arXiv subjects

George Younes

Publications and source records attributed to George Younes.

At least 37 records · Page 2Linked to original sources

arXiv2023

The High Energy X-ray Probe (HEX-P): Supernova remnants, pulsar wind nebulae, and nuclear astrophysics

HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging ($<10"$ full width at half maximum) and broad spectral coverage (0.2--80 keV) with an effective area far superior to current facilities (including XMM-Newton and NuSTAR) to enable revolutionary new insights into a variety of important astrophysical problems. HEX-P is ideally suited to address important problems in the physics and astrophysics of supernova remnants (SNRs) and pulsar-wind nebulae (PWNe). For shell SNRs, HEX-P can greatly improve our understanding via more accurate spectral characterization and localization of non-thermal X-ray emission from both non-thermal-dominated SNRs and those containing both thermal and non-thermal components, and can discover previously unknown non-thermal components in SNRs. Multi-epoch HEX-P observations of several young SNRs (e.g., Cas A and Tycho) are expected to detect year-scale variabilities of X-ray filaments and knots, thus enabling us to determine fundamental parameters related to diffusive shock acceleration, such as local magnetic field strengths and maximum electron energies. For PWNe, HEX-P will provide spatially-resolved, broadband X-ray spectral data separately from their pulsar emission, allowing us to study how particle acceleration, cooling, and propagation operate in different evolution stages of PWNe. HEX-P is also poised to make unique and significant contributions to nuclear astrophysics of Galactic radioactive sources by improving detections of, or limits on, $^{44}$Ti in the youngest SNRs and by potentially discovering rare nuclear lines as evidence of double neutron star mergers. Throughout the paper, we present simulations of each class of objects, demonstrating the power of both the imaging and spectral capabilities of HEX-P to advance our knowledge of SNRs, PWNe, and nuclear astrophysics.

astro-ph.HE↗

arXiv2023

Quasi-Periodic Peak Energy Oscillations in X-ray Bursts from SGR J1935+2154

Magnetars are young neutron stars powered by the strongest magnetic fields in the Universe (10$^{13-15}$ G). Their transient X-ray emission usually manifests as short (a few hundred milliseconds), bright, energetic ($\sim$ 10$^{40-41}$ erg) X-ray bursts. Since its discovery in 2014, magnetar J1935+2154 has become one of the most prolific magnetars, exhibiting very active bursting episodes, and other fascinating events such as pulse timing anti-glitches and Fast Radio Bursts. Here, we present evidence for possible 42 Hz (24 ms) quasi-periodic oscillations in the $νF_ν$ spectrum peak energy (Ep) identified in a unique burst detected with the Fermi Gamma-ray Burst Monitor in January 2022. While quasi-periodic oscillations have been previously reported in the intensity of magnetar burst lightcurves, quasi-periodic oscillations in the Ep have not. We also find an additional event from the same outburst that appears to exhibit similar character in Ep, albeit of lower statistical quality. For these two exceptional transients, such Ep oscillations can be explained by magnetospheric density and pressure perturbations. For burst-emitting plasma consisting purely of $e^+e^-$ pairs, these acoustic modes propagate along a highly magnetized flux tube of length up to around $L\sim 130$ neutron star radii, with $L$ being lower if ions are present in the emission zone. Detailed time-resolved analyses of other magnetar bursts are encouraged to evaluate the rarity of these events and their underlying mechanisms.

astro-ph.HE↗

arXiv2023

A NICER View on the 2020 Magnetar-Like Outburst of PSR J1846-0258

We report on our monitoring of the strong-field magnetar-like pulsar PSR J1846-0258 with the Neutron Star Interior Composition Explorer (NICER) and the timing and spectral evolution during its outburst in August 2020. Phase-coherent timing solutions were maintained from March 2017 through November 2021, including a coherent solution throughout the outburst. We detected a large spin-up glitch of magnitude Δν/ν= 3 X 10^{-6} at the start of the outburst and observed an increase in pulsed flux that reached a factor of more than 10 times the quiescent level, a behavior similar to that of the 2006 outburst. Our monitoring observations in June and July 2020 indicate that the flux was rising prior to the SWIFT announcement of the outburst on August 1, 2020. We also observed several sharp rises in the pulsed flux following the outburst and the flux reached quiescent level by November 2020. The pulse profile was observed to change shape during the outburst, returning to the pre-outburst shape by 2021. Spectral analysis of the pulsed emission of NICER data shows that the flux increases result entirely from a new black body component that gradually fades away while the power-law remains nearly constant at its quiescent level throughout the outburst. Joint spectral analysis of NICER and simultaneous NuSTAR data confirms this picture. We discuss the interpretation of the magnetar-like outburst and origin of the transient thermal component in the context of both a pulsar-like and a magnetar-like model.

astro-ph.HE↗

arXiv2023

XMM-Newton and Chandra observations of the candidate Fermi-LAT pulsar 4FGL J1015.5-6030

4FGL J1015.5-6030 is an unidentified Fermi-LAT source hosting a bright, extended X-ray source whose X-ray spectrum is consistent with that of a young pulsar, yet no pulsations have been found. Here we report on XMM-Newton timing and Chandra imaging observations of the X-ray counterpart of 4FGL J1015.5-6030. We find no significant periodicity from the source and place a 3$σ$ upper-limit on its pulsed fraction of 34$\%$. The Chandra observations resolve the point source from the extended emission. We find that the point source's spectrum is well fit by a blackbody model, with temperature $kT=0.205\pm0.009$ keV, plus a weak power-law component, which is consistent with a thermally emitting neutron star with a magnetospheric component. The extended emission spans angular scales of a few arcseconds up to about 30$''$ from the point source and its spectrum is well fit by a power-law model with a photon index $Γ=1.70\pm0.05$. The extended emission's spectrum and 0.5-10 keV luminosity of 4$\times10^{32}$ erg s$^{-1}$ (at a plausible distance of 2 kpc) are consistent with that of a pulsar wind nebula. Based on a comparison to other GeV and X-ray pulsars, we find that this putative pulsar is likely a middle-aged (i.e., $τ\sim 0.1$--1 Myr) radio-quiet pulsar with $\dot{E}\sim10^{34}-10^{35}$ erg s$^{-1}$.

astro-ph.HE↗

arXiv2023

The 2022 high-energy outburst and radio disappearing act of the magnetar 1E 1547.0-5408

We report the radio and high-energy properties of a new outburst from the radio-loud magnetar 1E 1547.0$-$5408. Following the detection of a short burst from the source with Swift-BAT on 2022 April 7, observations by NICER detected an increased flux peaking at $(6.0 \pm 0.4) \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ in the soft X-ray band, falling to the baseline level of $1.7\times10^{-11}$ erg s$^{-1}$ cm$^{-2}$ over a 17-day period. Joint spectroscopic measurements by NICER and NuSTAR indicated no change in the hard non-thermal tail despite the prominent increase in soft X-rays. Observations at radio wavelengths with Murriyang, the 64-m Parkes radio telescope, revealed that the persistent radio emission from the magnetar disappeared at least 22 days prior to the initial Swift-BAT detection and was re-detected two weeks later. Such behavior is unprecedented in a radio-loud magnetar, and may point to an unnoticed slow rise in the high-energy activity prior to the detected short-bursts. Finally, our combined radio and X-ray timing revealed the outburst coincided with a spin-up glitch, where the spin-frequency and spin-down rate increased by $0.2 \pm 0.1$ $μ$Hz and $(-2.4 \pm 0.1) \times 10^{-12}$ s$^{-2}$ respectively. A linear increase in spin-down rate of $(-2.0 \pm 0.1) \times 10^{-19}$ s$^{-3}$ was also observed over 147 d of post-outburst timing. Our results suggest that the outburst may have been associated with a reconfiguration of the quasi-polar field lines, likely signalling a changing twist, accompanied by spatially broader heating of the surface and a brief quenching of the radio signal, yet without any measurable impact on the hard X-ray properties.

astro-ph.HE↗

arXiv2023

GRB 221009A: Discovery of an Exceptionally Rare Nearby and Energetic Gamma-Ray Burst

We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image (MAXI), and Neutron Star Interior Composition Explorer Mission (NICER). This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (b = 4.3 degrees) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (~> 10kpc). We present analysis of the light curves and spectra at X-ray and UV/optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at T0 + 4.5 ks than any previous GRB observed by Swift. In its rest frame GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10^4 long GRBs were as energetic as GRB 221009A; such a large E_gamma,iso implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ~<1 per 1000 yr - making this a truly remarkable opportunity unlikely to be repeated in our lifetime.

astro-ph.HE↗

arXiv2023

A high-mass X-ray binary descended from an ultra-stripped supernova

Ultra-stripped supernovae are different from other terminal explosions of massive stars, as they show little or no ejecta from the actual supernova event. They are thought to occur in massive binary systems after the exploding star has lost its surface through interactions with its companion. Such supernovae produce little to no kick, leading to the formation of a neutron star without loss of the binary companion, which itself may also evolve into another neutron star. Here we show that a recently discovered high-mass X-ray binary, CPD -29 2176 (CD -29 5159; SGR 0755-2933), has an evolutionary history that shows the neutron star component formed during an ultra-stripped supernova. The binary has orbital elements that are similar both in period and in eccentricity to one of 14 Be X-Ray binaries that have both known orbital periods and eccentricities. The identification of the progenitors systems for ultra-stripped supernovae is necessary as their evolution pathways leads to the formation of a binary neutron star systems. Binary neutron stars, such as the system that produced the kilonova GW170817 that was observed with both electromagnetic and gravitational energy, are known to produce a large quantity of heavy elements.

astro-ph.HE↗

arXiv2022

Two decades of X-ray observations of the isolated neutron star RX J1856.5-3754: detection of thermal and non-thermal hard X-rays and refined spin-down measurement

The soft X-ray pulsar RX J1856.5-3754 is the brightest member of a small class of thermally-emitting, radio-silent, isolated neutron stars. Its X-ray spectrum is almost indistinguishable from a blackbody with $kT^\infty\approx 60$ eV, but evidence of harder emission above $\sim 1$ keV has been recently found. We report on a spectral and timing analysis of RX J1856.5-3754 based on the large amount of data collected by XMM-Newton in 2002--2022, complemented by a dense monitoring campaign carried out by NICER in 2019. Through a phase-coherent timing analysis we obtained an improved value of the spin-down rate $\dotν=-6.042(4)\times10^{-16}$ Hz s$^{-1}$, reducing by more than one order magnitude the uncertainty of the previous measurement, and yielding a characteristic spin-down field of $1.47\times10^{13}$ G. We also detect two spectral components above $\sim1$ keV: a blackbody-like one with $kT^\infty=138\pm13$ eV and emitting radius $31_{-16}^{+8}$ m, and a power law with photon index $Γ=1.4_{-0.4}^{+0.5}$. The power-law 2--8\,keV flux, $(2.5_{-0.6}^{+0.7})\times10{-15}$ erg cm$^{-2}$ s$^{-1}$, corresponds to an efficiency of $10^{-3}$, in line with that seen in other pulsars. We also reveal a small difference between the $0.1$--$0.3$ keV and $0.3$--$1.2$ keV pulse profiles, as well as some evidence for a modulation above $1.2$ keV. These results show that, notwithstanding its simple spectrum, \eighteen still has a non-trivial thermal surface distribution and features non-thermal emission as seen in other pulsars with higher spin-down power.

astro-ph.HE↗

arXiv2022

Polarized Radiation Signals from Highly Magnetized Neutron Star Surfaces

The surfaces of neutron stars are likely sources of strongly polarized soft X rays due to the presence of strong magnetic fields. Scattering transport in the surface layers is critical to the determination of the emergent anisotropy of light intensity, and is strongly influenced by the complicated interplay between linear and circular polarization information. We have developed a magnetic Thomson scattering simulation to model the outer layers of fully-ionized atmospheres in such compact objects. Here we summarize emergent intensities and polarizations from extended atmospheric simulations, spanning considerable ranges of magnetic colatitudes. General relativistic propagation of light from the surface to infinity is fully included. The net polarization degrees are moderate and not very small when summing over a variety of field directions. These results provide an important foundation for observations of magnetars to be acquired by NASA's new IXPE X-ray polarimeter and future X-ray polarimetry missions.

astro-ph.HE↗

arXiv2022

Limits on the hard X-ray emission from the periodic fast radio burst FRB 180916.J0158+65

FRB 180916.J0158+65 is one of the nearest, periodically repeating, and actively bursting fast radio burst (FRB) which has been localized to the outskirts of a spiral galaxy. In this work we study the FRB with the hard X-ray $14-195$ keV data from the Burst Alert Telescope (BAT) on board The Neil Gehrels Swift Observatory. BAT uses coded mask technology giving a localization of $\lesssim 3$ arc-minute in the hard X-ray band, along with an accurate background estimation. BAT has been observing the source location in survey mode since February 2020. The survey mode observations involves background subtracted spectra, integrated over a time span ranging $300-2000$ seconds, at the source location (from Feb 2020-Jan 2022). We analyzed all the $\sim 230$ survey mode observations from BAT and checked for any signal in any of the observations. We did not detect any signal at $>5σ$ confidence level in any of the observations. We could estimate a $5σ$ upper limit on the $14-195$ keV flux, which ranged between $4.5\times 10^{-10} - 7.6\times 10^{-9}\, \rm erg\, cm^{-2}\, s^{-1}$. At the source distance this relates to a $5σ$ upper limit on luminosity of $5.08\times 10^{44}- 8.5\times 10^{45} \rm erg\, s^{-1}$. With this estimate, we could rule out any persistent X-ray emission, at the source location for these snapshots of BAT observations.

astro-ph.HE↗

arXiv2022

Simultaneous view of the FRB~180301 with FAST and NICER during a bursting phase

FRB180301 is one of the most actively repeating fast radio bursts (FRBs) which has shown polarization angle changes in its radio burst emission, an indication for their likely origin in the magnetosphere of a highly-magnetized neutron star. We carried out a multi-wavelength campaign with the FAST radio telescope and NICER X-ray observatory to investigate any possible X-ray emission temporally coincident with the bright radio bursts. The observations took place on 2021 March 4, 9 and 19. We detected five bright radio bursts with FAST, four of which were strictly simultaneous with the NICER observations. The peak flux-density of the radio bursts ranged between $28-105$ mJy, the burst fluence between $27-170$ mJy-ms, and the burst durations between $1.7-12.3$ ms. The radio bursts from FRB~180301 exhibited complex time domain structure, and sub-pulses were detected in individual bursts, with no significant circular polarisation. The linear degree of polarisation in L-band reduced significantly compared to the 2019 observations. We do not detect any X-ray emission in excess of the background during the 5ms, 10ms, 100ms, 1sec and 100sec time intervals at/around the radio-burst barycenter-corrected arrival times, at a $>5σ$ confidence level. The $5σ$ upper limits on the X-ray a) persistent flux is $<7.64\times 10^{-12}\, \rm erg\, cm^{-2}\, s^{-1}$ , equivalent to $L_{\rm X}<2.50 \times 10^{45} \rm erg\, s^{-1}$ and b) 5 ms fluence is $<2\times 10^{-11} \rm erg\, cm^{-2}$, at the radio burst regions. Using the $5$ ms X-ray fluence upper limit, we can estimate the radio efficiency $η_{R/X} \equiv L_{\rm Radio}/L_{\rm X-ray} \gtrsim 10^{-8}$. The derived upper limit on $η_{R/X}$ is consistent with both magnetospheric models and synchrotron maser models involving relativistic shocks.

astro-ph.HE↗

arXiv2022

Intensity and Polarization Characteristics of Extended Neutron Star Surface Regions

The surfaces of neutron stars are sources of strongly polarized soft X rays due to the presence of strong magnetic fields. Radiative transfer mediated by electron scattering and free-free absorption is central to defining local surface anisotropy and polarization signatures. Scattering transport is strongly influenced by the complicated interplay between linear and circular polarizations. This complexity has been captured in a sophisticated magnetic Thomson scattering simulation we recently developed to model the outer layers of fully-ionized atmospheres in such compact objects, heretofore focusing on case studies of localized surface regions. Yet, the interpretation of observed intensity pulse profiles and their efficacy in constraining key neutron star geometry parameters is critically dependent upon adding up emission from extended surface regions. In this paper, intensity, anisotropy and polarization characteristics from such extended atmospheres, spanning considerable ranges of magnetic colatitudes, are determined using our transport simulation. These constitute a convolution of varied properties of Stokes parameter information at disparate surface locales with different magnetic field strengths and directions relative to the local zenith. Our analysis includes full general relativistic propagation of light from the surface to an observer at infinity. The array of pulse profiles for intensity and polarization presented highlights how powerful probes of stellar geometry are possible. Significant phase-resolved polarization degrees in the range of 10-60% are realized when summing over a variety of surface field directions. These results provide an important background for observations to be acquired by NASA's new IXPE X-ray polarimetry mission.

astro-ph.HE↗

arXiv2021

Fermi-GBM Observations of the SGR J1935+2154 Burst Forest

During 2020 April and May, SGR J1935+2154 emitted hundreds of short bursts and became one of the most prolific transient magnetars. At the onset of the active bursting period, a 130 s burst "forest," which included some bursts with peculiar time profiles, were observed with the Fermi/Gamma-ray Burst Monitor (GBM). In this Letter, we present the results of time-resolved spectral analysis of this burst "forest" episode, which occurred on 2020 April 27. We identify thermal spectral components prevalent during the entire 130 s episode; high-energy maxima appear during the photon flux peaks, which are modulated by the spin period of the source. Moreover, the evolution of the $νF_ν$ spectral hardness (represented by $E_{\rm peak}$ or blackbody temperature) within the lightcurve peaks is anti-correlated with the pulse phases extrapolated from the pulsation observed within the persistent soft X-ray emission of the source six hours later. Throughout the episode, the emitting area of the high-energy (hotter) component is 1-2 orders of magnitude smaller than that for the low-energy component. We interpret this with a geometrical viewing angle scenario, inferring that the high-energy component likely originates from a low-altitude hotspot located within closed toroidal magnetic field lines.

astro-ph.HE↗

arXiv2021

Search for Long-duration Gravitational-wave Signals Associated with Magnetar Giant Flares

Magnetar giant flares are rare and highly energetic phenomena observed in the transient sky whose emission mechanisms are still not fully understood. Depending on the nature of the excited modes of the magnetar, they are also expected to emit gravitational waves, which may bring unique information about the dynamics of the excitation. A few magnetar giant flares have been proposed to be associated to short gamma-ray bursts. In this paper we revisit, with a new gravitational-wave search algorithm, the possible emission of gravitational waves from four magnetar giant flares within 5 Mpc. While no gravitational-wave signals were observed, we discuss the future prospects of detecting signals with more sensitive gravitational-wave detectors. We in particular show that galactic magnetar giant flares that emit at least 1% of their electromagnetic energy as gravitational waves could be detected during the planned observing run of the LIGO and Virgo detectors at design sensitivity, with even better prospects for third generation detectors.

gr-qc↗

arXiv2021

A month of monitoring the new magnetar Swift J1555.2-5402 during an X-ray outburst

The soft gamma-ray repeater Swift J1555.2-5402 was discovered by means of a 12-ms duration short burst detected with Swift BAT on 2021 June 3. Then 1.6 hours after the first burst detection, NICER started daily monitoring of this X-ray source for a month. The absorbed 2-10 keV flux stays nearly constant at around 4e-11 erg/s/cm2 during the monitoring timespan, showing only a slight gradual decline. A 3.86-s periodicity is detected, and the time derivative of this period is measured to be 3.05(7)e-11 s/s. The soft X-ray pulse shows a single sinusoidal shape with a root-mean-square pulsed fraction that increases as a function of energy from 15% at 1.5 keV to 39% at 7 keV. The equatorial surface magnetic field, characteristic age, and spin-down luminosity are derived under the dipole field approximation to be 3.5e+14 G, 2.0 kyr, and 2.1e+34 erg/s, respectively. An absorbed blackbody with a temperature of 1.1 keV approximates the soft X-ray spectrum. Assuming a source distance of 10 kpc, the peak X-ray luminosity is ~8.5e+35 erg/s in the 2--10 keV band. During the period of observations, we detect 5 and 37 short bursts with Swift/BAT and NICER, respectively. Based on these observational properties, especially the inferred strong magnetic field, this new source is classified as a magnetar. We also coordinated hard X-ray and radio observations with NuSTAR, DSN, and VERA. A hard X-ray power-law component that extends up to at least 40 keV is detected at 3-sigma significance. The 10-60 keV flux, which is dominated by the power-law component, is ~9e-12 erg/s/cm2 with a photon index of ~1.2. The pulsed fraction has a sharp cutoff above 10 keV, down to ~10% in the hard-tail component band. No radio pulsations are detected during the DSN nor VERA observations. We place 7σ upper limits of 0.043mJy and 0.026 mJy on the flux density at S-band and X-band, respectively.

astro-ph.HE↗

arXiv2021

NuSTAR observation of LS 5039

LS 5039 is a high-mass gamma-ray binary hosting a compact object of unknown type. NuSTAR observed LS 5039 during its entire 3.9 day binary period. We performed a periodic signal search up to 1000 Hz which did not produce credible period candidates. We do see the 9.05 s period candidate, originally reported by Yoneda et al. 2020 using the same data, in the Fourier power spectrum, but we find that the statistical significance of this feature is too low to claim it as a real detection. We also did not find significant bursts or quasi-periodic variability. The modulation with the orbital period is clearly seen and remains unchanged over a decade long timescale when compared to the earlier Suzaku light curve. The joint analysis of the NuSTAR and Suzaku XIS data shows that the 0.7-70 keV spectrum can be satisfactory described by a single absorbed power-law model with no evidence of cutoff at higher energies. The slope of the spectrum anti-correlates with the flux during the binary orbit. Therefore, if LS 5039 hosts a young neutron star, its X-ray pulsations appear to be outshined by the intrabinary shock emission. The lack of spectral lines and/or an exponential cutoff at higher energies suggests that the putative neutron star is not actively accreting. Although a black hole scenario still remains a possibility, the lack of variability or Fe K$α$ lines, which typically accompany accretion, makes it less likely.

astro-ph.HE↗

arXiv2021

Back to quiescence: post-outburst evolution of the pulsar J1119-6127 and its wind nebula

We report on the analysis of a deep Chandra observation of the high-magnetic field pulsar (PSR) J1119-6127 and its compact pulsar wind nebula (PWN) taken in October 2019, three years after the source went into outburst. The 0.5-7 keV post-outburst (2019) spectrum of the pulsar is best described by a two-component blackbody plus powerlaw model with a temperature of 0.2\pm0.1 keV, photon index of 1.8\pm0.4 and X-ray luminosity of ~1.9e33 erg s^{-1}, consistent with its pre-burst quiescent phase. We find that the pulsar has gone back to quiescence. The compact nebula shows a jet-like morphology elongated in the north-south direction, similar to the pre-burst phase. The post-outburst PWN spectrum is best fit by an absorbed powerlaw with a photon index of 2.3\pm0.5 and flux of ~3.2e-14 erg cm^{-2} s^{-1} (0.5-7 keV). The PWN spectrum shows evidence of spectral softening in the post-outburst phase, with the pre-burst photon index of 1.2\pm0.4 changing to 2.3\pm0.5, and pre-burst luminosity of ~1.5e32 erg s^{-1} changing to 2.7e32 erg s^{-1} in the 0.5-7 keV band, suggesting magnetar outbursts can impact PWNe. The observed timescale for returning to quiescence, of just a few years, implies a rather fast cooling process and favors a scenario where J1119 is temporarily powered by magnetic energy following the magnetar outburst, in addition to its spin-down energy.

astro-ph.HE↗

arXiv2020

Persistent Emission Properties of SGR J1935+2154 During Its 2020 Active Episode

We present detailed spectral and temporal characteristics of the persistent X-ray emission of SGR J1935+2154 based on our XMM-Newton and Chandra observations taken in the aftermath of its April 2020 burst storm, during which hundreds of energetic X-ray bursts were emitted, including one associated with an extraordinary fast radio burst. We clearly detect the pulsed X-ray emission in the XMM-Newton data. An average spin-down rate of 1.6$\times$10$^{-11}$ s s$^{-1}$ is obtained using our spin period measurement combined with three earlier values reported from the same active episode. Our investigations of the XMM-Newton and Chandra spectra with a variety of phenomenological and physically-motivated models, concluded that the magnetic field topology of SGR J1935+2154 is most likely highly non-dipolar. The spectral models indicate that surface field strengths in somewhat localized regions substantially exceed the polar value of 4.4$\times$10$^{14}$ G inferred from a spin-down torque associated with a rotating magnetic dipole.

astro-ph.HE↗