arXiv subjects

George Younes

Publications and source records attributed to George Younes.

At least 55 records · Page 3Linked to original sources

arXiv2020

The NICER View of the 2020 Burst Storm and Persistent Emission of SGR 1935+2154

We report on NICER observations of the Magnetar SGR~1935+2154, covering its 2020 burst storm and long-term persistent emission evolution up to $\sim90$ days post outburst. During the first 1120~seconds taken on April 28 00:40:58 UTC we detect over 217 bursts, corresponding to a burst rate of $>0.2$ bursts s$^{-1}$. Three hours later the rate is at 0.008 bursts s$^{-1}$, remaining at a comparatively low level thereafter. The $T_{90}$ burst duration distribution peaks at 840~ms; the distribution of waiting times to the next burst is fit with a log-normal with an average of 2.1 s. The 1-10 keV burst spectra are well fit by a blackbody, with an average temperature and area of $kT=1.7$ keV and $R^2=53$ km$^2$. The differential burst fluence distribution over $\sim3$ orders of magnitude is well modeled with a power-law form $dN/dF\propto F^{-1.5\pm0.1}$. The source persistent emission pulse profile is double-peaked hours after the burst storm. We find that the bursts peak arrival times follow a uniform distribution in pulse phase, though the fast radio burst associated with the source aligns in phase with the brighter peak. We measure the source spin-down from heavy-cadence observations covering days 21 to 39 post-outburst, $\dotν=-3.72(3)\times10^{-12}$ Hz s$^{-1}$; a factor 2.7 larger than the value measured after the 2014 outburst. Finally, the persistent emission flux and blackbody temperature decrease rapidly in the early stages of the outburst, reaching quiescence 40 days later, while the size of the emitting area remains unchanged.

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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

NICER Observation of the Temporal and Spectral Evolution of Swift J1818.0-1607: a Missing Link between Magnetars and Rotation Powered Pulsars

We report on the hard X-ray burst and the first ~100 days NICER monitoring of the soft X-ray temporal and spectral evolution of the newly-discovered magnetar Swift J1818.0-1607. The burst properties are typical of magnetars with a duration of $T_{90}=10\pm4$ ms and a temperature of $kT=8.4\pm0.7$ keV. The 2--8 keV pulse shows a broad, single peak profile with a pulse fraction increasing with time from 30% to 43%. The NICER observations reveal strong timing noise with $\dotν$ varying erratically by a factor of 10, with an average long-term spin-down rate of $\dotν=(-2.48\pm0.03)\times10^{-11}$~s$^{-2}$, implying an equatorial surface magnetic field of $2.5\times10^{14}$ G and a young characteristic age of $\sim$470 yr. We detect a large spin-up glitch at MJD 58928.56 followed by a candidate spin-down glitch at MJD 58934.81, with no accompanying flux enhancements. The persistent soft X-ray spectrum of Swift~J1818.0-1607 can be modeled as an absorbed blackbody with a temperature of ~1 keV. Its flux decayed by ~60% while the modeled emitting area decreased by ~30% over the NICER observing campaign. This decrease, coupled with the increase in the pulse fraction points to a shrinking hot spot on the neutron star surface. Assuming a distance of 6.5 kpc, we measure a peak X-ray luminosity of $1.9\times10^{35}$ erg/s, lower than its spin-down luminosity of $7.2\times10^{35}$ erg/s. Its quiescent thermal luminosity is $\lesssim 1.7\times10^{34}$ erg/s, lower than those of canonical young magnetars. We conclude that Swift J1818.0-1607 is an important link between regular magnetars and high magnetic field rotation powered pulsars.

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arXiv2020

Burst properties of the most recurring transient magnetar SGR J1935+2154

We present timing and time-integrated spectral analysis of 127 bursts from SGR J1935+2154. These bursts were observed with the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope and the Burst Alert Telescope on the Neil Gehrels Swift Observatory during the source's four active episodes from 2014 to 2016. This activation frequency makes SGR J1935+2154 the most burst prolific transient magnetar. We find the average duration of all the detected bursts to be much shorter than the typical, anticipated value. We fit the burst time-integrated spectra with two black-body functions, a Comptonized model and three other simpler models. Bursts from SGR J1935+2154 exhibit similar spectral properties to other magnetars, with the exception of the power law index from the Comptonized model, which correlates with burst fluence. We find that the durations and both black-body temperatures of the bursts have significantly evolved across the four active episodes. We also find that the burst time history exhibits two trends, which are strongly correlated with the decay of the persistent emission in each outburst.

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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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arXiv2019

The X-ray Polarization Probe mission concept

The X-ray Polarization Probe (XPP) is a second generation X-ray polarimeter following up on the Imaging X-ray Polarimetry Explorer (IXPE). The XPP will offer true broadband polarimetery over the wide 0.2-60 keV bandpass in addition to imaging polarimetry from 2-8 keV. The extended energy bandpass and improvements in sensitivity will enable the simultaneous measurement of the polarization of several emission components. These measurements will give qualitatively new information about how compact objects work, and will probe fundamental physics, i.e. strong-field quantum electrodynamics and strong gravity.

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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

Prospects for Pulsar Studies at MeV Energies

Enabled by the Fermi Large Area Telescope, we now know young and recycled pulsars fill the gamma-ray sky, and we are beginning to understand their emission mechanism and their distribution throughout the Galaxy. However, key questions remain: Is there a large population of pulsars near the Galactic center? Why do the most energetic pulsars shine so brightly in MeV gamma rays but not always at GeV energies? What is the source and nature of the pair plasma in pulsar magnetospheres, and what role does the polar cap accelerator play? Addressing these questions calls for a sensitive, wide-field MeV telescope, which can detect the population of MeV-peaked pulsars hinted at by Fermi and hard X-ray telescopes and characterize their spectral shape and polarization.

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arXiv2019

Prospects for AGN Studies at Hard X-ray through MeV Energies

This White Paper explores advances in the study of Active Galaxies which will be enabled by new observing capabilities at MeV energies (hard X-rays to gamma-rays; 0.1-1000 MeV), with a focus on multi-wavelength synergies. This spectral window, covering four decades in energy, is one of the last frontiers for which we lack sensitive observations. Only the COMPTEL mission, which flew in the 1990s, has significantly probed this energy range, detecting a handful of AGN. In comparison, the currently active Fermi Gamma-ray Space Telescope, observing at the adjacent range of 0.1-100 GeV, is 100-1000 times more sensitive. This White Paper describes advances to be made in the study of sources as diverse as tidal disruption events, jetted AGN of all classes (blazars, compact steep-spectrum sources, radio galaxies and relics) as well as radio-quiet AGN, most of which would be detected for the first time in this energy regime. New and existing technologies will enable MeV observations at least 50-100 times more sensitive than COMPTEL, revealing new source populations and addressing several open questions, including the nature of the corona emission in non-jetted AGN, the precise level of the optical extragalactic background light, the accretion mode in low-luminosity AGN, and the structure and particle content of extragalactic jets.

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arXiv2019

XMM-Newton and Chandra observations of the unidentified Fermi-LAT source 3FGL J1016.5-6034: A Young Pulsar with a Nebula?

We report the discovery of a bright X-ray source in the XMM-Newton and Chandra X-ray Observatory (CXO) images of the unidentified Fermi-LAT source 3FGL J1016.5-6034. The XMM-Newton spectrum of the source is well fit by an absorbed blackbody+power-law model with a temperature, $kT=0.20\pm0.02$ keV, and photon index $Γ=1.8\pm0.1$. The CXO resolves the same source into a point source (CXOU J101546.0-602939) and a surrounding compact nebula seen up to about $30''$ from the point source. The CXO spectrum of the nebula can be described by an absorbed power-law with $Γ=1.7\pm0.3$ and is partly responsible for the non-thermal emission observed in the XMM-Newton spectrum. The XMM-Newton images also reveal faint extended emission on arcminute scales. These properties strongly suggest that the X-ray source and the accompanying extended emission are a newly discovered young pulsar with a pulsar wind nebula. We also analyze $\sim10$ years of Fermi-LAT data and find that the improved LAT source localization is consistent with the position of CXOU J101546.0-602939.

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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

Physics and astrophysics of strong magnetic field systems with eXTP

In this paper we present the science potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies of strongly magnetized objects. We will focus on the physics and astrophysics of strongly magnetized objects, namely magnetars, accreting X-ray pulsars, and rotation powered pulsars. We also discuss the science potential of eXTP for QED studies. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to be launched in the mid 2020s.

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arXiv2018

NuSTAR Hard X-ray View of Low-luminosity Active Galactic Nuclei: High-energy Cutoff and Truncated Thin Disk

We report the analysis of simultaneous XMM-Newton+NuSTAR observations of two low-luminosity Active Galactic Nuclei (LLAGN), NGC 3998 and NGC 4579. We do not detect any significant variability in either source over the ~3 day length of the NuSTAR observations. The broad-band 0.5-60 keV spectrum of NGC 3998 is best fit with a cutoff power-law, while the one for NGC 4579 is best fit with a combination of a hot thermal plasma model, a power-law, and a blend of Gaussians to fit an Fe complex observed between 6 and 7 keV. Our main spectral results are the following: (1) neither source shows any reflection hump with a $3σ$ reflection fraction upper-limits $R<0.3$ and $R<0.18$ for NGC 3998 and NGC 4579, respectively; (2) the 6-7 keV line complex in NGC 4579 could either be fit with a narrow Fe K line at 6.4 keV and a moderately broad Fe XXV line, or 3 relatively narrow lines, which includes contribution from Fe XXVI; (3) NGC 4579 flux is 60% brighter than previously detected with XMM-Newton, accompanied by a hardening in the spectrum; (4) we measure a cutoff energy $E_{\rm cut}=107_{-18}^{+27}$ keV in NGC 3998, which represents the lowest and best constrained high-energy cutoff ever measured for an LLAGN; (5) NGC 3998 spectrum is consistent with a Comptonization model with either a sphere ($τ\approx3\pm1$) or slab ($τ\approx1.2\pm0.6$) geometry, corresponding to plasma temperatures between 20 and 150 keV. We discuss these results in the context of hard X-ray emission from bright AGN, other LLAGN, and hot accretion flow models.

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arXiv2018

Compton echoes from nearby Gamma-Ray Bursts

The recent discovery of gravitational waves from GW170817, associated with a short Gamma-Ray Burst (GRB) at a distance of $40$Mpc, has demonstrated that short GRBs can occur locally and at a reasonable rate. Furthermore, gravitational waves enable us to detect close by GRBs, even when we are observing at latitudes far from the jet's axis. We consider here Compton echoes, the scattered light from the prompt and afterglow emission. Compton echoes, an as yet undetected counterpart of GRBs, peak in X-rays and maintain a roughly constant flux for hundreds to thousands of years after the burst. Though too faint to be detected in typical cosmological GRBs, a fraction of close by bursts with a sufficiently large energy output in X-rays, and for which the surrounding medium is sufficiently dense, may indeed be observed in this way. The detection of a Compton echo could provide unique insight into the burst properties and the environment's density structure. In particular, it could potentially determine whether or not there was a successful jet that broke through the compact binary merger ejecta. We discuss here the properties and expectations from Compton echoes and suggest methods for detectability.

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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 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

Burst and Outburst Characteristics of Magnetar 4U 0142+61

We have compiled the most comprehensive burst sample from magnetar 4U 0142+61, comprising 27 bursts from its three burst-active episodes in 2011, 2012 and the latest one in 2015 observed with Swift/BAT and Fermi/GBM. Bursts from 4U 0142+61 morphologically resemble typical short bursts from other magnetars. However, 4U 0142+61 bursts are less energetic compared to the bulk of magnetar bursts. We uncovered an extended tail emission following a burst on 2015 February 28, with a thermal nature, cooling over a time-scale of several minutes. During this tail emission, we also uncovered pulse peak phase aligned X-ray bursts , which could originate from the same underlying mechanism as that of the extended burst tail, or an associated and spatially coincident but different mechanism.

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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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