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Register a new userThe magnetar 1E 1547.0-5408: radio spectrum, polarimetry, and timing
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F. Camilo
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We have investigated the radio emission from the anomalous X-ray pulsar 1E 1547.0-5408 (PSR J1550-5418) using the Parkes telescope and the Australia Telescope Compact Array. The flux density of the pulsar is roughly the same between 1.4 and 45 GHz, but shows time variability. The radiation is nearly 100% linearly polarized between frequencies of 45 and 3.2 GHz, but from 2.3 to 1.4 GHz it becomes increasingly more depolarized. The rotation measure of -1860 rad/m^2 is the largest for any known pulsar, and implies an average magnetic field strength along the line of sight of 2.7 microG. The pulse profiles are circularly polarized at all frequencies observed, more so at lower frequencies, at the ~15% level. The observed swing of the position angle of linear polarization as a function of pulse phase suggests that in this neutron star the rotation and magnetic axes are nearly aligned, and that its radio emission is only detectable within a small solid angle. Timing measurements indicate that the period derivative of this 2 s pulsar has increased by nearly 40% in a 6-month period. The flat spectrum and variability in flux density and pulse profiles are reminiscent of the properties of XTE J1810-197, the only other known radio-emitting magnetar, and are anomalous by comparison with those of ordinary radio pulsars.
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We present the evolution of the X-ray emission properties of the magnetar 1E 1547.0-5408 since February 2004 over a time period covering three outbursts. We analyzed new and archival observations taken with the Swift, NuSTAR, Chandra and XMM-Newton X-ray satellites. The source has been observed at a relatively steady soft X-ray flux of $approx$ 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$ (0.3-10 keV) over the last 9 years, which is about an order of magnitude fainter than the flux at the peak of the last outburst in 2009, but a factor of $sim$ 30 larger than the level in 2006. The broad-band spectrum extracted from two recent NuSTAR observations in April 2016 and February 2019 showed a faint hard X-ray emission up to $sim$ 70 keV. Its spectrum is adequately described by a flat power law component, and its flux is $sim$ $7 times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ (10-70 keV), that is a factor of $sim$ 20 smaller than at the peak of the 2009 outburst. The hard X-ray spectral shape has flattened significantly in time, which is at variance with the overall cooling trend of the soft X-ray component. The pulse profile extracted from these NuSTAR pointings displays variability in shape and amplitude with energy (up to $approx$ 25 keV). Our analysis shows that the flux of 1E 1547.0-5408 is not yet decaying to the 2006 level and that the source has been lingering in a stable, high-intensity state for several years. This might suggest that magnetars can hop among distinct persistent states that are probably connected to outburst episodes and that their persistent thermal emission can be almost entirely powered by the dissipation of currents in the corona.
We investigated the radio spectra of two magnetars, PSR J1622$-$4950 and 1E 1547.0$-$5408, using observations from the Australia Telescope Compact Array and the Atacama Large Millimeter/submillimeter Array taken in 2017. Our observations of PSR J1622$-$4950 show a steep spectrum with a spectral index of $-$1.3 $pm$ 0.2 in the range of 5.5-45 GHz during its re-activating X-ray outburst in 2017. By comparing the data taken at different epochs, we found significant enhancement in the radio flux density. The spectrum of 1E 1547.0$-$5408 was inverted in the range of 43-95 GHz, suggesting a spectral peak at a few hundred gigahertz. Moreover, we obtained the X-ray and radio data of radio magnetars, PSR J1622$-$4950 and SGR J1745$-$2900, from literature and found two interesting properties. First, radio emission is known to be associated with X-ray outburst but has different evolution. We further found that the rising time of the radio emission is much longer than that of the X-ray during the outburst. Second, the radio magnetars may have double peak spectra at a few GHz and a few hundred GHz. This could indicate that the emission mechanism is different in the cm and the sub-mm bands. These two phenomenons could provide a hint to understand the origin of radio emission and its connection with the X-ray properties.
A bright burst, followed by an X-ray tail lasting ~10 ks, was detected during an XMM-Newton observation of the magnetar 1E 1547.0-5408 carried out on 2009 February 3. The burst, also observed by SWIFT/BAT, had a spectrum well fit by the sum of two blackbodies with temperatures of ~4 keV and 10 keV and a fluence in the 0.3-150 keV energy range of ~1e-5 erg/cm2. The X-ray tail had a fluence of ~4e-8 erg/cm2. Thanks to the knowledge of the distances and relative optical depths of three dust clouds between us and 1E 1547.0-5408, we show that most of the X-rays in the tail can be explained by dust scattering of the burst emission, except for the first ~20-30 s. We point out that other X-ray tails observed after strong magnetar bursts may contain a non-negligible contribution due to dust scattering.
The Suzaku data of the highly variable magnetar 1E 1547.0$-$5408, obtained during the 2009 January activity, were reanalyzed. The 2.07 s pulsation of the 15--40 keV emission detected with the HXD was found to be phase modulated, with a period of $36.0^{+4.5}_{-2.5}$ ks and an amplitude of $0.52 pm 0.14$ s. The modulation waveform is suggested to be more square-wave like rather than sinusoidal. While the effect was confirmed with the 10--14 keV XIS data, the modulation amplitude decreased towards lower energies, becoming consistent with 0 below 4 keV. After the case of 4U 0142+61, this makes the 2nd example of this kind of behavior detected from magnetars. The effect can be interpreted as a manifestation of free precession of this magnetar, which is suggested to be oblately deformed under the presence of strong toroidal field of $sim 10^{16}$ G.
This paper describes an analysis of the NuSTAR data of the fastest-rotating magnetar 1E 1547$-$5408, acquired in 2016 April for a time lapse of 151 ks. The source was detected with a 1-60 keV flux of $1.7 times 10^{-11}$ ergs s$^{-1}$ cm$^{-2}$, and its pulsation at a period of $2.086710(5)$ sec. In 8-25 keV, the pulses were phase-modulated with a period of $T=36.0 pm 2.3$ ks, and an amplitude of $sim 0.2$ sec. This reconfirms the Suzaku discovery of the same effect at $T=36.0 ^{+4.5}_{-2.5} $ ks, made in the 2009 outburst. These results strengthen the view derived from the Suzaku data, that this magnetar performs free precession as a result of its axial deformation by $sim 0.6 times 10^{-4}$, possibly caused by internal toroidal magnetic fields reaching $sim 10^{16}$ G. Like in the Suzaku case, the modulation was not detected in energies below $sim 8$ keV. Above 10 keV, the pulse-phase behaviour, including the 36 ks modulation parameters, exhibited complex energy dependences: at $sim 22$ keV, the modulation amplitude increased to $sim 0.5$ sec, and the modulation phase changed by $sim 65^circ$ over 10--27 keV, followed by a phase reversal. Although the pulse significance and pulsed fraction were originally very low in $>10$ keV, they both increased noticeably, when the arrival times of individual photons were corrected for these systematic pulse-phase variations. Possible origins of these complex phenomena are discussed, in terms of several physical processes that are specific to ultra-strong magnetic fields.
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