arXiv subjects

Neil Gehrels

Publications and source records attributed to Neil Gehrels.

At least 55 records · Page 3Linked to original sources

arXiv2013

The Swift/BAT Hard X-ray Transient Monitor

The Swift/Burst Alert Telescope (BAT) hard X-ray transient monitor provides near real-time coverage of the X-ray sky in the energy range 15-50 keV. The BAT observes 88% of the sky each day with a detection sensitivity of 5.3 mCrab for a full-day observation and a time resolution as fine as 64 seconds. The three main purposes of the monitor are (1) the discovery of new transient X-ray sources, (2) the detection of outbursts or other changes in the flux of known X-ray sources, and (3) the generation of light curves of more than 900 sources spanning over eight years. The primary interface for the BAT transient monitor is a public web page. Between 2005 February 12 and 2013 April 30, 245 sources have been detected in the monitor, 146 of them persistent and 99 detected only in outburst. Among these sources, 17 were previously unknown and were discovered in the transient monitor. In this paper, we discuss the methodology and the data processing and filtering for the BAT transient monitor and review its sensitivity and exposure. We provide a summary of the source detections and classify them according to the variability of their light curves. Finally, we review all new BAT monitor discoveries; for the new sources that are previously unpublished, we present basic data analysis and interpretations.

astro-ph.HE↗

arXiv2013

Probing the Gamma-Ray Burst Rate with Trigger Simulations of the Swift Burst Alert Telescope

The long gamma-ray burst (GRB) rate is essential for revealing the connection between GRBs, supernovae and stellar evolution. Additionally, the GRB rate at high redshift provides a strong probe of star formation history in the early universe. While hundreds of GRBs are observed by Swift, it remains difficult to determine the intrinsic GRB rate due to the complex trigger algorithm of Swift. Current studies usually approximate the Swift trigger algorithm by a single detection threshold. However, unlike the previously flown GRB instruments, Swift has over 500 trigger criteria based on photon count rate and additional image threshold for localization. To investigate possible systematic biases and explore the intrinsic GRB properties, we developed a program that is capable of simulating all the rate trigger criteria and mimicking the image trigger threshold. We use this program to search for the intrinsic GRB rate. Our simulations show that adopting the complex trigger algorithm of Swift increases the detection rate of dim bursts. As a result, we find that either the GRB rate is much higher than previously expected at large redshift, or the luminosity evolution is non-negligible. We will discuss the best results of the GRB rate in our search, and their impact on the star-formation history.

astro-ph.CO↗

arXiv2013

A bright ultraluminous X-ray source in NGC 5907

We present a multi-mission X-ray analysis of a bright (peak observed 0.3-10 keV luminosity of ~ 6x10^{40} erg s^{-1}), but relatively highly absorbed ULX in the edge-on spiral galaxy NGC 5907. The ULX is spectrally hard in X-rays (Gamma ~ 1.2-1.7, when fitted with an absorbed power-law), and has a previously-reported hard spectral break consistent with it being in the ultraluminous accretion state. It is also relatively highly absorbed for a ULX, with a column of ~ 0.4-0.9x10^{22} atom cm^{-2} in addition to the line-of-sight column in our Galaxy. Although its X-ray spectra are well represented by accretion disc models, its variability characteristics argue against this interpretation. The ULX spectra instead appear dominated by a cool, optically-thick Comptonising corona. We discuss how the measured 9 per cent rms variability and a hardening of the spectrum as its flux diminishes might be reconciled with the effects of a very massive, radiatively-driven wind, and subtle changes in the corona respectively. We speculate that the cool disc-like spectral component thought to be produced by the wind in other ULXs may be missing from the observed spectrum due to a combination of a low temperature (~ 0.1 keV), and the high column to the ULX. We find no evidence, other than its extreme X-ray luminosity, for the presence of an intermediate mass black hole (~ 10^2 - 10^4 Msun) in this object. Rather, the observations can be consistently explained by a massive (greater than ~ 20 Msun) stellar remnant black hole in a super-Eddington accretion state.

astro-ph.HE↗

arXiv2013

The Burst Cluster: Dark Matter in a Cluster Merger Associated with the Short Gamma Ray Burst, GRB 050509B

We have identified a merging galaxy cluster with evidence of two distinct sub-clusters. The X-ray and optical data suggest that the subclusters are moving away from each other after closest approach. This cluster merger was discovered from observations of the well localized short-duration gamma-ray burst (GRB), GRB 050509B. The Swift/Burst Alert Telescope (BAT) source position is coincident with a cluster of galaxies ZwCl 1234.0+02916. The subsequent Swift/X-Ray Telescope (XRT) localization of the X-ray afterglow found the GRB coincident with 2MASX J12361286+2858580, a giant red elliptical galaxy in the cluster. Deep multi-epoch optical images were obtained to constrain the evolution of the GRB afterglow, including a 27480s exposure in the F814W band with Hubble Space Telescope Advanced Camera for Surveys (ACS), among the deepest imaging ever obtained towards a known galaxy cluster in a single passband. We perform a weak gravitational lensing analysis, including mapping the total mass distribution of the merger system. Combined with Chandra X-ray Observatory and Swift/XRT observations, we investigate the dynamical state of the merger to probe the nature of the dark matter component. Our weak gravitational lensing measurements reveal a separation of the X-ray centroid of the western subcluster from the center of the mass and galaxy light distributions, similar to that of the famous "Bullet cluster". We conclude that the "Burst cluster" is another candidate merger system for determining the nature of dark matter and for studying the environment of short GRBs. We discuss connections between the cluster dynamical state and/or matter composition and compact object mergers, the leading model for the origin of short GRBs. Finally, we present results from a weak lensing survey based on archival Very Large Telescope (VLT) images in the areas of 5 other short GRBs.

astro-ph.CO↗

arXiv2013

Using ISS Telescopes for Electromagnetic Follow-up of Gravitational Wave Detections of NS-NS and NS-BH Mergers

The International Space Station offers a unique platform for rapid and inexpensive deployment of space telescopes. A scientific opportunity of great potential later this decade is the use of telescopes for the electromagnetic follow-up of ground-based gravitational wave detections of neutron star and black hole mergers. We describe this possibility for OpTIIX, an ISS technology demonstration of a 1.5 m diffraction limited optical telescope assembled in space, and ISS-Lobster, a wide-field imaging X-ray telescope now under study as a potential NASA mission. Both telescopes will be mounted on pointing platforms, allowing rapid positioning to the source of a gravitational wave event. Electromagnetic follow-up rates of several per year appear likely, offering a wealth of complementary science on the mergers of black holes and neutron stars.

gr-qc↗

arXiv2013

G306.3-0.9: A newly discovered young galactic supernova remnant

We present X-ray and radio observations of the new Galactic supernova remnant (SNR) G306.3-0.9, recently discovered by Swift. Chandra imaging reveals a complex morphology, dominated by a bright shock. The X-ray spectrum is broadly consistent with a young SNR in the Sedov phase, implying an age of 2500 yr for a distance of 8 kpc, plausibly identifying this as one of the 20 youngest Galactic SNRs. Australia Telescope Compact Array (ATCA) imaging reveals a prominent ridge of radio emission that correlates with the X-ray emission. We find a flux density of ~ 160 mJy at 1 GHz, which is the lowest radio flux recorded for a Galactic SNR to date. The remnant is also detected at 24microns, indicating the presence of irradiated warm dust. The data reveal no compelling evidence for the presence of a compact stellar remnant.

astro-ph.HE↗

arXiv2013

Gamma Ray Bursts in the Swift-Fermi Era

Gamma-ray bursts (GRBs) are among the most violent occurrences in the universe. They are powerful explosions, visible to high redshift, and thought to be the signature of black hole birth. They are highly luminous events and provide excellent probes of the distant universe. GRB research has greatly advanced over the past 10 years with the results from Swift, Fermi and an active follow-up community. In this review we survey the interplay between these recent observations and the theoretical models of the prompt GRB emission and the subsequent afterglows.

astro-ph.HE↗

arXiv2013

A Universal Scaling for the Energetics of Relativistic Jets From Black Hole Systems

Black holes generate collimated, relativistic jets which have been observed in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies (active galactic nuclei; AGN). How jet physics scales from stellar black holes in GRBs to the supermassive ones in AGNs is still unknown. Here we show that jets produced by AGNs and GRBs exhibit the same correlation between the kinetic power carried by accelerated particles and the gamma-ray luminosity, with AGNs and GRBs lying at the low and high-luminosity ends, respectively, of the correlation. This result implies that the efficiency of energy dissipation in jets produced in black hole systems is similar over 10 orders of magnitude in jet power, establishing a physical analogy between AGN and GRBs.

astro-ph.HE↗

arXiv2012

Exploring the NRO Opportunity for a Hubble-sized Wide-field Near-IR Space Telescope -- NEW WFIRST

We discuss scientific, technical and programmatic issues related to the use of an NRO 2.4m telescope for the WFIRST initiative of the 2010 Decadal Survey. We show that this implementation of WFIRST, which we call "NEW WFIRST," would achieve the goals of the NWNH Decadal Survey for the WFIRST core programs of Dark Energy and Microlensing Planet Finding, with the crucial benefit of deeper and/or wider near-IR surveys for GO science and a potentially Hubble-like Guest Observer program. NEW WFIRST could also include a coronagraphic imager for direct detection of dust disks and planets around neighboring stars, a high-priority science and technology precursor for future ambitious programs to image Earth-like planets around neighboring stars.

astro-ph.IM↗

arXiv2012

Flat-spectrum radio sources as likely counterparts of unidentified INTEGRAL sources (Research Note)

Many sources in the fourth INTEGRAL/IBIS catalogue are still unidentified, since they lack an optical counterpart. An important tool that can help in identifying/classifying these sources is the cross-correlation with radio catalogues, which are very sensitive and positionally accurate. Moreover, the radio properties of a source, such as the spectrum or morphology, could provide further insight into its nature. Flat-spectrum radio sources at high Galactic latitudes are likely to be AGN, possibly associated to a blazar or to the compact core of a radio galaxy. Here we present a small sample of 6 sources extracted from the fourth INTEGRAL/IBIS catalogue that are still unidentified/unclassified, but which are very likely associated with a bright, flat-spectrum radio object. To confirm the association and to study the source X-ray spectral parameters, we performed X-ray follow-up observations with Swift/XRT. We report the results obtained from this search and discuss the nature of each source. 5 of the 6 radio associations are also detected in X-rays; in 3 cases they are the only counterpart found. IGR J06073--0024 is a flat-spectrum radio quasar at z=1.08, IGR J14488--4008 is a newly discovered radio galaxy, while IGR J18129--0649 is an AGN of a still unknown type. The nature of IGR J07225--3810 and IGR J19386--4653 is less well defined, since in both cases we find another X-ray source in the INTEGRAL error circle; nevertheless, the flat-spectrum radio source, likely to be a radio loud AGN, remains a viable and more convincing association in both cases. Only for IGR J11544--7618 could we not find any convincing counterpart since the radio association is not an X-ray emitter.

astro-ph.HE↗

arXiv2012

Seeking Counterparts to Advanced LIGO/Virgo Transients with Swift

Binary neutron star (NS) mergers are among the most promising astrophysical sources of gravitational wave emission for Advanced LIGO and Advanced Virgo, expected to be operational in 2015. Finding electromagnetic counterparts to these signals will be essential to placing them in an astronomical context. The Swift satellite carries a sensitive X-ray telescope (XRT), and can respond to target-of-opportunity requests within 1-2 hours, and so is uniquely poised to find the X-ray counterparts to LIGO/Virgo triggers. Assuming NS mergers are the progenitors of short gamma-ray bursts (GRBs), some percentage of LIGO/Virgo triggers will be accompanied by X-ray band afterglows that are brighter than 10^-12 erg/s/cm^2 in the XRT band one day after the trigger time. We find that a soft X-ray transient of this flux is bright enough to be extremely rare, and so could be confidently associated with even a moderately localized GW signal. We examine two possible search strategies with the Swift XRT to find bright transients in LIGO/Virgo error boxes. In the first strategy, XRT could search a volume of space with a ~100 Mpc radius by observing ~30 galaxies over the course of a day, with sufficient depth to observe the expected X-ray afterglow. For an extended LIGO/Virgo horizon distance, the XRT could employ 100 s exposures to cover an area of ~ 35 square degrees in about a day, remain sensitive enough to image GW discovered GRB afterglows. These strategies demonstrate that discovery of X-ray band counterparts to GW triggers will be possible.

astro-ph.IM↗

arXiv2012

Gamma Ray Bursts and their links with Supernovae and Cosmology

Gamma-ray bursts are the most luminous explosions in the Universe, whose origin and mechanism is the focus of intense interest. They appear connected to supernova remnants from massive stars or the merger of their remnants, and their brightness makes them temporarily detectable out to the larges distances yet explored in the Universe. After pioneering breakthroughs from space and ground experiments, their study is entering a new phase with observations from the recently launched \fermi satellite, as well as the prospect of detections or limits from large neutrino and gravitational wave detectors. The interplay between such observations and theoretical models of gamma-ray bursts is reviewed, as well as their connections to supernovae and cosmology.

astro-ph.HE↗

arXiv2012

Gamma Ray Bursts

Gamma-ray bursts (GRBs) are bright flashes of gamma-rays coming from the cosmos. They occur roughly once per day, last typically 10s of seconds and are the most luminous events in the universe. More than three decades after their discovery, and after pioneering advances from space and ground experiments, they still remain mysterious. The launch of the Swift and Fermi satellites in 2004 and 2008 brought in a trove of qualitatively new data. In this review we survey the interplay between these recent observations and the theoretical models of the prompt GRB emission and the subsequent afterglows.

astro-ph.HE↗

arXiv2012

High Energy Transients

We present an overview of high energy transients in astrophysics, highlighting important advances over the past 50 years. We begin with early discoveries of gamma-ray transients, and then delve into physical details associated with a variety of phenomena. We discuss some of the unexpected transients found by Fermi and Swift, many of which are not easily classifiable or in some way challenge conventional wisdom. These objects are important insofar as they underscore the necessity of future, more detailed studies.

astro-ph.HE↗

arXiv2012

GRB 120422A: A Low-luminosity Gamma-ray Burst Driven by Central Engine

GRB 120422A is a low-luminosity Gamma-ray burst (GRB) associated with a bright supernova, which distinguishes itself by its relatively short T90 ~ 5 s and an energetic X-ray tail. We analyze the Swift BAT and XRT data and discuss the physical implications. We show that the early steep decline in the X-ray light curve can be interpreted as the curvature tail of a late emission episode around 58-86 s, with a curved instantaneous spectrum at the end of the emission episode. Together with the main activity in the first ~ 20 s and the weak emission from 40 s to 60 s, the prompt emission is variable, which points towards a central engine origin, in contrast to the shock breakout origin as invoked to interpret some other nearby low-luminosity supernova GRBs. The curvature effect interpretation and interpreting the early shallow decay as the coasting external forward shock emission in a wind medium both give a constraint on the bulk Lorentz factor Γto be around several. Comparing the properties of GRB 120422A and other supernova GRBs, we found that the main criterion to distinguish engine-driven GRBs from the shock breakout GRBs is the time-averaged luminosity, with a separation line of about ~ 10^48 erg s^-1.

astro-ph.HE↗

arXiv2012

Broadband Spectral Investigations of SGR J1550-5418 Bursts

We present the results of our broadband spectral analysis of 42 SGR J1550-5418 bursts simultaneously detected with the Swift/X-ray Telescope (XRT) and the Fermi/Gamma-ray Burst Monitor (GBM), during the 2009 January active episode of the source. The unique spectral and temporal capabilities of the XRT Windowed Timing mode have allowed us to extend the GBM spectral coverage for these events down to the X-ray domain (0.5-10 keV). Our earlier analysis of the GBM data found that the SGR J1550-5418 burst spectra were described equally well with a Comptonized model or with two blackbody functions; the two models were statistically indistinguishable. Our new broadband (0.5 - 200 keV) spectral fits show that, on average, the burst spectra are better described with two blackbody functions than with the Comptonized model. Thus, our joint XRT/GBM analysis clearly shows for the first time that the SGR J1550-5418 burst spectra might naturally be expected to exhibit a more truly thermalized character, such as a two-blackbody or even a multi-blackbody signal. Using the Swift and RXTE timing ephemeris for SGR J1550-5418 we construct the distribution of the XRT burst counts with spin phase and find that it is not correlated with the persistent X-ray emission pulse phase from SGR J1550-5418. These results indicate that the burst emitting sites on the neutron star need not be co-located with hot spots emitting the bulk of the persistent X-ray emission. Finally, we show that there is a significant pulse phase dependence of the XRT burst counts, likely demonstrating that the surface magnetic field of SGR J1550-5418 is not uniform over the emission zone, since it is anticipated that regions with stronger surface magnetic field could trigger bursts more efficiently.

astro-ph.HE↗

arXiv2012

Particle acceleration in relativistic outflows

In this review we confront the current theoretical understanding of particle acceleration at relativistic outflows with recent observational results on various source classes thought to involve such outflows, e.g. gamma-ray bursts, active galactic nuclei, and pulsar wind nebulae. We highlight the possible contributions of these sources to ultra-high-energy cosmic rays.

astro-ph.HE↗

arXiv2012

A Jet Break in the X-ray Light Curve of Short GRB 111020A: Implications for Energetics and Rates

We present broad-band observations of the afterglow and environment of the short GRB 111020A. An extensive X-ray light curve from Swift/XRT, XMM-Newton and Chandra, spanning ~100 seconds to 10 days after the burst, reveals a significant break at t~2 days with pre- and post-break decline rates of alphaX,1 ~ -0.78 and alphaX,2<-1.7, respectively. Interpreted as a jet break, we infer a collimated outflow with an opening angle of ~3-8 degrees. The resulting beaming-corrected gamma-ray (10-1000 keV band) and blastwave kinetic energies are (2-3)e48 erg and (0.3-2)e49 erg, respectively, with the range depending on the unknown redshift of the burst. We report a radio afterglow limit of <39 microJy (3-sigma) from EVLA observations which, along with our finding that vc 24.4 mag at 18 hours after the burst, and reveal a potential host galaxy with i~24.3 mag. The sub-arcsecond localization from Chandra provides a precise offset of 0.80"+/-0.11" (1-sigma) from this galaxy corresponding to an offset of 5-7 kpc for z=0.5-1.5. We find a high excess neutral Hydrogen column density of (7.5+/-2.0)e21 cm^(-2) (z=0). Our observations demonstrate that a growing fraction of short GRBs are collimated which may lead to a true event rate of >100-1000 Gpc^(-3) yr^(-1), in good agreement with the NS-NS merger rate of ~200-3000 Gpc^(-3) yr^(-1). This consistency is promising for coincident short GRB-gravitational wave searches in the forthcoming era of Advanced LIGO/VIRGO.

astro-ph.HE↗