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Register a new userThe X-ray reactivation of the radio bursting magnetar SGR 1935+2154
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A few years after its discovery as a magnetar, SGR J1935+2154 started a new burst-active phase on 2020 April 27, accompanied by a large enhancement of its X-ray persistent emission. Radio single bursts were detected during this activation, strengthening the connection between magnetars and fast radio bursts. We report on the X-ray monitoring of SGR J1935+2154 from ~3 days prior to ~3 weeks after its reactivation, using Swift, NuSTAR, and NICER. We detected X-ray pulsations in the NICER and NuSTAR observations, and constrained the spin period derivative to |Pdot| < 3e-11 s/s (3 sigma c.l.). The pulse profile showed a variable shape switching between single and double-peaked as a function of time and energy. The pulsed fraction decreased from ~34% to ~11% (5-10 keV) over ~10 days. The X-ray spectrum was well fit by an absorbed blackbody model with temperature decreasing from kT ~ 1.6 to 0.45-0.6 keV, plus a non-thermal component (Gamma ~ 1.2) observed up to ~25 keV with NuSTAR. The 0.3-10 keV X-ray luminosity (at 6.6 kpc) increased in less than four days from ~ 6e33 erg/s to about 3e35 erg/s and then decreased again to 2.5e34 erg/s over the following three weeks of the outburst. We also detected several X-ray bursts, with properties typical of short magnetar bursts.
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Fast radio bursts (FRBs) are millisecond-duration, bright radio signals (fluence $mathrm{0.1 - 100,Jy,ms}$) emitted from extragalactic sources of unknown physical origin. The recent CHIME/FRB and STARE2 detection of an extremely bright (fluence $sim$MJy$,$ms) radio burst from the Galactic magnetar SGR~1935$+$2154 supports the hypothesis that (at least some) FRBs are emitted by magnetars at cosmological distances. In follow-up observations totalling 522.7$,$hrs on source, we detect two bright radio bursts with fluences of $112pm22mathrm{,Jy,ms}$ and $24pm5mathrm{,Jy,ms}$, respectively. Both bursts appear affected by interstellar scattering and we measure significant linear and circular polarisation for the fainter burst. The bursts are separated in time by $sim$1.4$,$s, suggesting a non-Poissonian, clustered emission process -- similar to what has been seen in some repeating FRBs. Together with the burst reported by CHIME/FRB and STARE2, as well as a much fainter burst seen by FAST (fluence 60$mathrm{,mJy,ms}$), our observations demonstrate that SGR 1935+2154 can produce bursts with apparent energies spanning roughly seven orders of magnitude, and that the burst rate is comparable across this range. This raises the question of whether these four bursts arise from similar physical processes, and whether the FRB population distribution extends to very low energies ($sim10^{30},$erg, isotropic equivalent).
We report on INTEGRAL observations of the soft $gamma$-ray repeater SGR 1935+2154 performed between 2020 April 28 and May 3. Several short bursts with fluence of $sim10^{-7}-10^{-6}$ erg cm$^{-2}$ were detected by the IBIS instrument in the 20-200 keV range. The burst with the hardest spectrum, discovered and localized in real time by the INTEGRAL Burst Alert System, was spatially and temporally coincident with a short and very bright radio burst detected by the CHIME and STARE2 radio telescopes at 400-800 MHz and 1.4 GHz, respectively. Its lightcurve shows three narrow peaks separated by $sim$29 ms time intervals, superimposed on a broad pulse lasting $sim$0.6 s. The brightest peak had a delay of 6.5$pm$1.0 ms with respect to the 1.4 GHz radio pulse (that coincides with the second and brightest component seen at lower frequencies). The burst spectrum, an exponentially cut-off power law with photon index $Gamma=0.7_{-0.2}^{+0.4}$ and peak energy $E_p=65pm5$ keV, is harder than those of the bursts usually observed from this and other magnetars. By the analysis of an expanding dust scattering ring seen in X-rays with the {it Neil Gehrels Swift Observatory} XRT instrument, we derived a distance of 4.4$_{-1.3}^{+2.8}$ kpc for SGR 1935+2154, independent of its possible association with the supernova remnant G57.2+0.8. At this distance, the burst 20-200 keV fluence of $(6.1pm 0.3)times10^{-7}$ erg cm$^{-2}$ corresponds to an isotropic emitted energy of $sim1.4times10^{39}$ erg. This is the first burst with a radio counterpart observed from a soft $gamma$-ray repeater and it strongly supports models based on magnetars that have been proposed for extragalactic fast radio bursts.
Very recently, an extremely bright fast radio burst (FRB) 200428 with two sub-millisecond pulses was discovered to come from the direction of the Galactic magnetar SGR 1935+2154, and an X-ray burst (XRB) counterpart was detected simultaneously. These observations favor magnetar-based interior-driven models. In this Letter, we propose a different model for FRB 200428 associated with an XRB from SGR 1935+2154, in which a magnetar with high proper velocity encounters an asteroid of mass $sim10^{20},$g. This infalling asteroid in the stellar gravitational field is first possibly disrupted tidally into a great number of fragments at radius $sim {rm a,,few}$ times $10^{10},$cm, and then slowed around the Alfv$acute{rm e}$n radius by an ultra-strong magnetic field and in the meantime two major fragments of mass $sim 10^{17},$g that cross magnetic field lines produce two pulses of FRB 200428. The whole asteroid is eventually accreted onto the poles along magnetic field lines, impacting the stellar surface, creating a photon-e$^pm$ pair fireball trapped initially in the stellar magnetosphere, and further leading to an XRB. We show that this gravitationally-powered model can interpret all of the observed features self-consistently.
Recently, a bright coherent radio burst with millisecond duration, reminiscent of cosmological fast radio bursts (FRBs), was co-detected with an anomalously-hard X-ray burst from a Galactic magnetar SGR 1935$+$2154. We investigate the possibility that the event was triggered by a deposition of a magnetic energy in a localized region of the magnetosphere, thereby producing a so-called trapped fireball (FB) and simultaneously launching relativistic outflows. We show that the thermal component of the X-ray burst spectrum is consistent with a trapped FB with an average temperature of a few hundred keV and a size of $sim10^5$ cm. Meanwhile, the non-thermal component of the X-ray burst and the coherent radio burst may arise from relativistic outflows. We calculate the dynamical evolution of the outflow, launched with an energy budget $sim10^{39}mbox{-}10^{40}$ erg comparable to that of the trapped FB, for a variety of baryon load $eta$ and initial magnetization $sigma_0$ parameters. If both the hard X-ray and radio bursts are produced by the energy dissipation of the outflow, the properties can be constrained by the conditions for photon escape and the intrinsic timing offset of $lesssim 10$ ms among the radio and X-ray burst spikes. We show that the hard X-ray bursts need to be generated at $r_{rm X}gtrsim10^{8}$ cm from the stellar surface, irrespective of the emission mechanism. Particularly in the case of shock dissipation, the outflow should accelerate up to a Lorentz factor of $Gamma gtrsim 10^3$ by the time it reaches the outer edge of the magnetosphere and the shock dissipation should take place at $10^{12},mathrm{cm} lesssim r_{rm radio, X} lesssim 10^{14},mathrm{cm}$. In this case, extremely clean ($etagtrsim10^4$) and/or highly magnetized ($sigma_0gtrsim10^3$) outflows are implied, which may be consistent with the rarity of this phenomenon.
We analyzed broad-band X-ray and radio data of the magnetar SGR J1935+2154 taken in the aftermath of its 2014, 2015, and 2016 outbursts. The source soft X-ray spectrum <10 keV is well described with a BB+PL or 2BB model during all three outbursts. NuSTAR observations revealed a hard X-ray tail, $Gamma=0.9$, extending up to 79 keV, with flux larger than the one detected <10 keV. Imaging analysis of Chandra data did not reveal small-scale extended emission around the source. Following the outbursts, the total 0.5-10 keV flux from SGR J1935+2154 increased in concordance to its bursting activity, with the flux at activation onset increasing by a factor of $sim7$ following its strongest June 2016 outburst. A Swift/XRT observation taken 1.5 days prior to the onset of this outburst showed a flux level consistent with quiescence. We show that the flux increase is due to the PL or hot BB component, which increased by a factor of $25$ compared to quiescence, while the cold BB component $kT=0.47$ keV remained more or less constant. The 2014 and 2015 outbursts decayed quasi-exponentially with time-scales of $sim40$ days, while the stronger May and June 2016 outbursts showed a quick short-term decay with time-scales of $sim4$ days. Our Arecibo radio observations set the deepest limits on the radio emission from a magnetar, with a maximum flux density limit of 14 $mu$Jy for the 4.6 GHz observations and 7 $mu$Jy for the 1.4 GHz observations. We discuss these results in the framework of the current magnetar theoretical models.
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