high radio frequencies (e.g., [7, 11, 12]). The flux densities, spectral indices, and pulse shapes of radio magnetars can also change on short timescales (e.g., [8, 10, 13–18]), and their radio pulse profiles often display multiple emission components, which can vary significantly across multiple radio frequencies (e.g., [6, 13, 14, 19–21]). Individual pulses from radio magnetars are typically composed of narrow subpulses, which can be exceptionally bright. However, they are unlike the giant pulses emitted by the Crab pulsar [6, 13, 22]. The morphology of these pulses can also change substantially between rotations (e.g., [6, 13]). Irregular timing behavior, including glitches (sudden increases in the pulsar’s rotation frequency), is also commonly observed from radio magnetars (e.g., [14, 20, 21, 23–26]), and their radio emission has been reported to episodically disappear and suddenly reactivate [21, 27–32]. Radio active and quiescent magnetars can also emit short X-ray bursts [25, 33–35]. The assortment of behavior listed here points to an underlying connection between high magnetic field radio pulsars and magnetars [6, 13, 25, 31].

In this paper, we discuss recent observations of three radio magnetars, PSR J1745–2900, PSR J1622–4950, XTE J1810–197, and the transitional magnetar candidate, PSR J1119–6127, using the Deep Space Network (DSN) radio telescopes. This paper is not intended to be a comprehensive review of the vast literature on radio magnetars. Instead, we focus on recent observational results on these particular radio magnetars using the DSN antennas. For a more complete review of magnetars, we refer the interested reader to some of the available review articles on the subject (e.g., [4, 41–46]). In Section 2, we describe the DSN radio dishes and the system’s observing capabilities. We discuss our observational results on each of the four magnetars listed above in Sections 3–6. A summary is provided in Section 7. This paper was prepared in response to an invited solicitation for a dedicated special issue on magnetars.

2. The Deep Space Network

The DSN consists of an array of radio telescopes at three locations (Goldstone, California; Madrid, Spain; and Canberra, Australia). Each of these sites is approximately equally separated in terrestrial longitude and situated in a relatively remote location to shield against radio-frequency interference (RFI). With multiple radio antennas at each site, the DSN covers both celestial hemispheres and serves as the spacecraft tracking and communication infrastructure for NASA’s deep space missions. The three DSN complexes each include a 70 m diameter antenna, with a surface suitable for radio observations at frequencies up to 27 GHz. In addition, each site hosts a number of smaller 34 m diameter radio telescopes, which are capable of observations as high as 32 GHz. Each antenna is equipped with multiple high efficiency feeds, highly sensitive cryogenically cooled receivers, and dual (circular) polarization capabilities. When the DSN antennas are not communicating with spacecraft, they may be used for radio astronomy and other radio science applications.

Recently, all three sites have been upgraded with state-of-the-art pulsar processing backends that enable data recording with high time and frequency resolution. The DSN telescopes are able to perform radio observations at the following standard frequency bands: -band (centered at 1.5 GHz), -band (centered at 2.3 GHz), -band (centered at 8.4 GHz), and -band (centered at 32 GHz). In addition, the 70 m radio dish in Canberra (see Figure 4) is outfitted with a dual beam -band feed covering 17–27 GHz. These capabilities are currently being used in various pulsar-related programs, which include high frequency, ultrawide bandwidth searches for pulsars in the Galactic Center (GC), high frequency monitoring of radio magnetars [13, 17, 18, 31, 32, 39, 40, 47, 48], multifrequency studies of giant pulses from the Crab pulsar [49, 50], and high frequency searches for fast radio bursts (FRBs).

The DSN radio telescopes are particularly well-suited for monitoring radio magnetars. These instruments allow for high cadence observations, which are important for tracking changes in the flux densities, pulse profile shapes, spectral indices, and single pulse behavior of radio magnetars, all of which can vary on daily timescales. High frequency observations are also essential because the spectral indices of radio magnetars are quite flat or inverted on average. In fact, the GC magnetar, PSR J1745–2900 (see Section 3), has been detected at record high radio frequencies [11, 12]. The DSN antennas are also capable of providing simultaneous, dual band observations with both circular polarizations, which is essential for accurate spectral index measurements and polarimetric studies. Additionally, since the large 70 m dishes have very low system temperatures, they are ideal for studying the morphology of single pulses from radio magnetars. We refer the reader to several of our recent papers (see [13, 17, 18, 31, 32, 39, 40, 47–50]), which are summarized in Sections 3–6.

3. PSR J1745–2900: The Galactic Center Magnetar

The GC magnetar, PSR J1745–2900, was serendipitously discovered by the Neil Gehrels Swift Observatory after a hard X-ray burst was detected on 2013 April 24 [33, 34]. The magnetar has DM (1778 3 pc ) and RM (–66960 50 rad ) values that are larger in magnitude than any known pulsar [51] (see Figure 3). It is located 0.1 pc from the Galaxy’s central 4 1 black hole, Sagittarius (Sgr ) [52], making it an excellent probe of the magneto-ionic environment near the inner region of the Galaxy. We recently carried out simultaneous radio observations of PSR J1745–2900 at 2.3 and 8.4 GHz during four separate epochs between 2015 July 30 and 2016 August 20 using the 70 m DSN radio telescope, DSS-43 (see Section 2) [13]. The observational parameters used for this study are provided by Pearlman et al. [13]. Here, we discuss our measurements of the magnetar’s radio profile shape, flux density, radio spectrum, and single pulse behavior, which are also described in detail in Pearlman et al. [13].

Figure 3: Image of the Canberra Deep Space Communication Complex (CDSCC). The 70 m telescope (DSS-43) is shown in the foreground, and the three 34 m beam waveguide antennas (DSS-34, DSS-35, and DSS-36) are shown in the background. Image credit: CDSCC (see https://www.cdscc.nasa.gov/Pages/antennas.html).

Figure 4: Average -band pulse profiles of the GC magnetar from observations with the 70 m DSN radio telescope, DSS-43, carried out on (a) 2015 July 30, (b) 2015 August 15, (c) 2016 April 1, and (d) 2016 August 20. This figure was adapted from Pearlman et al. [13].

Radio pulsations were detected at a period of ≈ 3.77 s in all of our observations [13]. The average -band pulse profiles, shown in Figure 4, are single-peaked during epochs 1–3 and double-peaked during epoch 4. The -band pulse profiles are not shown here since the pulsed emission was significantly weaker at this frequency. The mean flux density at -band was noticeably variable [13], and the -band flux densities on 2015 July 30 and August 15 were smaller by a factor of 7.5 compared to measurements performed 5 months earlier by Torne et al. [12]. Our measurements on 2016 April 1 and August 20 indicated that the -band flux density more than doubled since 2015 August 15 [13].

Multifrequency radio observations of PSR J1745–2900 revealed that its radio spectrum is often relatively flat or inverted [11, 12, 51], which is typical of most radio magnetars. However, its radio spectrum can also significantly steepen to levels comparable to ordinary radio pulsars [13, 53], which have an average spectral index of [54]. During epochs 1–3, Pearlman et al. [13] found that the magnetar exhibited a significantly negative average spectral index of when the average pulse profile was single-peaked, which is comparable to the steep spectrum derived by Pennucci et al. [53] between 2 and 9 GHz. The spectral index then significantly flattened to during epoch 4 when the profile displayed an additional component [13].

Pearlman et al. [13] also performed an analysis of single pulses detected at 8.4 GHz during epoch 3, which displayed the brightest pulses. They found that the single pulse structure observed from the GC magnetar was extremely variable in time, and the pulse morphology can be entirely different between successive rotations [13] (e.g., see Figure 5). Giant pulses, with flux densities more than ten times the mean flux level, and pulses with multiple emission components were detected during many of the magnetar’s rotations [13]. These giant pulses are different in nature from those emitted by the Crab pulsar [22]. There is also some evidence that the brightest emission component appears first during a given rotation and may trigger additional weaker outbursts [13].

Figure 5: Examples of bright single pulses detected from the GC magnetar at 8.4 GHz during epoch 3 (2016 April 1) with the 70 m DSN radio telescope, DSS-43. We show the single pulse profiles measured in the left circular polarization (LCP) and right circular polarization (RCP) channels during pulse cycles (a) 237 and (b) 239, where pulse numbers are referenced with respect to the start of the observation. The best-fit thin scattering screen model, described in the Appendix of Pearlman et al. [13], is overlaid in red on the single pulses in the top row. A pulse broadening timescale of 7.1 0.2 ms and 6.7 0.3 ms was measured in the corresponding polarization channels [13]. This figure was adapted from Pearlman et al. [13].

The observed flux density distribution of the single pulses could not be described by a log-normal distribution due to these giant pulses, and bright single pulses can sometimes form a high flux tail [13, 55]. This is similar to the single pulse behavior reported from the transient radio magnetar, XTE J1810–197, whose pulse-energy distribution is well described by a log-normal distribution at lower energies and a power-law at higher energies [56]. In particular, Pearlman et al. [13] measured a scaling exponent of from a power-law fit to the distribution of single pulse flux densities with peak fluxes greater 15 times than the mean level. The pulse intensity distribution is likely variable in time since a high flux tail has not persistently been observed from the GC magnetar [13, 55, 57, 58]. No correlation has been found between the peak flux density and the number of emission components in the single pulses [13]. In addition, an earlier single pulse analysis by Yan et al. [57] at 8.6 GHz revealed no obvious correlation between the width and the peak flux density of the GC magnetar’s strongest pulses, and there was no evidence of subpulse drifting during their observations.

Pearlman et al. [13] found that the typical intrinsic pulse width of the emission components was 1.8 ms, and they reported a prevailing delay time of 7.7 ms between successive components. Additionally, their analysis showed that some of the emission components at later pulse phases were detected more strongly in one of the circular polarization channels, which suggests that some of the magnetar’s emission components may be more polarized than others (e.g., see Figure 5(b)) [13]. The overall single pulse behavior during epoch 3 can be explained by fan beam emission with a width of , and tapering of the fan beam may give rise to fainter emission components at later pulse phases [13].

The GC magnetar’s emission region is thought to emit pulses fairly regularly since bright single pulses were detected during almost all rotations. During epoch 3, bright pulses were detected during 70% of the GC magnetar’s rotations, but often not at precisely the same phase [13]. At higher radio frequencies (45 GHz), Gelfand et al. [58] found that bright pulses, with an average width of 4.6 ms, were produced during a similar fraction of the magnetar’s rotations. However, radio observations at 3.1 GHz have shown that the GC magnetar can exhibit brief periods of pulsar nulling [59]. Strong single pulses have also been detected at radio frequencies up to 154 GHz [11], which suggests a broadband underlying emission mechanism.

Pearlman et al. [13] discovered significant frequency structure over bandwidths of 100 MHz in many of the single pulse emission components, which is the first time such behavior has been observed from a radio magnetar. They argued that these features could be produced by strong lensing from refractive plasma structures located near the inner part of the Galaxy, but may also be intrinsic to the magnetar and possibly similar to the banded structures observed in the Crab pulsar’s High-Frequency Interpulse [60]. Diffractive interstellar scintillation and instrumental effects were both ruled out as possible origins of this behavior [13]. If these features are indeed due to interstellar plasma lensing, then this suggests that a magneto-ionic medium close to the GC can boost the observed flux densities of pulses by more than an order of magnitude [13]. This behavior is reminiscent of the structure observed in pulses from the repeating FRB 121102 [61–63] and may indicate a connection with the larger population of FRBs. Furthermore, the GC magnetar and the repeating FRB 121102 have similar DM and RM values (see Figure 2) and emit pulses with similar morphology. An extragalactic magnetar near a massive black hole, perhaps not unlike PSR J1745–2900, is one of the currently favored progenitor theories for FRBs (e.g., [63–65]).

Recently, Pearlman et al. [13] showed that the emission components comprising the GC magnetar’s single pulses can be substantially broadened (e.g., see Figure 5(a)). A characteristic single pulse broadening timescale of 6.9 0.2 ms was reported at 8.4 GHz [13]. The pulse broadening magnitude was also found to be variable between pulses detected during consecutive pulse cycles and between pulse components in the same pulsar rotation [13]. Pearlman et al. [13] argued that this behavior could be intrinsic, a result of multiple successive unresolved low amplitude emission components, or extrinsic to the magnetar, possibly produced by high density plasma clouds traversing the radio beam at high velocities in the pulsar magnetosphere.

Multifrequency radio pulse profiles and single pulses from the GC magnetar revealed that the interstellar scattering is several orders of magnitude smaller than predicted by the NE2001 electron density model [53, 66, 67]. Spitler et al. [67] derived a scatter broadening timescale of 1.3 0.2 s at 1 GHz and a scatter broadening spectral index of –3.8 0.2 from the scattered pulse shapes of single pulses and average pulse profiles between 1.19 and 18.95 GHz. These results were used to argue for the existence of a single uniform, thin scattering screen at a distance of 5.8 0.3 kpc from the magnetar [68]. Subsequent radio interferometric measurements determined that the angular and temporal broadening are both produced by a single thin scattering screen located 4.2 kpc from the magnetar [69]. However, Pearlman et al. [13] showed that individual single pulses from the GC magnetar at 8.4 GHz can be broadened by more than an order of magnitude compared to what is predicted by Spitler et al. [67], which is incompatible with a static, thin scattering screen at distances 1 kpc. A secondary local screen (e.g., 0.1 pc from the magnetar [70]) is not expected to contribute significantly to the temporal broadening [71]. We refer the interested reader to Sections 4.2 and 4.3 of Pearlman et al. [13] for a detailed discussion of mechanisms which might reconcile the observed pulse broadening behavior.

4. PSR J1119–6127: A Transitional Magnetar

PSR J1119–6127 is a high magnetic field ( ≈ 4 1 G), rotation-powered pulsar with a spin period of P ≈ 410 ms [32]. The pulsar resides at the center of the supernova remnant (SNR) G292.2–0.5 [72–74], which lies in the Galactic plane at a distance of 8.4 kpc [75]. The pulsar’s DM (707.4 1.3 pc [76]) and RM (+853 2 rad [77]) are both large compared to other radio pulsars (see Figure 2), and it has one of the largest period derivatives ( ≈ 4 1 [24]), which implies a characteristic age of 2 kyr. PSR J1119–6127 is the first rotation-powered radio pulsar to display magnetar-like activity. In fact, only one other rotation-powered X-ray pulsar, PSR J1846–0258, has previously displayed similar behavior [78], but no radio pulsations have yet been detected from that object. Short X-ray outbursts were detected from PSR J1119–6127 on 2016 July 27 and 2016 July 28 with the Fermi Gamma-Ray Burst Monitor (GBM) and Swift Burst Alert Telescope (BAT), respectively [79, 80]. A large spin-up glitch [25, 81], hardening of the X-ray spectrum [25], additional X-ray bursts [26, 35], and irregular post-outburst timing behavior [26, 82] were subsequently reported.

Shortly after the first X-ray outbursts, we used the DSN’s 70 m radio telescope, DSS-43, to monitor changes in the pulsar’s radio emission during this period of magnetar-like activity. Simultaneous observations were regularly performed at 2.3 and 8.4 GHz over the course of 5 months following the initial X-ray bursts. After this period, the pulsar was intermittently monitored. Several of these observations are described in detail in Majid et al. [32], and additional results will be presented in an upcoming paper by Pearlman et al. [39].

We found that radio pulsations from PSR J1119–6127 disappeared after the initial X-ray outbursts, and the emission reactivated approximately two weeks later [18, 29–32, 39, 83]. The pulse profiles at 2.3 and 8.4 GHz dramatically evolved over a time period of several months after the radio emission resumed, which is atypical of ordinary radio pulsars [18, 32, 39]. The 2.3 GHz pulse profiles developed a multicomponent emission structure, while the 8.4 GHz profile showed a single emission peak that varied in strength during this time period [18, 32, 39]. Previous radio observations performed before the bursts indicated that the pulse profile was predominantly single-peaked, and an extremely rare double-peaked structure was seen only once after a strong glitch in 2007 [23, 77, 84, 85]. In contrast, our post-outburst radio observations showed that the 2.3 GHz pulse profile developed multiple emission components, two of which significantly weakened several weeks after the magnetar-like activity had subsided (see Figure 6) [18, 32, 39].

Figure 6: Average -band pulse profiles of PSR J1119–6127, obtained with the 70 m DSN radio telescope (DSS-43) on (a) 2016 August 19 and (b) 2016 September 1, during the pulsar’s recent episode of magnetar-like activity. This figure was adapted from Majid et al. [32]. Additional observations during the pulsar’s post-outburst recovery period will be presented in Pearlman et al. [39].

Simultaneous radio observations at 2.3 and 8.4 GHz during this period revealed that the spectral index was unusually steep and comparable to the average spectral index of normal radio pulsars when the pulse profiles were multi-peaked. As the profiles stabilized and became single-peaked, the pulsar’s radio spectrum flattened considerably [17, 18, 32, 39]. Significant changes in the pulsar’s rotational frequency, flux density, and single pulse behavior were also observed [17, 18, 31, 32, 39]. These results demonstrate that PSR J1119–6127 is a transitional object, i.e., a high magnetic field, rotation-powered radio pulsar capable of exhibiting transient magnetar-like behavior.

The detection of pulsed radio emission from several magnetars, along with the behavior observed from PSR J1119–6127 during its 2016 outburst, has weakened the notion that there is a sharp separation between magnetars and high magnetic field, rotation-powered radio pulsars. Historically, surface dipolar magnetic fields above the quantum critical value ( ≈ 4.4 1 G) and persistent X-ray luminosities exceeding rotational energy losses () were interpreted as some of the defining observational characteristics of magnetars, but these criteria are not always reliable predictors of magnetar-like behavior [86, 87]. In the case of PSR J1119–6127, the pulsar’s quiescent X-ray luminosity in the 0.5–10 keV energy band ( 0.9 1 erg ; Gonzalez et al. [88]) is several orders of magnitude smaller than its spin-inferred rotational energy ( 2.3 1 erg ), i.e., ≈ 0.0004, which suggests that the pulsar is often primarily powered by its rotation. However, during the 2016 outburst, the pulsar’s X-ray luminosity rose to ~ 0.1 [25]. Other radio magnetars, e.g., PSR J1622–4950, XTE J1810–197, and 1E 1547.0–5408, also have X-ray conversion efficiencies below unity ( 1) [87] and display transient high-energy emission. This suggests that some magnetars and high magnetic field pulsars may be powered through a combination of magnetic and rotational energy.

5. PSR J1622–4950

PSR J1622–4950 was first detected using the Parkes radio telescope, and it is the only magnetar discovered at radio wavelengths without prior knowledge of a corresponding X-ray counterpart [8]. The pulsar has a spin period of 4.33 s and a DM of 820 pc [8]. The timing solution, reported by Levin et al. [8], implies a very high surface magnetic field of 2.8 1 G and a characteristic age of 4 kyr. PSR J1622–4950 has a flat radio spectrum and a highly variable flux density and pulse profile [8–10, 16, 89], which is similar to other radio magnetars.

After its initial discovery, the magnetar was regularly monitored and detected with variable flux densities at the Parkes Observatory until 2014 March [16]. Scholz et al. [16] resumed monitoring the magnetar in 2015 January, but they failed to detect the pulsar through 2016 September. After remaining in a dormant state for roughly 2 years, PSR J1622–4950 resumed its radio emission sometime between January and April of 2017 [90]. Shortly after the radio reactivation, we carried out simultaneous observations of PSR J1622–4950 at -band and -band using the 70 m radio telescope, DSS-43, on 2017 May 23 [48]. Pearlman et al. [48] found that the magnetar’s radio spectrum had significantly steepened between 2.3 and 8.4 GHz, and its spectral behavior was consistent with the majority of ordinary pulsars.

Beginning in 2017 April, we used the 34 m DSN radio dishes near Canberra, Australia to initiate a monitoring program to observe PSR J1622–4950 over 30 epochs to date, already lasting more than a year. The observations were carried out using simultaneous S/X-band receivers. The data processing steps and observational setup were described in detail by Majid et al. [32]. The details of this monitoring campaign will be presented in Pearlman et al. [40]. The observing epochs were not regularly spaced during the year-long monitoring campaign due to various logistical issues, including scheduling constraints. Each observing epoch ranged in duration from roughly half an hour to four hours in length. The duration of each epoch was sufficient to obtain accurate measurements of the flux densities in both observing bands, spectral index, pulsar spin period, rate of single pulse emission, and pulse scatter broadening.

Our results indicate that the magnetar exhibited noticeable changes in its pulse profile at S-band (e.g., see Figure 7), but also particularly at X-band. PSR J1622–4950’s pulse profiles sometimes showed evidence of the emergence of a new pulse component. The magnetar’s flux density also showed variability in the range of a few mJy to tens of mJy. In addition, we also observed remarkable short-term changes in the magnetar’s emission behavior during a few observing epochs and detected bright single pulses with varying pulse morphology (e.g., see Figure 8).

Figure 7: Average -band pulse profiles of the radio magnetar, PSR J1622–4950, obtained using the 34 m radio telescopes near Canberra, Australia on (a) 2018 April 26 and (b) 2018 May 10. Additional details on these observations will be presented in Pearlman et al. [40].

Figure 8: Example of a bright single pulse detected at -band from PSR J1622–4950 during the observation on 2018 May 10 (see Figure 7(b)). In the top panel, we show the integrated single pulse profile with a time resolution of ∼5 ms. The dynamic spectrum, dedispersed at the magnetar's nominal DM of 820 pc , is shown in the bottom panel. This event has a S/N of 10 and was detected by convolving the dedispersed time-series with a 23 ms wide boxcar template. The shape of the single pulse is noticeably scatter-broadened.

6. XTE J1810–197

In July 2003, XTE J1810–197 was serendipitously discovered using the Rossi X-ray Timing Explorer (RXTE) after the magnetar underwent a transient X-ray outburst [91]. The pulsar’s rotational period was determined to be 5.54 s, and a high spin-down rate of 1 was measured, which suggests that the neutron star is young ( 7.6 kyr) and possesses a large spin-inferred dipolar magnetic field ( 2.6 1 G). Approximately one year later, a coincident radio source was found at the location of the pulsar [92]. Subsequently, pulsed radio emission was detected from the magnetar, making XTE J1810–197 the first magnetar with detected radio pulsations [6]. Multifrequency radio observations between 0.7 and 42 GHz revealed that the magnetar emits bright, highly linearly polarized radio pulses, comprised of narrow subpulses with widths ≤ 10 ms, during each rotation [6]. These results demonstrated that there is an underlying connection between magnetars and the larger population of ordinary radio pulsars.

Radio pulsations from XTE J1810–197 suddenly ceased without warning in late 2008, despite continued X-ray activity [21, 93, 94]. After more than 10 years in quiescence, bright radio pulsations were detected again on 2018 December 8 with the 76 m Lovell Telescope at Jodrell Bank [95]. Since the magnetar’s reactivation, numerous radio observatories have carried out follow-up observations of the magnetar [47, 96–100]. In particular, we observed XTE J1810–197 continuously for 5.5 hours on 2018 December 25 (MJD 58477.05623) using one of the DSN’s 34 m radio telescopes near Canberra, Australia [47]. Right circular polarization data were simultaneously recorded at center frequencies of 8.4 and 32 GHz, with roughly 500 MHz of bandwidth at each frequency band, using the JPL ultra-wideband pulsar machine. Our best estimate of the barycentric spin period and DM is 5.5414471(5) s and 178 9 pc , respectively. The average pulse profiles were noticeably variable, and we detected bright, multicomponent single pulses at both frequency bands. We measured mean flux densities of 4.0 0.8 mJy at 8.4 GHz and 1.7 0.3 mJy at 32 GHz, which yielded a spectral index of –0.7 0.2 over this wide frequency range [47]. Additional multifrequency observations are needed to study the magnetar’s behavior after its recent outburst. Towards this end, we are continuing to carry out high frequency radio observations of XTE J1810–197 using the DSN’s 70 and 34 m radio dishes near Canberra. A regular monitoring program of this magnetar is also planned.

7. Discussion and Conclusions

We have presented an overview of recent results from observations of the radio magnetars, PSR J1745–2900, PSR J1622–4950, and XTE J1810–197, and the transitional magnetar candidate, PSR J1119–6127, obtained using the DSN radio telescopes near Canberra, Australia. These studies provide further evidence of the variable nature of these objects. Each of these radio magnetars exhibited remarkable pulse profile changes over timescales of weeks to months, with large accompanied flux and spectral index variations. In combination with results at X-ray wavelengths, the variability observed in radio magnetars may be explained by the conditions of the magnetosphere [4, 45, 101], though the level of variability in each object likely depends on the size and geometry of magnetospheric deformations. Toroidal oscillations in the star may be excited during an outburst, which then modify the magnetospheric structure and allow radio emission to be produced. Since this emission behavior is clearly transitory, further radio monitoring of these objects is needed to study their long-term radiative and timing behavior.

The DSN has served as an excellent facility for performing state-of-the-art pulsar observations, as we have demonstrated through the study of the four magnetars discussed in this paper. The combination of the excellent sensitivity of the DSN antennas, particularly with the presence of a large 70 m diameter dish at each of the DSN complexes, multifrequency receivers, and the recent deployment of modern pulsar machines, offers an opportunity for pulsar observations that will be a significant addition to the already existing resources in pulsar astronomy. The availability of the 70 m antenna in Canberra, with its southern location, makes it an ideal resource, complementing the Parkes telescope, for observations of Galactic plane sources, including the Galactic Center. In search mode, the DSN’s pulsar machines offer high frequency and timing resolution with the ability to record multiple frequencies and incoming polarization bands simultaneously. This allows for observations with a high instantaneous sensitivity, which are quite useful for studies of single pulses. With precision tracking capabilities available at multiple frequencies, the DSN is particularly well-suited for carrying out observations at shorter wavelengths, which have proven useful for studying objects such as magnetars with flatter spectral indices and high DM pulsars.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

A. B. Pearlman acknowledges support by the Department of Defense (DoD) through the National Defense Science and Engineering Graduate (NDSEG) Fellowship Program and by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No. DGE-1144469. We thank the Jet Propulsion Laboratory’s Research and Technology Development Program and Caltech’s President’s and Director’s Fund for partial support at JPL and the Caltech campus. A portion of this research was performed at the Jet Propulsion Laboratory, California Institute of Technology and the Caltech campus, under a Research and Technology Development Grant through a contract with the National Aeronautics and Space Administration. US government sponsorship is acknowledged.

References

R. C. Duncan and C. Thompson, “Formation of very strongly magnetized neutron stars: implications for gamma-ray bursts,” The Astrophysical Journal Letters, vol. 392, no. 1, pp. L9–L13, 1992.

C. Thompson and R. C. Duncan, “The soft gamma repeaters as very strongly magnetized neutron stars - I. Radiative mechanism for outbursts,” Monthly Notices of the Royal Astronomical Society, vol. 275, no. 2, pp. 255–300, 1995.

C. Thompson and R. C. Duncan, “The soft gamma repeaters as very strongly magnetized neutron stars. II. quiescent neutrino, X-ray, and alfven wave emission,” The Astrophysical Journal Letters, vol. 473, no. 1, pp. 322–342, 1996.

V. M. Kaspi and A. M. Beloborodov, “Magnetars,” Annual Review of Astronomy and Astrophysics, vol. 55, pp. 261–301, 2017.

S. A. Olausen and V. M. Kaspi, “The McGill magnetar catalog,” The Astrophysical Journal Supplement Series, vol. 212, no. 1, p. 6, 2014.

F. Camilo, S. M. Ransom, J. P. Halpern et al., “Transient pulsed radio emission from a magnetar,” Nature, vol. 442, no. 7105, pp. 892–895, 2006.

F. Camilo, S. M. Ransom, J. Peñalver et al., “The variable radio-to-X-ray spectrum of the magnetar XTE J1810-197,” The Astrophysical Journal, vol. 669, no. 1, pp. 561–569, 2007.

L. Levin, M. Bailes, S. Bates et al., “A radio-loud magnetar in X-ray quiescence,” The Astrophysical Journal Letters, vol. 721, no. 1, pp. L33–L37, 2010.

M. J. Keith, S. Johnston, L. Levin, and M. Bailes, “17- and 24-GHz observations of southern pulsars,” Monthly Notices of the Royal Astronomical Society, vol. 416, pp. 346–354, 2011.

L. Levin, M. Bailes, S. D. Bates et al., “Radio emission evolution, polarimetry and multifrequency single pulse analysis of the radio magnetar PSR J1622-4950,” Monthly Notices of the Royal Astronomical Society, vol. 422, no. 3, pp. 2489–2500, 2012.

P. Torne, R. P. Eatough, R. Karuppusamy et al., “Simultaneous multifrequency radio observations of the galactic centre magnetar SGR J1745-2900,” Monthly Notices of the Royal Astronomical Society, vol. 451, no. 1, pp. L50–L54, 2015.

P. Torne, G. Desvignes, R. P. Eatough et al., “Detection of the magnetar SGR J1745-2900 up to 291 GHz with evidence of polarized millimetre emission,” Monthly Notices of the Royal Astronomical Society, vol. 465, no. 1, pp. 242–247, 2017.

A. B. Pearlman, W. A. Majid, T. A. Prince, J. Kocz, and S. Horiuchi, “Pulse morphology of the galactic center magnetar PSR J1745–2900,” The Astrophysical Journal, vol. 866, no. 2, p. 160, 2018.

F. Camilo, I. Cognard, S. M. Ransom et al., “The magnetar XTE J1810-197: variations in torque, radio flux density, and pulse profile morphology,” The Astrophysical Journal, vol. 663, no. 1 I, pp. 497–504, 2007.

K. Lazaridis, A. Jessner, M. Kramer et al., “Radio spectrum of the AXP J1810-197 and of its profile components,” Monthly Notices of the Royal Astronomical Society, vol. 390, no. 2, pp. 839–846, 2008.

P. Scholz, F. Camilo, J. Sarkissian et al., “Spin-down evolution and radio disappearance of the magnetar PSR J1622-4950,” The Astrophysical Journal, vol. 841, no. 2, p. 126, 2017.

A. B. Pearlman, W. A. Majid, S. Horiuchi et al., “Magnetar-like spectral index flattening of the high magnetic field pulsar PSR J1119-6127,” The Astronomer’s Telegram, no. 9870, 2016.

A. B. Pearlman, W. A. Majid, S. Horiuchi, J. Kocz, J. Lippuner, and T. A. Prince, “Magnetar-like activity and radio emission variability from the high magnetic field pulsar PSR J1119-6127,” AAS/High Energy Astrophysics Division, vol. 16, p. 109, 2017.

F. Camilo, J. Reynolds, S. Johnston, J. P. Halpern, S. M. Ransom, and W. Van Straten, “Polarized radio emission from the magnetar XTE J1810-197,” The Astrophysical Journal, vol. 659, no. 1, pp. L37–L40, 2007.

F. Camilo, J. Reynolds, S. Johnston, J. P. Halpern, and S. M. Ransom, “The magnetar 1E 1547.0-5408: radio spectrum, polarimetry, and timing,” The Astrophysical Journal, vol. 679, no. 1, pp. 681–686, 2008.

F. Camilo, S. M. Ransom, J. P. Halpern et al., “Radio disappearance of the magnetar XTE J1810-197 and continued X-ray timing,” The Astrophysical Journal, vol. 820, p. 110, 2016.

J. M. Cordes, N. D. R. Bhat, T. H. Hankins, M. A. McLaughlin, and J. Kern, “The brightest pulses in the universe: Multifrequency observations of the Crab pulsar’s giant pulses,” The Astrophysical Journal, vol. 612, no. 1 I, pp. 375–388, 2004.

P. Weltevrede, S. Johnston, and C. M. Espinoza, “The glitch-induced identity changes of PSR J1119-6127,” Monthly Notices of the Royal Astronomical Society, vol. 411, no. 3, pp. 1917–1934, 2011.

D. Antonopoulou, P. Weltevrede, C. M. Espinoza et al., “The unusual glitch recoveries of the high-magnetic-field pulsar J1119-6127,” Monthly Notices of the Royal Astronomical Society, vol. 447, no. 4, pp. 3924–3935, 2015.

R. F. Archibald, V. M. Kaspi, S. P. Tendulkar, and P. Scholz, “A magnetar-like outburst from a High-B radio pulsar,” The Astrophysical Journal, vol. 829, p. L21, 2016.

R. F. Archibald, V. M. Kaspi, S. P. Tendulkar, and P. Scholz, “The 2016 outburst of PSR J1119-6127: cooling and a spin-down-dominated glitch,” The Astrophysical Journal, vol. 869, no. 2, p. 180, 2018.

M. Burgay, G. L. Israel, A. Possenti et al., “Back to radio: parkes detection of radio pulses from the transient AXP 1E1547.0-5408,” The Astronomer’s Telegram, no. 1913, 2009.

F. Camilo, J. P. Halpern, and S. M. Ransom, “Radio pulsations not detected from AXP/SGR 1E1547.0-5408 following recent outburst,” The Astronomer’s Telegram, no. 1907, 2009.

M. Burgay, A. Possenti, M. Kerr et al., “Pulsed radio emission from PSR J1119-6127 disappeared,” The Astronomer’s Telegram, no. 9286, 2016.

M. Burgay, A. Possenti, M. Kerr et al., “Pulsed radio emission from PSR J1119-6127 re-activated,” The Astronomer’s Telegram, no. 9366, 2016.

W. Majid, T. Dobreva, J. Kocz et al., “High frequency radio observations of PSR J1119-6127,” The Astronomer’s Telegram, no. 9321, 2016.

W. A. Majid, A. B. Pearlman, T. Dobreva et al., “Post-outburst radio observations of the high magnetic field pulsar PSR J1119-6127,” The Astrophysical Journal Letters, vol. 834, p. L2, 2017.

N. Degenaar, M. T. Reynolds, J. M. Miller, J. A. Kennea, and R. Wijnands, “Large flare from Sgr A∗ detected by Swift,” The Astronomer’s Telegram, no. 5006, 2013.

J. A. Kennea, D. N. Burrows, C. Kouveliotou et al., “Swift discovery of a new soft gamma repeater, SGR J1745-29, near Sagittarius A∗,” The Astrophysical Journal Letters, vol. 770, p. L24, 2013.

E. Göǧüş, L. Lin, Y. Kaneko et al., “Magnetar-like X-ray bursts from a rotation-powered pulsar, PSR J1119-6127,” The Astrophysical Journal Letters, vol. 829, p. L25, 2016.

R. N. Manchester, G. B. Hobbs, A. Teoh, and M. Hobbs, “The Australia telescope national facility pulsar catalogue,” The Astronomical Journal, vol. 129, no. 4, pp. 1993–2006, 2005.

B. Zhang, A. K. Harding, and A. G. Muslimov, “Radio pulsar death line revisited: Is PSR J2144-3933 anomalous?” The Astrophysical Journal, vol. 531, no. 2, pp. L135–L138, 2000.

E. Petroff, E. D. Barr, A. Jameson et al., “FRBCAT: the fast radio burst catalogue,” Publications of the Astronomical Society of Australia (PASA), vol. 33, article e045, 2016.

A. B. Pearlman, W. A. Majid, T. A. Prince et al., “In preparation,” 2019.

A. B. Pearlman, A. Cook, A. Amsellem et al., “In preparation,” 2019.

P. M. Woods and C. Thompson, “Soft gamma repeaters and anomalous X-ray pulsars: magnetar candidates,” in Compact Stellar X-Ray Sources, pp. 547–586, 2006.

V. M. Kaspi, “Grand unification of neutron stars,” Proceedings of the National Acadamy of Sciences of the United States of America (PNAS), vol. 107, no. 16, pp. 7147–7152, 2010.

N. Rea and P. Esposito, “Magnetar outbursts: an observational review,” Astrophysics and Space Science Proceedings, vol. 21, pp. 247–273, 2011.

S. Mereghetti, J. A. Pons, and A. Melatos, “Magnetars: properties, origin and evolution,” Space Science Reviews, vol. 191, no. 1-4, pp. 315–338, 2015.

R. Turolla, S. Zane, and A. L. Watts, “Magnetars: the physics behind observations. A review,” Reports on Progress in Physics, vol. 78, no. 11, p. 116901, 2015.

V. M. Kaspi and M. Kramer, “Radio pulsars: the neutron star population fundamental physics,” 2016, https://arxiv.org/abs/1602.07738.

W. A. Majid, A. B. Pearlman, T. A. Prince et al., “High frequency radio observations of the reactivated magnetar XTE J1810-197,” The Astronomer’s Telegram, no. 12353, p. 1, 2019.

A. B. Pearlman, W. A. Majid, T. A. Prince et al., “High frequency radio observations of the reactivated magnetar PSR J1622-4950,” The Astronomer’s Telegram, no. 10581, 2017.

W. A. Majid, C. J. Naudet, S. T. Lowe, and T. B. Kuiper, “Statistical studies of giant pulse emission from the Crab pulsar,” The Astrophysical Journal, vol. 741, p. 53, 2011.

W. Majid, “A multi-wavelength campaign to study giant pulses from the Crab pulsar,” in Neutron Stars and Pulsars: Challenges and Opportunities after 80 years, J. van Leeuwen, Ed., vol. 291 of IAU Symposium, pp. 322–322, 2013.

R. P. Eatough, H. Falcke, R. Karuppusamy et al., “A strong magnetic field around the supermassive black hole at the centre of the galaxy,” Nature, vol. 501, no. 7467, pp. 391–393, 2013.

G. C. Bower, A. Deller, P. Demorest et al., “The proper motion of the galactic center pulsar relative to Sagittarius A∗,” The Astrophysical Journal, vol. 798, no. 2, p. 120, 2015.

T. T. Pennucci, A. Possenti, P. Esposito et al., “Simultaneous multi-band radio and X-ray observations of the galactic center magnetar SGR 1745-2900,” The Astrophysical Journal, vol. 808, no. 1, p. 81, 2015.

O. Maron, J. Kijak, M. Kramer, and R. Wielebinski, “Pulsar spectra of radio emission,” Astronomy and Astrophysics Supplement Series, vol. 147, no. 2, pp. 195–203, 2000.

R. S. Lynch, R. F. Archibald, V. M. Kaspi, and P. Scholz, “Green Bank Telescope and Swift X-ray telescope observations of the galactic center radio magnetar SGR J1745–2900,” The Astrophysical Journal, vol. 806, no. 2, article 266, 2015.

M. Serylak, B. W. Stappers, P. Weltevrede et al., “Simultaneous multifrequency single-pulse properties of AXP XTE J1810-197,” Monthly Notices of the Royal Astronomical Society, vol. 394, no. 1, pp. 295–308, 2009.

Z. Yan, Z. Shen, X. Wu et al., “Single-pulse radio observations of the galactic center magnetar PSR J1745–2900,” The Astrophysical Journal, vol. 814, no. 1, article 5, 2015.

J. D. Gelfand, S. Ransom, C. Kouveliotou et al., “The high-frequency radio emission of the galactic center magnetar SGR J1745–29 during a transitional period,” The Astrophysical Journal, vol. 850, no. 1, article 53, 2017.

W. M. Yan, N. Wang, R. N. Manchester, Z. G. Wen, and J. P. Yuan, “Single-pulse observations of the galactic center magnetar PSR J1745−2900 at 3.1 GHz,” Monthly Notices of the Royal Astronomical Society, vol. 476, no. 3, pp. 3677–3687, 2018.

T. H. Hankins, J. A. Eilek, and G. Jones, “The Crab pulsar at centimeter wavelengths II: single pulses,” The Astrophysical Journal, vol. 833, no. 1, article 47, 2016.

L. G. Spitler, P. Scholz, J. W. T. Hessels et al., “A repeating fast radio burst,” Nature, vol. 531, pp. 202–205, 2016.

V. Gajjar, A. P. V. Siemion, D. C. Price et al., “Highest-frequency detection of FRB 121102 at 4-8 GHz using the breakthrough listen digital backend at the Green Bank Telescope,” The Astrophysical Journal, vol. 863, no. 1, 2018.

D. Michilli, A. Seymour, J. W. T. Hessels et al., “An extreme magneto-ionic environment associated with the fast radio burst source FRB 121102,” Nature, vol. 553, pp. 182–185, 2018.

U.-L. Pen and L. Connor, “Local circumnuclear magnetar solution to extragalactic fast radio bursts,” The Astrophysical Journal, vol. 807, no. 2, article 179, 2015.

B. D. Metzger, E. Berger, and B. Margalit, “Millisecond magnetar birth connects FRB 121102 to superluminous supernovae and long-duration gamma-ray bursts,” The Astrophysical Journal, vol. 841, no. 1, article 14, 2017.

J. M. Cordes and T. J. W. Lazio, “NE2001. I. A new model for the galactic distribution of free electrons and its fluctuations,” 2002, https://arxiv.org/abs/astro-ph/0207156v3.

L. G. Spitler, K. J. Lee, R. P. Eatough et al., “Pulse broadening measurements from the Galactic center pulsar J1745-2900,” The Astrophysical Journal Letters, vol. 780, no. 1, p. L3, 2014.

G. C. Bower, A. Deller, P. Demorest et al., “The angular broadening of the galactic center pulsar SGR J1745-29: a new constraint on the scattering medium,” The Astrophysical Journal Letters, vol. 780, no. 1, p. L2, 2014.

O. Wucknitz, “Probing interstellar scattering towards the Galactic centre with pulsar VLBI,” in Proceedings of the 12th European VLBI Network Symposium and Users Meeting, A. Tarchi, M. Giroletti, and L. Feretti, Eds., vol. 66, pp. 1–6, Trieste, Italy, 2014.

G. Desvignes, R. P. Eatough, U. L. Pen et al., “Large magneto-ionic variations toward the galactic center magnetar, PSR J1745-2900,” The Astrophysical Journal Letters, vol. 852, no. 1, p. L12, 2018.

J. Dexter, A. Deller, G. C. Bower et al., “Locating the intense interstellar scattering towards the inner Galaxy,” Monthly Notices of the Royal Astronomical Society, vol. 471, no. 3, pp. 3563–3576, 2017.

F. Crawford, B. M. Gaensler, V. M. Kaspi et al., “A radio supernova remnant associated with the young pulsar J1119-6127,” The Astrophysical Journal, vol. 554, no. 1, pp. 152–160, 2001.

M. J. Pivovaroff, V. M. Kaspi, F. Camilo, B. M. Gaensler, and F. Crawford, “X-ray observations of the new pulsar-supernova remnant system PSR J1119-6127 and supernova remnant G292.2-0.5,” The Astrophysical Journal, vol. 554, no. 1, pp. 161–172, 2001.

H. S. Kumar, S. Safi-Harb, and M. E. Gonzalez, “Chandra and XMM-Newton studies of the supernova remnant G292.2-0.5 associated with the pulsar J1119-6127,” The Astrophysical Journal, vol. 754, no. 2, article 96, 2012.

J. L. Caswell, N. M. McClure-Griffiths, and M. C. M. Cheung, “Supernova remnant G292.2-0.5, its pulsar, and the Galactic magnetic field,” Monthly Notices of the Royal Astronomical Society, vol. 352, no. 4, pp. 1405–1412, 2004.

C. He, C. Ng, and V. M. Kaspi, “The correlation between dispersion measure and X-ray column density from radio pulsars,” The Astrophysical Journal, vol. 768, no. 1, article 64, 2013.

S. Johnston and J. M. Weisberg, “Profile morphology and polarization of young pulsars,” Monthly Notices of the Royal Astronomical Society, vol. 368, no. 4, pp. 1856–1870, 2006.

F. P. Gavriil, M. E. Gonzalez, E. V. Gotthelf et al., “Magnetar-like emission from the young pulsar in Kes 75,” Science, vol. 319, no. 5871, pp. 1802–1805, 2008.

J. A. Kennea, A. Y. Lien, F. E. Marshall, D. M. Palmer, T. G. R. Roegiers, and B. Sbarufatti, “Swift detection of a SGR burst from PSR 1119-6127,” The Astronomer’s Telegram, no. 9274, 2016.

G. Younes, C. Kouveliotou, and O. Roberts, “GBM observation of SGR-like burst from the direction of PSR 1119-6127,” GRB Coordinates Network, Circular Service, vol. 19736, no. 1, 2016.

R. F. Archibald, S. P. Tendulkar, P. Scholz, and V. M. Kaspi, “Glitch observation and hard X-ray power law measurement in PSR J1119-6127,” The Astronomer’s Telegram, no. 9284, 2016.

L. C.-C. Lin, H.-H. Wang, K.-L Li et al., “Investigation of the high-energy emission from the magnetar-like pulsar PSR J1119–6127 after the 2016 outburst,” The Astrophysical Journal, vol. 866, no. 1, article 6, 2018.

S. Dai, S. Johnston, P. Weltevrede et al., “Peculiar spin frequency and radio profile evolution of PSR J1119-6127 following magnetar-like X-ray bursts,” Monthly Notices of the Royal Astronomical Society, vol. 480, no. 3, pp. 3584–3594, 2018.

F. Camilo, V. M. Kaspi, A. G. Lyne et al., “Discovery of two high magnetic field radio pulsars,” The Astrophysical Journal, vol. 541, no. 1, pp. 367–374, 2000.

F. Crawford and N. C. Keim, “Radio polarization of the young high-magnetic-field pulsar PSR J1119-6127,” The Astrophysical Journal, vol. 590, no. 2 I, pp. 1020–1025, 2003.

S. Mereghetti, “The strongest cosmic magnets: soft gamma-ray repeaters and anomalous X-ray pulsars,” The Astronomy and Astrophysics Review, vol. 15, no. 4, pp. 225–287, 2008.

N. Rea, J. A. Pons, D. F. Torres, and R. Turolla, “The fundamental plane for radio magnetars,” The Astrophysical Journal, vol. 748, no. 1, p. L12, 2012.

M. E. Gonzalez, V. M. Kaspi, F. Camilo, B. M. Gaensler, and M. J. Pivovaroff, “Unusual pulsed X-ray emission from the young, high magnetic field pulsar PSR J1119–6127,” The Astrophysical Journal, vol. 630, no. 1, article 489, 2005.

G. E. Anderson, B. M. Gaensler, P. O. Slane et al., “Multi-wavelength observations of the radio magnetar PSR J1622-4950 and discovery of its possibly associated supernova remnant,” The Astrophysical Journal, vol. 751, no. 1, article 53, 2012.

F. Camilo, P. Scholz, M. Serylak et al., “Revival of the magnetar PSR J1622-4950: observations with MeerKAT, Parkes, XMM-Newton, Swift, Chandra, and NuSTAR,” The Astrophysical Journal, vol. 856, article 180, 2018.

A. I. Ibrahim, C. B. Markwardt, J. H. Swank et al., “Discovery of a transient magnetar: XTE J1810-197,” The Astrophysical Journal, vol. 609, no. 1, pp. L21–L24, 2004.

J. P. Halpern, E. V. Gotthelf, R. H. Becker, D. J. Helfand, and R. L. White, “Discovery of radio emission from the transient anomalous X-ray pulsar XTE J1810-197,” The Astrophysical Journal Letters, vol. 632, no. 1, pp. L29–L32, 2005.

F. Pintore, F. Bernardini, S. Mereghetti et al., “The variable spin-down rate of the transient magnetar XTE J1810−197,” Monthly Notices of the Royal Astronomical Society, vol. 458, no. 2, pp. 2088–2093, 2016.

F. Pintore, S. Mereghetti, P. Esposito et al., “The 11 yr of low activity of the magnetar XTE J1810−197,” Monthly Notices of the Royal Astronomical Society, vol. 483, no. 3, pp. 3832–3838, 2019.

A. Lyne, L. Levin, B. Stappers, M. Mickaliger, G. Desvignes, and M. Kramer, “Intense radio flare from the magnetar XTE J1810–197,” The Astronomer’s Telegram, no. 12284, 2018.

G. Desvignes, R. Eatough, M. Kramer et al., “High-frequency observations of the radio magnetar XTE J1810–197,” The Astronomer’s Telegram, no. 12285, 2018.

M. E. Lower, C. Flynn, M. Bailes et al., “Detection of low-frequency radio emission from the magnetar XTE J1810–197,” The Astronomer’s Telegram, no. 12288, 2018.

B. C. Joshi, Y. Maan, M. P. Surnis, M. Bagchi, and P. K. Manoharan, “Detection of pulsed radio emission from bursting magnetar XTE J1810-197 below 750 MHz with the uGMRT,” The Astronomer’s Telegram, no. 12312, 2018.

S. Del Palacio, F. Garcia, L. Combi et al., “Follow up of the radio flare from the magnetar XTE J1810-197 at 1.4 GHz,” The Astronomers Telegram, no. 12323, 2018.

S. A. Trushkin, N. N. Bursov, P. G. Tsybulev, N. A. Nizhelskij, and A. Erkenov, “RATAN-600 multi-frequency measurements of the magnetar XTE J1810-197,” The Astronomer’s Telegram, no. 12372, 2019.

R. F. Archibald, M. Burgay, M. Lyutikov et al., “Magnetar-like X-ray bursts suppress pulsar radio emission,” The Astrophysical Journal Letters, vol. 849, no. 2, p. L20, 2017.