arXiv subjects

Matthew G. Baring

Publications and source records attributed to Matthew G. Baring.

At least 55 records · Page 3Linked to original sources

arXiv2020

Fermi/GBM View of the 2019 and 2020 Burst Active Episodes of SGR J1935+2154

We present temporal and time-integrated spectral analyses of 148 bursts from the latest activation of SGR J1935+2154, observed with Fermi/GBM from October 4th 2019 through May 20th 2020, excluding a ~130 s segment with a very high burst density on April 27th 2020. The 148 bursts presented here, are slightly longer and softer than bursts from earlier activations of SGR J1935+2154, as well as from other magnetars. The long-term spectral evolution trend is interpreted as being associated with an increase in the average plasma loading of the magnetosphere during bursts. We also find a trend of increased burst activity from SGR J1935+2154 since its discovery in 2014. Finally, we find no association of typical radio bursts with X-ray bursts from the source; this contrasts the association of FRB 200428 with an SGR J1935+2154 X-ray burst, to date unique among the magnetar population.

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arXiv2020

X-ray Through Very-High-Energy Intrabinary Shock Emission from Black Widows and Redbacks

Black widow and redback systems are compact binaries in which a millisecond pulsar heats and may even ablate its low-mass companion by its intense wind of relativistic particles and radiation. In such systems, an intrabinary shock can form as a site of particle acceleration and associated non-thermal emission. We model the X-ray and gamma-ray synchrotron and inverse-Compton spectral components for select spider binaries, including diffusion, convection and radiative energy losses in an axially-symmetric, steady-state approach. Our new multi-zone code simultaneously yields energy-dependent light curves and orbital phase-resolved spectra. Using parameter studies and matching the observed X-ray spectra and light curves, and Fermi Large Area Telescope spectra where available, with a synchrotron component, we can constrain certain model parameters. For PSR J1723--2837 these are notably the magnetic field and bulk flow speed of plasma moving along the shock tangent, the shock acceleration efficiency, and the multiplicity and spectrum of pairs accelerated by the pulsar. This affords a more robust prediction of the expected high-energy and very-high-energy gamma-ray flux. We find that nearby pulsars with hot or flaring companions may be promising targets for the future Cherenkov Telescope Array. Moreover, many spiders are likely to be of significant interest to future MeV-band missions such as AMEGO and e-ASTROGAM.

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arXiv2020

Simultaneous magnetic polar cap heating during a flaring episode from the magnetar 1RXS J170849.0-400910

During a pointed 2018 NuSTAR observation, we detected a flare with a 2.2 hour duration from the magnetar 1RXS J170849.0$-$400910. The flare, which rose in $\sim25$ seconds to a maximum flux 6 times larger than the persistent emission, is highly pulsed with an rms pulsed fraction of $53\%$. The pulse profile shape consists of two peaks separated by half a rotational cycle, with a peak flux ratio of $\sim$2. The flare spectrum is thermal with an average temperature of 2.1 keV. Phase resolved spectroscopy show that the two peaks possess the same temperature, but differ in size. These observational results along with simple light curve modeling indicate that two identical antipodal spots, likely the magnetic poles, are heated simultaneously at the onset of the flare and for its full duration. Hence, the origin of the flare has to be connected to the global dipolar structure of the magnetar. This might best be achieved externally, via twists to closed magnetospheric dipolar field lines seeding bombardment of polar footpoint locales with energetic pairs. Approximately 1.86 hours following the onset of the flare, a short burst with its own 3-minute thermal tail occurred. The burst tail is also pulsating at the spin period of the source and phase-aligned with the flare profile, implying an intimate connection between the two phenomena. The burst may have been caused by a magnetic reconnection event in the same twisted dipolar field lines anchored to the surface hot spots, with subsequent return currents supplying extra heat to these polar caps.

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arXiv2020

The Fast Radio Burst Luminosity Function and Death Line in the Low-Twist Magnetar Model

We explore the burst energy distribution of fast radio bursts (FRBs) in the low-twist magnetar model of Wadiasingh and Timokhin (2019). Motivated by the power-law fluence distributions of FRB 121102, we propose an elementary model for the FRB luminosity function of individual repeaters with an inversion protocol which directly relates the power-law distribution index of magnetar short burst fluences to that for FRBs. The protocol indicates the FRB energy scales virtually linearly with crust/field dislocation amplitude, if magnetar short bursts prevail in the magnetoelastic regime. Charge starvation in the magnetosphere during bursts (required in WT19) for individual repeaters implies the predicted burst fluence distribution is narrow, $\lesssim 3$ decades for yielding strains and oscillation frequencies feasible in magnetar crusts. Requiring magnetic confinement and charge starvation, we obtain a death line for FRBs which segregates magnetars from the normal pulsar population, suggesting only the former will host recurrent FRBs. We convolve the burst energy distribution for individual magnetars to define the distribution of luminosities in evolved magnetar populations. The broken power-law luminosity function's low energy character depends on the population model, while the high energy index traces that of individual repeaters. Independent of the evolved population, the broken power-law isotropic-equivalent energy/luminosity function peaks at $\sim10^{37}-10^{40}$ erg with a low-energy cutoff at $\sim 10^{37}$ erg. Lastly, we consider the local fluence distribution of FRBs, and find that it can constrain the subset of FRB-producing magnetar progenitors. Our model suggests that improvements in sensitivity may reveal flattening of the global FRB fluence distribution and saturation in FRB rates.

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arXiv2019

Time-Dependent, Multi-Wavelength Shock Acceleration Models for Active Flares of 3C 279

Jets in blazars are an excellent forum for studying acceleration at relativistic shocks using the highly-variable emission seen across the electromagnetic spectrum. Our recent work on combining multi-wavelength leptonic emission models with simulated thermal+non-thermal distributions from shock acceleration theory has resulted in new insights into plasma conditions in blazars. This has demonstrated the ability to infer the cyclotron frequency, the plasma density and thus also the Alfven speed, thereby determining the rapidity of particle energization. An important inference was that MHD turbulence levels decline with remoteness from jet shocks. This paper outlines new results from our recent extension of this program to a two-zone, time-evolving construction, modeling together both extended, enhanced emission states from larger radiative regions, and prompt flare events from compact acceleration zones. These are applied to flares in the FSRQ blazar 3C 279 monitored by Fermi-LAT in gamma-rays in late 2013. With impulsive injection episodes from the shock zone, as the acceleration first proceeds and then abates, the radiative simulations obtain a pronounced spectral hardening in the optical and gamma-ray bands as the flare grows, followed by a softening during the decay phase. For 3C 279, while model radio and X-ray synchrotron flares are temporally correlated, there is a lag in both bands relative to GeV gamma rays and optical emission on timescales of a number of hours. This delay is governed by the short cooling time associated with the bright external Compton signal.

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arXiv2019

Discovery and Identification of MAXI J1621-501 as a Type I X-ray Burster with a Super-Orbital Period

MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up with a multitude of space missions (NuSTAR, Swift, Chandra, NICER, INTEGRAL, and MAXI) and ground-based observatories (Gemini, IRSF, and ATCA). The source was discovered with MAXI on 2017 October 19 as a new, unidentified transient. Further observations with NuSTAR revealed 2 Type I X-ray bursts, identifying MAXI J1621-501 as a Low Mass X-ray Binary (LMXB) with a neutron star primary. Overall, 24 Type I bursts were detected from the source during a 15 month period. At energies below 10 keV, the source spectrum was best fit with three components: an absorbed blackbody with kT = 2.3 keV, a cutoff power law with index $Γ$ = 0.7, and an emission line centered on 6.3 keV. Timing analysis of the X-ray persistent emission and burst data has not revealed coherent pulsations from the source or an orbital period. We identified, however, a super-orbital period $\sim{}$78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of $\sim{}$82 days. Thus, MAXI J1621-501 joins a small group of sources characterized with super-orbital periods.

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arXiv2019

Astro2020 Science White Paper: Using X-Ray Polarimetry to Probe the Physics of Black Holes and Neutron Stars

This white paper highlights compact object and fundamental physics science opportunities afforded by high-throughput broadband (0.1-60 keV) X-ray polarization observations. X-ray polarimetry gives new observables with geometric information about stellar remnants which are many orders of magnitude too small for direct imaging. The X-ray polarimetric data also reveal details about the emission mechanisms and the structure of the magnetic fields in and around the most extreme objects in the Universe. Whereas the Imaging X-ray Polarimetry Explorer (IXPE) to be launched in 2021 will obtain first results for bright objects, a follow-up mission could be one order of magnitude more sensitive and would be able to use a broader bandpass to perform physics type experiments for representative samples of sources.

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arXiv2019

Opacities for Photon Splitting and Pair Creation in Neutron Star Magnetospheres

Over the last four decades, persistent and flaring emission of magnetars observed by various telescopes has provided us with a suite of light curves and spectra in soft and hard X-rays, with no emission yet detected above around 1 MeV. Attenuation of such high-energy photons by magnetic pair creation and photon splitting is expected to be active in the magnetospheres of magnetars, possibly accounting for the paucity of gamma-rays in their signals. This paper explores polarization-dependent opacities for these two QED processes in static vacuum dipole magnetospheres of highly-magnetized neutron stars, calculating attenuation lengths and determining escape energies, which are the maximum photon energies for transparency out to infinity. The numerical trajectory integral analysis in flat and curved spacetimes provides upper bounds of a few MeV or less to the visible energies for magnetars for locales proximate to the stellar surface. Photon splitting opacity alone puts constraints on the possible emission locales in their magnetospheres: regions within field loops of maximum altitudes 2-4 stellar radii are not commensurate with maximum detected energies of around 250 keV. These constraints apply not only to magnetar flares but also to their quiescent hard X-ray tail emission. An exploration of photon splitting attenuation in the context of a resonant inverse Compton scattering model for the hard X-ray tails derives distinctive phase-resolved spectroscopic and polarimetric signatures, of significant interest for future MeV-band missions such as AMEGO and e-ASTROGAM.

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arXiv2019

Spectral Variability Signatures of Relativistic Shocks in Blazars

Mildly relativistic, oblique shocks are frequently invoked as possible sites of relativistic particle acceleration and production of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars, via diffusive shock acceleration (DSA). In recent work, we had self-consistently coupled DSA and radiation transfer simulations in blazar jets. These one-zone models determined that the observed spectral energy distributions (SEDs) of blazars strongly constrain the nature of the hydromagnetic turbulence responsible for pitch-angle scattering. In this paper, we expand our previous work by including full time dependence and treating two emission zones, one being the site of acceleration. This modeling is applied to a multiwavelength flare of the flat spectrum radio quasar 3C~279, fitting snap-shot SEDs and light curves. We predict spectral hysteresis patterns in various energy bands as well as cross-band time lags with optical and GeV gamma-rays as well as radio and X-rays tracing each other closely with zero time lag, but radio and X-rays lagging behind the optical and gamma-ray variability by several hours.

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arXiv2019

Magnetars as Astrophysical Laboratories of Extreme Quantum Electrodynamics: The Case for a Compton Telescope

A next generation of Compton and pair telescopes that improve MeV-band detection sensitivity by more than a decade beyond current instrumental capabilities will open up new insights into a variety of astrophysical source classes. Among these are magnetars, the most highly magnetic of the neutron star zoo, which will serve as a prime science target for a new mission surveying the MeV window. This paper outlines the core questions pertaining to magnetars that can be addressed by such a technology. These range from global magnetar geometry and population trends, to incisive probes of hard X-ray emission locales, to providing cosmic laboratories for spectral and polarimetric testing of exotic predictions of QED, principally the prediction of the splitting of photons and magnetic pair creation. Such fundamental physics cannot yet be discerned in terrestrial experiments. State of the art modeling of the persistent hard X-ray tail emission in magnetars is presented to outline the case for powerful diagnostics using Compton polarimeters. The case highlights an inter-disciplinary opportunity to seed discovery at the interface between astronomy and physics.

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arXiv2018

Resonant Inverse Compton Scattering Spectra from Highly-magnetized Neutron Stars

Hard, non-thermal, persistent pulsed X-ray emission extending between 10 keV and $\sim 150$ keV has been observed in nearly ten magnetars. For inner-magnetospheric models of such emission, resonant inverse Compton scattering of soft thermal photons by ultra-relativistic charges is the most efficient production mechanism. We present angle-dependent upscattering spectra and pulsed intensity maps for uncooled, relativistic electrons injected in inner regions of magnetar magnetospheres, calculated using collisional integrals over field loops. Our computations employ a new formulation of the QED Compton scattering cross section in strong magnetic fields that is physically correct for treating important spin-dependent effects in the cyclotron resonance, thereby producing correct photon spectra. The spectral cut-off energies are sensitive to the choices of observer viewing geometry, electron Lorentz factor, and scattering kinematics. We find that electrons with energies $\lesssim 15$ MeV will emit most of their radiation below 250 keV, consistent with inferred turnovers for magnetar hard X-ray tails. More energetic electrons still emit mostly below 1 MeV, except for viewing perspectives sampling field line tangents. Pulse profiles may be singly- or doubly-peaked dependent upon viewing geometry, emission locale, and observed energy band. Magnetic pair production and photon splitting will attenuate spectra to hard X-ray energies, suppressing signals in the Fermi-LAT band. The resonant Compton spectra are strongly polarized, suggesting that hard X-ray polarimetry instruments such as X-Calibur, or a future Compton telescope, can prove central to constraining model geometry and physics.

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arXiv2017

The Sleeping Monster: NuSTAR observations of SGR 1806-20, 11 years after the Giant Flare

We report the analysis of 5 NuSTAR observations of SGR 1806-20 spread over a year from April 2015 to April 2016, more than 11 years following its Giant Flare (GF) of 2004. The source spin frequency during the NuSTAR observations follows a linear trend with a frequency derivative $\dotν=(-1.25\pm0.03)\times10^{-12}$ Hz s$^{-1}$, implying a surface dipole equatorial magnetic field $B\approx7.7\times10^{14}$ G. Thus, SGR 1806-20 has finally returned to its historical minimum torque level measured between 1993 and 1998. The source showed strong timing noise for at least 12 years starting in 2000, with $\dotν$ increasing one order of magnitude between 2005 and 2011, following its 2004 major bursting episode and GF. SGR 1806-20 has not shown strong transient activity since 2009 and we do not find short bursts in the NuSTAR data. The pulse profile is complex with a pulsed fraction of $\sim8\%$ with no indication of energy dependence. The NuSTAR spectra are well fit with an absorbed blackbody, $kT=0.62\pm0.06$ keV, plus a power-law, $Γ=1.33\pm0.03$. We find no evidence for variability among the 5 observations, indicating that SGR 1806-20 has reached a persistent and potentially its quiescent X-ray flux level after its 2004 major bursting episode. Extrapolating the NuSTAR model to lower energies, we find that the 0.5-10 keV flux decay follows an exponential form with a characteristic timescale $τ=543\pm75$ days. Interestingly, the NuSTAR flux in this energy range is a factor of $\sim2$ weaker than the long-term average measured between 1993 and 2003, a behavior also exhibited in SGR $1900+14$. We discuss our findings in the context of the magnetar model.

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arXiv2017

X-ray Synchrotron Polarization from Turbulent Plasmas in Supernova Remnants

As supernova remnants (SNRs) age, they become efficient cosmic ray accelerators at their outer shell shocks. The current paradigm for shock acceleration theory favors turbulent field environs in the proximity of these shocks, turbulence driven by current instabilities involving energetic ions. With the imminent prospect of dedicated X-ray polarimeters becoming a reality, the possibility looms of probing turbulence on scales that couple to the super-TeV electrons that emit X-rays. This paper presents model X-ray polarization signatures from energetic electrons moving in simulated MHD turbulence of varying levels of "chaos." The emission volumes are finite slabs that represent the active regions of young SNR shells. We find that the turbulent field energy must be quite limited relative to that of the total field in order for the X-ray polarization degree to be as strong as the radio measures obtained in some remnants. Results presented are pertinent to the planned IXPE and XIPE polarimeters.

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arXiv2017

Magnetar Hard Spectral Tails

Pulsed non-thermal quiescent emission between 10 keV and around 150 keV has been observed in $\sim10$ magnetars. For inner magnetospheric models of such hard X-ray signals, resonant Compton upscattering of soft thermal photons from the neutron star surface is the most efficient radiative process. We present angle-dependent hard X-ray upscattering model spectra for uncooled monoenergetic relativistic electrons. The spectral cut-off energies are critically dependent on the observer viewing angles and electron Lorentz factor. We find that electrons with energies less than around 15 MeV will emit most of their radiation below 250 keV, consistent with the observed turnovers in magnetar hard X-ray tails. Moreover, electrons of higher energy still emit most of the radiation below around 1 MeV, except for quasi-equatorial emission locales for select pulses phases. Our spectral computations use new state-of-the-art, spin-dependent formalism for the QED Compton scattering cross section in strong magnetic fields.

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arXiv2017

Constraining Relativistic Bow Shock Properties in Rotation-Powered Millisecond Pulsar Binaries

Multiwavelength followup of unidentified Fermi sources has vastly expanded the number of known galactic-field "black widow" and "redback" millisecond pulsar binaries. Focusing on their rotation-powered state, we interpret the radio to X-ray phenomenology in a consistent framework. We advocate the existence of two distinct modes differing in their intrabinary shock orientation, distinguished by the phase-centering of the double-peaked X-ray orbital modulation originating from mildly-relativistic Doppler boosting. By constructing a geometric model for radio eclipses, we constrain the shock geometry as functions of binary inclination and shock stand-off $R_0$. We develop synthetic X-ray synchrotron orbital light curves and explore the model parameter space allowed by radio eclipse constraints applied on archetypal systems B1957+20 and J1023+0038. For B1957+20, from radio eclipses the stand-off is $R_0 \sim 0.15$ -- $0.3$ fraction of binary separation from the companion center, depending on the orbit inclination. Constructed X-ray light curves for B1957+20 using these values are qualitatively consistent with those observed, and we find occultation of the shock by the companion as a minor influence, demanding significant Doppler factors to yield double peaks. For J1023+0038, radio eclipses imply $R_0 \lesssim 0.4$ while X-ray light curves suggest $0.1\lesssim R_0 \lesssim 0.3$ (from the pulsar). Degeneracies in the model parameter space encourage further development to include transport considerations. Generically, the spatial variation along the shock of the underlying electron power-law index should yield energy-dependence in the shape of light curves motivating future X-ray phase-resolved spectroscopic studies to probe the unknown physics of pulsar winds and relativistic shock acceleration therein.

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arXiv2017

X-ray and radio observations of the magnetar SGR J1935+2154 during its 2014, 2015, and 2016 outbursts

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, $Γ=0.9$, 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 $μ$Jy for the 4.6 GHz observations and 7 $μ$Jy for the 1.4 GHz observations. We discuss these results in the framework of the current magnetar theoretical models.

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arXiv2016

Learning About the Magnetar Swift J1834.9-0846 from its Wind Nebula

The first wind nebula around a magnetar was recently discovered in X-rays around Swift~J1834.9$-$0846. We study this magnetar's global energetics and the properties of its particle wind or outflows. At a distance of $\sim4\;$kpc, Swift~J1834.9$-$0846 is located at the center of the supernova remnant (SNR) W41 whose radius is $\sim 19\;$pc, an order of magnitude larger than that of the X-ray nebula ($\sim2\;$pc). The association with SNR W41 suggests a common age of $\sim5-100\;$kyr, while its spin-down age is $4.9$~kyr. A small natal kick velocity may partly explain why a wind nebula was detected around this magnetar but not around other magnetars, most of which appear to have larger kick velocities and may have exited their birth SNR. We find that the GeV and TeV source detected by Fermi/LAT and H.E.S.S., respectively, of radius $\sim11\;$pc is most likely of hadronic origin. The dynamics and internal structure of the nebula are examined analytically to explain the nebula's current properties. Its size may naturally correspond to the diffusion-dominated cooling length of the X-ray emitting $e^+e^-$ pairs. This may also account for the spectral softening of the X-ray emission from the nebula's inner to outer parts. Analysis of the X-ray synchrotron nebula implies that (i) the nebular magnetic field is $\gtrsim 11\;μ$G (and likely $\lesssim30\;μ$G), and (ii) the nebula is not powered predominantly by the magnetar's quiescent spin-down-powered MHD wind, but by other outflows that contribute most of its energy. The latter are most likely associated with the magnetar's bursting activity, and possibly dominated by outflows associated with its past giant flares. The energy source for the required outflows cannot be the decay of the magnetar's dipole field alone, and is most likely the decay of its much stronger internal magnetic field.

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arXiv2016

Probing Acceleration and Turbulence at Relativistic Shocks in Blazar Jets

Diffusive shock acceleration (DSA) at relativistic shocks is widely thought to be an important acceleration mechanism in various astrophysical jet sources, including radio-loud active galactic nuclei such as blazars. Such acceleration can produce the non-thermal particles that emit the broadband continuum radiation that is detected from extragalactic jets. An important recent development for blazar science is the ability of Fermi-LAT spectroscopy to pin down the shape of the distribution of the underlying non-thermal particle population. This paper highlights how multi-wavelength spectra spanning optical to X-ray to gamma-ray bands can be used to probe diffusive acceleration in relativistic, oblique, magnetohydrodynamic (MHD) shocks in blazar jets. Diagnostics on the MHD turbulence near such shocks are obtained using thermal and non-thermal particle distributions resulting from detailed Monte Carlo simulations of DSA. These probes are afforded by the characteristic property that the synchrotron $νF_ν$ peak energy does not appear in the gamma-ray band above 100 MeV. We investigate self-consistently the radiative synchrotron and inverse Compton signatures of the simulated particle distributions. Important constraints on the diffusive mean free paths of electrons, and the level of electromagnetic field turbulence are identified for three different case study blazars, Mrk 501, BL Lacertae and AO 0235+164. The X-ray excess of AO 0235+164 in a flare state can be modelled as the signature of bulk Compton scattering of external radiation fields, thereby tightly constraining the energy-dependence of the diffusion coefficient for electrons. The concomitant interpretations that turbulence levels decline with remoteness from jet shocks, and the probable significant role for non-gyroresonant diffusion, are posited.

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