arXiv subjects

N. Gehrels

Publications and source records attributed to N. Gehrels.

At least 253 records · Page 14Linked to original sources

arXiv2008

Monitoring Supergiant Fast X-ray Transients with Swift. Rise to the outburst in IGR J16479-4514

IGR J16479-4514 is a Supergiant Fast X-ray Transient (SFXT), a new class of High Mass X-ray Binaries, whose number is rapidly growing thanks to the observations of the Galactic plane performed with the INTEGRAL satellite. IGR J16479-4514 has been regularly monitored with Swift/XRT since November 2007, to study the quiescent emission, the outburst properties and their recurrence. A new bright outburst, reaching fluxes above 10$^{-9}$ erg cm$^{-2}$ s$^{-1}$, was caught by the Swift/BAT. Swift immediately re-pointed at the target with the narrow-field instruments so that, for the first time, an outburst from a SFXT where a periodicity in the outburst recurrence is unknown could be observed simultaneously in the 0.2--150 keV energy band. The X-ray emission is highly variable and spans almost four orders of magnitude in count rate during the Swift/XRT observations covering a few days before and after the bright peak. The X-ray spectrum in outburst is hard and highly absorbed. The power-law fit resulted in a photon index of 0.98$\pm{0.07}$, and in an absorbing column density of $\sim5\times10^{22}$ cm$^{-2}$. These observations demonstrate that in this source (similarly to what was observed during the 2007 outburst from the periodic SFXT IGR J11215-5952), the accretion phase lasts much longer than a few hours.

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arXiv2008

Monitoring Supergiant Fast X-ray Transients with Swift. I. Behavior outside outbursts

Supergiant Fast X-ray Transients (SFXTs) are a new class of HMXBs discovered thanks to the monitoring of the Galactic plane performed with the INTEGRAL satellite in the last 5 years. These sources display short outbursts (significantly shorter than typical Be/X-ray binaries) with a peak luminosity of a few 1E36 erg/s. The quiescent level, measured only in a few sources, is around 1E32 erg/s. We are performing a monitoring campaign with Swift of four SFXTs (IGRJ16479-4514, XTEJ1739-302, IGRJ17544-2619 and AXJ1841.0-0536/IGRJ18410-0535). We report on the first four months of Swift observations, started on 2007 October 26. We detect a low level X-ray activity in all four SFXTs which demonstrates that these transient sources accrete matter even outside their outbursts. This fainter X-ray activity is composed of many flares with a large flux variability, on timescales of thousands of seconds. The lightcurve variability is also evident on larger timescales of days, weeks and months, with a dynamic range of more than one order of magnitude in all four SFXTs. The X-ray spectra are typically hard, with an average 2-10 keV luminosity during this monitoring of about 1E33-1E34 erg/s. We detected pulsations from the pulsar AXJ1841.0-0536, with a period of 4.7008+/-0.0004 s. This monitoring demonstrates that these transients spend most of the time accreting matter, although at a much lower level (~100-1000 times lower than during the bright outbusts), and that the true quiescence, characterized by a soft spectrum and a luminosity of a few 1E32 erg/s, observed in the past only in a couple of members of this class, is probably a very rare state.

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arXiv2008

GRB 080319B: A Naked-Eye Stellar Blast from the Distant Universe

Long duration gamma-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and so provide a window into the process of black hole formation from the collapse of a massive star. Over the last forty years, our understanding of the GRB phenomenon has progressed dramatically; nevertheless, fortuitous circumstances occasionally arise that provide access to a regime not yet probed. GRB 080319B presented such an opportunity, with extraordinarily bright prompt optical emission that peaked at a visual magnitude of 5.3, making it briefly visible with the naked eye. It was captured in exquisite detail by wide-field telescopes, imaging the burst location from before the time of the explosion. The combination of these unique optical data with simultaneous gamma-ray observations provides powerful diagnostics of the detailed physics of this explosion within seconds of its formation. Here we show that the prompt optical and gamma-ray emissions from this event likely arise from different spectral components within the same physical region located at a large distance from the source, implying an extremely relativistic outflow. The chromatic behaviour of the broadband afterglow is consistent with viewing the GRB down the very narrow inner core of a two-component jet that is expanding into a wind-like environment consistent with the massive star origin of long GRBs. These circumstances can explain the extreme properties of this GRB.

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arXiv2008

An extremely luminous X-ray outburst at the birth of a supernova

Massive stars end their short lives in spectacular explosions, supernovae, that synthesize new elements and drive galaxy evolution. Throughout history supernovae were discovered chiefly through their delayed optical light, preventing observations in the first moments (hours to days) following the explosion. As a result, the progenitors of some supernovae and the events leading up to their violent demise remain intensely debated. Here we report the serendipitous discovery of a supernova at the time of explosion, marked by an extremely luminous X-ray outburst. We attribute the outburst to the break-out of the supernova shock-wave from the progenitor, and show that the inferred rate of such events agrees with that of all core-collapse supernovae. We forecast that future wide-field X-ray surveys will catch hundreds of supernovae each year in the act of explosion, and thereby enable crucial neutrino and gravitational wave detections that may ultimately unravel the explosion mechanism.

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arXiv2008

Bright flares from the X-ray pulsar SWIFT J1626.6-5156

We have performed a timing and spectral analysis of the X-ray pulsar SWIFT J1626.6-5156 during a major X-ray outburst in order to unveil its nature and investigate its flaring activity. Epoch- and pulse-folding techniques were used to derive the spin period. Time-average and pulse-phase spectroscopy were employed to study the spectral variability in the flare and out-of-flare states and energy variations with pulse phase. Power spectra were obtained to investigate the periodic and aperiodic variability. Two large flares, with a duration of ~450 seconds were observed on 24 and 25 December 2005. During the flares, the X-ray intensity increased by a factor of 3.5, while the peak-to-peak pulsed amplitude increased from 45% to 70%. A third, smaller flare of duration ~180 s was observed on 27 December 2005. The flares seen in SWIFT J1626.6-5156 constitute the shortest events of this kind ever reported in a high-mass X-ray binary. In addition to the flaring activity, strong X-ray pulsations with Pspin=15.3714+-0.0003 s characterise the X-ray emission in SWIFT J1626.6-5156. After the major outburst, the light curve exhibits strong long-term variations modulated with a 45-day period. We relate this modulation to the orbital period of the system or to a harmonic. Power density spectra show, in addition to the harmonic components of the pulsation, strong band-limited noise with an integrated 0.01-100 Hz fractional rms of around 40% that increased to 64% during the flares. A weak QPO (fractional rms 4.7%) with characteristic frequency of 1 Hz was detected in the non-flare emission. The timing (short X-ray pulsations, long orbital period) and spectral (power-law with cut off energy and neutral iron line) properties of SWIFT J1626.6-5156 are characteristic of Be/X-ray binaries.

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arXiv2008

The Gamma Ray Burst section of the White Paper on the Status and Future of Very High Energy Gamma Ray Astronomy: A Brief Preliminary Report

This is a short report on the preliminary findings of the gamma ray burst (GRB) working group for the white paper on the status and future of very high energy (VHE; >50 GeV) gamma-ray astronomy. The white paper discusses the status of past and current attempts to observe GRBs at GeV-TeV energies, including a handful of low-significance, possible detections. The white paper concentrates on the potential of future ground-based gamma-ray experiments to observe the highest energy emission ever recorded for GRBs, particularly for those that are nearby and have high Lorentz factors in the GRB jet. It is clear that the detection of VHE emission would have strong implications for GRB models, as well as cosmic ray origin. In particular, the extended emission phase (including both afterglow emission and possible flaring) of nearby long GRBs could provide the best possibility for detection. The difficult-to-obtain observations during the prompt phase of nearby long GRBs and short GRBs could also provide particularly strong constraints on the opacity and bulk Lorentz factors surrounding the acceleration site. The synergy with upcoming and existing observatories will, of course, be critical for both identification of GRBs and for multiwavelength/multimessenger studies.

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arXiv2008

Observations of X-ray Flares from Gamma Ray Bursts

Swift-XRT observations of the X-ray emission from gamma ray bursts (GRBs) and during the GRB afterglow have led to many new results during the past two years. One of these exciting results is that approximately 1/3-1/2 of GRBs contain detectable X-ray flares. The mean fluence of the X-ray flares is ~10 times less than that of the initial prompt emission, but in some cases the flare is as energetic as the prompt emission itself. The flares display fast rises and decays, and they sometimes occur at very late times relative to the prompt emission (sometimes as late as 10^5 s after T_0) with very high peak fluxes relative to the underlying afterglow decay that has clearly begun prior to some flares. The temporal and spectral properties of the flares are found to favor models in which flares arise due to the same GRB internal engine processes that spawned the prompt GRB emission. Therefore, both long and short GRB internal engine models must be capable of producing high fluences in the X-ray band at very late times.

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arXiv2008

Swift BAT Survey of AGN

We present the results of the analysis of the first 9 months of data of the Swift BAT survey of AGN in the 14-195 keV band. Using archival X-ray data or follow-up Swift XRT observations, we have identified 129 (103 AGN) of 130 objects detected at |b|> 15 deg and with significance >4.8 sigma. One source remains unidentified. These same X-ray data have allowed measurement of the X-ray properties of the objects. We fit a power law to the log N - log S distribution, and find the slope to be 1.42+/-0.14. Characterizing the differential luminosity function data as a broken power law, we find a break luminosity log L_*(erg/s) = 43.85+/-0.26, a low luminosity power law slope a=0.84^{+0.16}_{-0.22}, and a high luminosity power law slope b=2.55^{+0.43}_{-0.30}, similar to the values that have been reported based on INTEGRAL data. We obtain a mean photon index 1.98 in the 14-195 keV band, with an rms spread of 0.27. Integration of our luminosity function gives a local volume density of AGN above 10^{41} erg/s of 2.4x10^{-3}/Mpc^3, which is about 10% of the total luminous local galaxy density above M_*=-19.75. We have obtained X-ray spectra from the literature and from Swift XRT follow-up observations. These show that the distribution of log n_H is essentially flat from n_H=10^{20}/cm^{2} to 10^{24}/cm^2, with 50% of the objects having column densities of less than 10^{22}/cm^{2}. BAT Seyfert galaxies have a median redshift of 0.03, a maximum log luminosity of 45.1, and approximately half have log n_H > 22.

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arXiv2008

The Spectral Lag of GRB060505: A Likely Member of the Long Duration Class

Two long gamma-ray bursts, GRB 060505 and GRB 060614, occurred in nearby galaxies at redshifts of 0.089 and 0.125 respectively. Due to their proximity and durations, deep follow-up campaigns to search for supernovae (SNe) were initiated. However none were found in either case, to limits more than two orders of magnitude fainter than the prototypical GRB-associated SN, 1998bw. It was suggested that the bursts, in spite of their durations (4 and 102 s), belonged to the population of short GRBs which has been shown to be unrelated to SNe. In the case of GRB 060614 this argument was based on a number of indicators, including the negligible spectral lag, which is consistent with that of short bursts. GRB 060505 has a shorter duration, but no spectral lag was measured. We present the spectral lag measurements of GRB 060505 using Suzakus Wide Area Monitor and the Swift Burst Alert Telescope. We find that the lag is 0.36+/- 0.05 s, inconsistent with the lags of short bursts and consistent with the properties of long bursts and SN-GRBs. These results support the association of GRB 060505 with other low-luminosity GRBs also found in star-forming galaxies and indicates that at least some massive stars may die without bright SNe.

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arXiv2008

Screening High-z GRBs with BAT Prompt Emission Properties

Detecting high-z GRBs is important for constraining the GRB formation rate, and tracing the history of re-ionization and metallicity of the universe. Based on the current sample of GRBs detected by Swift with known redshifts, we investigated the relationship between red-shift, and spectral and temporal characteristics, using the BAT event-by-event data. We found red-shift trends for the peak-flux-normalized temporal width T90, the light curve variance, the peak flux, and the photon index in simple power-law fit to the BAT event data. We have constructed criteria for screening GRBs with high red-shifts. This will enable us to provide a much faster alert to the GRB community of possible high-z bursts.

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arXiv2008

Glimm's Method for Relativistic Hydrodynamics

We present the results of standard one-dimensional test problems in relativistic hydrodynamics using Glimm's (random choice) method, and compare them to results obtained using finite differencing methods. For problems containing profiles with sharp edges, such as shocks, we find Glimm's method yields global errors ~1-3 orders of magnitude smaller than the traditional techniques. The strongest differences are seen for problems in which a shear field is superposed. For smooth flows, Glimm's method is inferior to standard methods. The location of specific features can be off by up to two grid points with respect to an exact solution in Glimm's method, and furthermore curved states are not modeled optimally since the method idealizes solutions as being composed of piecewise constant states. Thus although Glimm's method is superior at correctly resolving sharp features, especially in the presence of shear, for realistic applications in which one typically finds smooth flows plus strong gradients or discontinuities, standard FD methods yield smaller global errors. Glimm's method may prove useful in certain applications such as GRB afterglow shock propagation into a uniform medium.

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arXiv2008

Unveiling the nature of INTEGRAL objects through optical spectroscopy. VI. A multi-observatory identification campaign

Using 8 telescopes in the northern and southern hemispheres, plus archival data from two on-line sky surveys, we performed a systematic optical spectroscopic study of 39 putative counterparts of unidentified or poorly studied INTEGRAL sources in order to determine or at least better assess their nature. This was implemented within the framework of our campaign to reveal the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 29 of these objects are active galactic nuclei (13 of which are of Seyfert 1 type, 15 are Seyfert 2 galaxies and one is possibly a BL Lac object) with redshifts between 0.011 and 0.316, 7 are X-ray binaries (5 with high-mass companions and 2 with low-mass secondaries), one is a magnetic cataclysmic variable, one is a symbiotic star and one is possibly an active star. Thus, the large majority (74%) of the identifications in this sample belongs to the AGN class. When possible, the main physical parameters for these hard X-ray sources were also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in studying the hard X-ray spectra of all classes of X-ray emitting objects, and the effectiveness of a strategy of multi-catalogue cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of still unidentified hard X-ray sources.

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arXiv2008

Line Searches in Swift X-ray Spectra

Prior to the launch of the Swift mission several X-ray line detections were reported in Gamma Ray Burst afterglow spectra. To date, these pre-Swift era results have not been conclusively confirmed. The most contentious issue in this area is the choice of statistical method used to evaluate the significance of these features. In this paper we compare three different methods already extant in the literature for assessing the significance of possible line features and discuss their relative advantages and disadvantages. The methods are demonstrated by application to observations of 40 bursts from the archive of Swift XRT at early times (< few ks post trigger in the rest frame of the burst). Based on this thorough analysis we found no strong evidence for emission lines. For each of the three methods we have determined detection limits for emission line strengths in bursts with spectral parameters typical of the Swift-era sample. We also discuss the effects of the current calibration status on emission line detection.

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arXiv2008

Improving Swift-XRT positions of GRBs

Since GRBs fade rapidly, it is important to publish accurate, precise positions at early times. For Swift-detected bursts, the best promptly available position is most commonly the X-ray Telescope (XRT) position. We present two processes, developed by the Swift team at Leicester, which are now routinely used to improve the precision and accuracy of the XRT positions reported by the Swift team. Both methods, which are fully automated, make use of a PSF-fitting approach which accounts for the bad columns on the CCD. The first method yields positions with 90% error radii <4.4" 90% of the time, within 10--20 minutes of the trigger. The second method astrometrically corrects the position using UVOT field stars and the known mapping between the XRT and UVOT detectors, yielding enhanced positions with 90% error radii of <2.8" 90% of the time, usually ~2 hours after the trigger.

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arXiv2008

Global Properties of X-Ray Flashes and X-Ray-Rich Gamma-Ray Bursts Observed by Swift

We describe and discuss the spectral and temporal characteristics of the prompt emission and X-ray afterglow emission of X-ray flashes (XRFs) and X-ray-rich gamma-ray bursts (XRRs) detected and observed by Swift between December 2004 and September 2006. We compare these characteristics to a sample of conventional classical gamma-ray bursts (C-GRBs) observed during the same period. We confirm the correlation between Epeak_obs and fluence noted by others and find further evidence that XRFs, XRRs and C-GRBs form a continuum. We also confirm that our known redshift sample is consistent with the correlation between the peak energy in the GRB rest frame (Epeak_src) and the isotropic radiated energy (Eiso), so called the Epeak_src-Eiso relation. The spectral properties of X-ray afterglows of XRFs and C-GRBs are similar, but the temporal properties of XRFs and C-GRBs are quite different. We found that the light curves of C-GRB afterglows show a break to steeper indices (shallow-to-steep break) at much earlier times than do XRF afterglows. Moreover, the overall luminosity of XRF X-ray afterglows is systematically smaller by a factor of two or more compared to that of C-GRBs. These distinct differences between the X-ray afterglows of XRFs and C-GRBs may be the key to understanding not only the mysterious shallow-to-steep break in X-ray afterglow light curves, but also the unique nature of XRFs.

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arXiv2008

High-redshift blazar identification for Swift J1656.3-3302

We report on the high-redshift blazar identification of a new gamma-ray source, Swift J1656.3-3302, detected with the BAT imager onboard the Swift satellite and the IBIS instrument on the INTEGRAL satellite. Follow-up optical spectroscopy has allowed us to identify the counterpart as an R-band 19 mag source that shows broad Lyman-alpha, Si IV, He II, C IV, and C III] emission lines at redshift z = 2.40+-0.01. Spectral evolution is observed in X-rays when the INTEGRAL/IBIS data are compared to the Swift/BAT results, with the spectrum steepening when the source gets fainter. The 0.7-200 keV X-ray continuum, observed with Swift/XRT and INTEGRAL/IBIS, shows the power law shape typical of radio loud (broad emission line) active galactic nuclei (with a photon index around 1.6) and a hint of spectral curvature below 2 keV, possibly due to intrinsic absorption (N_H about 7e22 cm-2) local to the source. Alternatively, a slope change (of about 1 in terms of photon index) around 2.7 keV can describe the X-ray spectrum equally well. At this redshift, the observed 20-100 keV luminosity of the source is about 1e48 erg s-1 (assuming isotropic emission), making Swift J1656.3-3302 one of the most X-ray luminous blazars. This source is yet another example of a distant gamma-ray loud quasar discovered above 20 keV. It is also the farthest object, among the previously unidentified INTEGRAL sources, whose nature has been determined a posteriori through optical spectroscopy.

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arXiv2008

Swift and Chandra Detections of Supernova 2006jc: Evidence for Interaction of the Supernova Shock with a Circumstellar Shell

The peculiar Type Ib supernova (SN) 2006jc has been observed with the UV/Optical Telescope (UVOT) and X-Ray Telescope (XRT) on board the Swift observatory over a period of 19 to 183 days after the explosion. Signatures of interaction of the outgoing SN shock with dense circumstellar material (CSM) are detected, such as strong X-ray emission (L_{0.2-10} > E39 erg/s) and the presence of MgII 2800A line emission visible in the UV spectra. In combination with a Chandra observation obtained on day 40 after the explosion, the X-ray light curve is constructed, which shows a unique rise of the X-ray emission by a factor of ~5 over a period of ~4 months, followed by a rapid decline. We interpret the unique X-ray and UV properties as a result of the SN shock interacting with a shell of material that was deposited by an outburst of the SN progenitor two years prior to the explosion. Our results are consistent with the explosion of a Wolf-Rayet star that underwent an episodic mass ejection qualitatively similar to those of luminous blue variable stars prior to its explosion. This led to the formation of a dense (>E7 cm**-3) shell at a distance of ~E16 cm from the site of the explosion, which expands with the WR wind at a velocity of (1300+/-300) km/s.

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arXiv2007

GRB 070724B: the first Gamma Ray Burst localized by SuperAGILE and its Swift X-ray Afterglow

GRB 070724B is the first Gamma Ray Burst localized by SuperAGILE, the hard X-ray monitor aboard the AGILE satellite. The coordinates of the event were published $\sim 19$ hours after the trigger. The Swift X-Ray Telescope pointed at the SuperAGILE location and detected the X-ray afterglow inside the SuperAGILE error circle. The AGILE gamma-ray Tracker and Minicalorimeter did not detect any significant gamma ray emission associated with GRB 070724B in the MeV and GeV range, neither prompt nor delayed. Searches of the optical afterglow were performed by the Swift UVOT and the Palomar automated 60-inch telescopes without any significant detection. Similarly the Very Large Array did not detect a radio afterglow. This is the first GRB event with a firm upper limit in the 100 MeV -- 30 GeV energy range, associated with an X-ray afterglow.

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