arXiv subjects

C. Kouveliotou

Publications and source records attributed to C. Kouveliotou.

At least 145 records · Page 8Linked to original sources

arXiv2006

Evidence for a Supernova Associated with the X-ray Flash 020903

We present ground-based and Hubble Space Telescope optical observations of the X-ray flash (XRF) 020903, covering 300 days. The afterglow showed a very rapid rise in the first day, followed by a relatively slow decay in the next few days. There was a clear bump in the light curve after ~25 days, accompanied by a drastic change in the spectral energy distribution. The light curve and the spectral energy distribution are naturally interpreted as the emergence -- and subsequent decay -- of a supernova (SN), similar to SN 1998bw. At peak luminosity, the SN is estimated to be 0.8 +/- 0.1 mag fainter than SN1998bw. This argues in favor of the existence of a supernova associated with this X-ray flash. A spectrum obtained 35 days after the burst shows emission lines from the host galaxy. We use this spectrum to put an upper limit on the oxygen abundance of the host at [O/H] < -0.6 dex. We also discuss a possible trend between the softness of several bursts and the early behavior of the optical afterglow, in the sense that XRFs and X-ray rich GRBs seem to have a plateau phase or even a rising light curve. This can be naturally explained in models where XRFs are similar to GRBs but seen off the jet axis.

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arXiv2006

Evidence for a Canonical GRB Afterglow Light Curve in the Swift/XRT Data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) detected with the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments: (i) an initial very steep decay (t^{-alpha} with 3 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.

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arXiv2005

An origin for short g-ray bursts unassociated with current star formation

Two short (<2 s) g-ray bursts (GRBs) have recently been localized and fading afterglow counterparts detected. The combination of these two results left unclear the nature of the host galaxies of the bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual g-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z=0.258, coincident with the position determined by ground-based optical and radio observations. The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated n a supernova explosion. A supernova origin was also ruled out for GRB 050709, even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2-3 orders of magnitude lower han that for the long bursts. Our results therefore suggest that an alternative source of bursts -- the of binary systems of neutron stars or a neutron star-black hole pair -- are the progenitors of short bursts.

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arXiv2005

Discovery of an Afterglow Extension of the Prompt Phase of Two Gamma Ray Bursts Observed by Swift

BAT and XRT observations of two recent well-covered GRBs observed by Swift, GRB 050315 and GRB 050319, show clearly a prompt component joining the onset of the afterglow emission. By fitting a power law form to the gamma-ray spectrum, we extrapolate the time dependent fluxes measured by the BAT, in the energy band 15-350 keV, into the spectral regime observed by the XRT 0.2-10 keV, and examine the functional form of the rate of decay of the two light curves. We find that the BAT and XRT light curves merge to form a unified curve. There is a period of steep decay up to ~300 s, followed by a flatter decay. The duration of the steep decay, ~100 s in the source frame after correcting for cosmological time dilation, agrees roughly with a theoretical estimate for the deceleration time of the relativistic ejecta as it interacts with circumstellar material. For GRB 050315, the steep decay can be characterized by an exponential form, where one e-folding decay time t_e(BAT) ~ 24+/-2 s, and t_e(XRT) ~ 35+/-2 s. For GRB 050319, a power law decay -(d ln f/d ln t) = n, where n is about 3, provides a reasonable fit. The early time X-ray fluxes are consistent with representing the lower energy tail of the prompt emission, and provide our first quantitative measure of the decay of the prompt gamma-ray emission over a large dynamic range in flux. The initial steep decay is expected due to the delayed high latitude photons from a curved shell of relativistic plasma illuminated only for a short interval. The overall conclusion is that the prompt phase of GRBs remains observable for hundreds of seconds longer than previously thought.

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arXiv2005

Structure in the radio counterpart to the 2004 Dec 27 giant flare from SGR 1806-20

On Dec 27, 2004, the magnetar SGR 1806-20 underwent an enormous outburst resulting in the formation of an expanding, moving, and fading radio source. We report observations of this radio source with the Multi-Element Radio-Linked Interferometer Network (MERLIN) and the Very Long Baseline Array (VLBA). The observations confirm the elongation and expansion already reported based on observations at lower angular resolutions, but suggest that at early epochs the structure is not consistent with the very simplest models such as a smooth flux distribution. In particular there appears to be significant structure on small angular scales, with ~10% of the radio flux arising on angular scales <100 milliarcsec. This structure may correspond to localised sites of particle acceleration during the early phases of expansion and interaction with the ambient medium.

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arXiv2005

A Re-brightening of the Radio Nebula associated with the 2004 December 27 giant flare from SGR 1806--20

The 2004 Dec. 27 giant Gamma-ray flare detected from the magnetar SGR 1806-20 created an expanding radio nebula which we have monitored with the Australia Telescope Compact Array and the Very Large Array. These data indicate that there was an increase in the observed flux ~25 days after the initial flare that lasted for ~8 days, which we believe is the result of ambient material swept-up and shocked by this radio nebula. For a distance to SGR 1806-20 of 15 kpc, using the properties of this rebrightening we infer that the initial blast wave was dominated by baryonic material of mass M>10^{24.5} g. For an initial expansion velocity v~0.7c (as derived in an accompanying paper), we infer this material had an initial kinetic energy E>10^{44.5} ergs. If this material originated from the magnetar itself, it may have emitted a burst of ultra-high energy (E > 1 TeV) neutrinos far brighter than that expected from other astrophysical sources.

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arXiv2005

Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211

We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at z=0.80 and z=1.02, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at z=0.77. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0.6" away. The spectrum shows a depression in flux in between 4000 A and 5500 A. This could be the result of a 2175 A-type extinction feature in the host of GRB991216, but at a rather red wavelength of 2360 A. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB011211 are identified with lines originating in a single absorption system at z=2.142+/-0.002, the redshift of the GRB011211 host galaxy. We also detect the Lyman alpha absorption line in the host, to which we fit a neutral hydrogen column density of log N(HI)=20.4+/-0.2, which indicates that it is a damped Lyman alpha system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB011211 redshift. For GRB021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [OII] 3727 at z=1.006. The corresponding unobscured [OII] star-formation rate is 1.4 Msun/yr.

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arXiv2005

Diagnosing the outflow from the SGR 1806-20 Giant Flare with Radio Observations

On 2004 Dec. 27, the soft gamma repeater (SGR) 1806-20 emitted the brightest giant flare (GF) ever detected from an SGR, with an (isotropic) energy release $\sim 100$ times greater than the only two other known SGR GFs. It was followed by a very bright, fading radio afterglow. Extensive follow-up radio observations provided a wealth of information with unprecedented astrometric precision, revealing the temporal evolution of the source size, along with densely sampled light curves and spectra. Here we expand on our previous work on this source, by explaining these observations within one self-consistent dynamical model. In this scenario, the early radio emission is due to the outflow ejected during the GF energizing a thin shell surrounding a pre-existing cavity, where the observed steep temporal decay of the radio emission seen beginning on day 9 is attributed to the adiabatic cooling of the shocked shell. The shocked ejecta and external shell move outward together, driving a forward shock into the ambient medium, and are eventually decelerated by a reverse shock. As we show in Gelfand et al. (2005), the radio emission from the shocked external medium naturally peaks when significant deceleration occurs, and then decays relatively slowly. The dynamical modeling of the collision between the ejecta and the external shell together with the observed evolution of the source size (which is nicely reproduced in our model) suggest that most of the energy in the outflow was in mildly relativistic material, with an initial expansion velocity $v/c \lesssim 0.7d_{15}$, for a distance of $15d_{15}$ kpc to SGR 1806-20. An initially highly relativistic outflow would not have produced a long coasting phase at a mildly relativistic expansion velocity, as was observed.

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arXiv2005

The Growth, Polarization, and Motion of the Radio Afterglow from the Giant Flare from SGR 1806-20

The extraordinary giant flare (GF) of 2004 December 27 from the soft gamma repeater (SGR) 1806-20 was followed by a bright radio afterglow. We present an analysis of VLA observations of this radio afterglow from SGR 1806-20, consisting of previously reported 8.5 GHz data covering days 7 to 20 after the GF, plus new observations at 8.5 and 22 GHz from day 24 to 81. We detect motion in the flux centroid of the afterglow, at an average velocity of 0.26 +/- 0.03 c (assuming a distance of 15 kpc) at a position angle of -45 degrees. This motion, in combination with the growth and polarization measurements, suggests an asymmetric outflow, mainly from one side of the magnetar. We find a deceleration in the expansion, from ~9 mas/day to <5 mas/day. The time of deceleration is roughly coincident with the rebrightening in the radio light curve, as expected to result when the ejecta from the GF sweeps up enough of the external medium, and transitions from a coasting phase to the Sedov-Taylor regime. The radio afterglow is elongated and maintains a 2:1 axis ratio with an average position angle of -40 degrees (north through east), oriented perpendicular to the average intrinsic linear polarization angle.

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arXiv2005

Discovery and identification of the very high redshift afterglow of GRB 050904

In 2000, Lamb and Reichart predicted that gamma-ray bursts (GRBs) and their afterglows occur in sufficient numbers and at sufficient brightnesses at very high redshifts (z > 5) to eventually replace quasars as the preferred probe of element formation and reionization in the early universe and to be used to characterize the star-formation history of the early universe, perhaps back to when the first stars formed. Here we report the discovery of the afterglow of GRB 050904 and the identification of GRB 050904 as the first very high redshift GRB. We measure its redshift to be 6.39(+0.11,-0.12), which is consistent with the reported spectroscopic redshift (6.29 +/- 0.01). Furthermore, just redward of Ly-alpha the flux is suppressed by a factor of three on the first night, but returns to expected levels by the fourth night. We propose that this is due to absorption by molecular hydrogen that was excited to rovibrational states by the GRB's prompt emission, but was then overtaken by the jet. Now that very high redshift GRBs have been shown to exist, and at least in this case the afterglow was very bright, observing programs that are designed to capitalize on this science will likely drive a new era of study of the early universe, using GRBs as probes.

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arXiv2005

Gas and dust properties in the afterglow spectra of GRB 050730

We present early WHT ISIS optical spectroscopy of the afterglow of gamma-ray burst GRB 050730. The spectrum shows a DLA system with the highest measured hydrogen column to date: N(HI) = 22.1 +/- 0.1 at the third-highest GRB redshift z = 3.968. Our analysis of the Swift XRT X-ray observations of the early afterglow show X-ray flares accompanied by decreasing X-ray absorption. From both the optical and the X-ray spectra we constrain the dust and gas properties of the host galaxy. We find the host to be a low metallicity galaxy, with low dust content. Much of the X-ray absorbing gas is situated close to the GRB, whilst the HI absorption causing the DLA is most likely located further out.

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arXiv2005

Two years of INTEGRAL monitoring of the Soft Gamma-Ray Repeater SGR 1806-20: from quiescence to frenzy

SGR 1806-20 has been observed for more than 2 years with the INTEGRAL satellite. In this period the source went from a quiescent state into a very active one culminating in a giant flare on December 27 2004. Here we report on the properties of all the short bursts detected with INTEGRAL before the giant flare. We derive their number-intensity distribution and confirm the hardness-intensity correlation for the bursts found by Gotz et al. (2004a). Our sample includes a very bright outburst that occurred on October 5 2004, during which over one hundred bursts were emitted in 10 minutes, involving an energy release of 3 x 1E42 erg. We present a detailed analysis of it and discuss our results in the framework of the magnetar model.

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arXiv2005

The radio afterglow of GRB 030329 at centimeter wavelengths: evidence for a structured jet or non-relativistic expansion

We present our centimeter wavelength (1.4, 2.3 and 4.8 GHz) light curves of the afterglow of GRB 030329, which were obtained with the Westerbork Synthesis Radio Telescope. Modeling the data according to a collimated afterglow results in a jet-break time of 10 days. This is in accordance with earlier results obtained at higher radio frequencies. However, with respect to the afterglow model, some additional flux at the lower frequencies is present when these light curves reach their maximum after 40-80 days. We show that this additional flux can be modeled with two or more components with progressively later jet breaks. From these results we infer that the jet is in fact a structured or a layered jet, where the ejecta with lower Lorentz factors produce additional flux that becomes visible at late times in the lowest frequency bands. We show that a transition to non-relativistic expansion of the fireball at late times can also account for the observed flux excess, except for the lowest frequency (1.4 GHz) data.

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arXiv2005

Gamma-Ray Burst Selected High Redshift Galaxies: Comparison to Field Galaxy Populations to z~3

We study the internal structural properties of 37 long duration gamma-ray burst (GRB) host galaxies imaged with the Hubble Space Telescope. Fitting exponential disk and r^1/4 models to the surface brightness profiles of eight z < 1.2 bright host galaxies, we find that the disk model is slightly preferred for most hosts, although two galaxies are fit best with an r^1/4 profile. We furthermore measure the central concentrations and asymmetries of all 37 host galaxies using the CAS system, and compare with field galaxies in the Hubble Deep Field. Our first main conclusion is that GRB hosts exhibit a surprisingly high diversity of galaxy types. A significant fraction (68%) of host galaxies are situated in a region of the concentration-asymmetry diagram occupied by spirals or peculiar/merging galaxies. Twelve hosts (32%) are situated in the region occupied by elliptical galaxies, having high concentration indices. We also find some evidence for evolution in GRB host galaxy morphology, such that hosts at z > 1 have a relatively high light concentration, indicating that these systems are perhaps progenitors of massive galaxies, or are compact blue star forming galaxies. We find that GRB hosts at z > 1 are different from the general field population at z > 1 in terms of light concentration at >99.5% confidence, yet have sizes similar to the general z > 1 population. This is the opposite of the effect at z < 1 where GRB hosts are smaller than average. We argue that GRB hosts trace the starburst population at high redshift, as similarly concentrated galaxies at z > 1 are undergoing a disproportionate amount of star formation for their luminosities. Our results show that GRBs are perhaps ideal tracers of typical galaxies undergoing star formation at any epoch.

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arXiv2005

GRB 050509B: Constraints on short gamma-ray burst models

We have obtained deep optical images with the Very Large Telescope at ESO of the first well-localized short-duration gamma-ray burst, GRB 050509B. From V and R imaging, initiated ~2 days after the GRB trigger and lasting up to three weeks, we detect no variable object inside the small Swift/XRT X-ray error circle down to 2sigma limits of V = 26.5 and R = 25.1. The X-ray error circle includes a giant elliptical galaxy at z = 0.225, which has been proposed as the likely host of this GRB. Our limits indicate that if the GRB originated at z = 0.225, any supernova-like event accompanying the GRB would have to be over 100 times fainter than normal Type Ia SNe or Type Ic hypernovae, 5 times fainter than the faintest known Ia or Ic SNe, and fainter than the faintest known Type II SNe. Moreover, we use the optical limits to constrain the energetics of the GRB outflow. Simple models indicate that, unless the intrinsic energy in the outflow from GRB 050509B was << 10^51 erg, there was very little radioactive material with efficient decay timescales for generating a large luminosity. These limits strongly constrain progenitor models for this short GRB.

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arXiv2005

On the Afterglow and Host Galaxy of GRB021004: A Comprehensive Study with the Hubble Space Telescope

(Abridged) We report on HST observations of the late-time afterglow and host galaxy of GRB021004 (z=2.33). Although this GRB is one of the best observed so far, there is substantial disagreement between different interpretations of data sets on this burst in the literature. We have observed the field of GRB021004 with the HST at multiple epochs from 3 days until almost 10 months after the burst. With STIS spectroscopy we cover the spectral region 600-1700AA in the restframe. From the limit on the flux recovery bluewards of the Lyman-limit we constrain the HI column density to be above 1x10^18 cm-2 Based on ACS and NICMOS imaging we find that the afterglow evolved achromatically within the errors. The color changes observed by other authors during the first four days must be related to a stochastic phenomenon superimposed on an afterglow component with a constant spectral shape. This achromaticity implies that the cooling break has remained on the blue side of the optical part of the spectrum for at least two weeks after the burst. The optical-to-X-ray slope is consistent with being the same at 1.4 and 52.4 days after the burst. This indicates that the cooling frequency is constant and hence, according to fireball models, that the circumburst medium has a constant density profile. The late-time slope of the lightcurve is in the range alpha_2 = 1.8-1.9, and is inconsistent with a single power-law. This could be due to a late-time flattening caused by the transition to non-relativistic expansion or due to excess emission (a lightcurve `bump') 7 days after burst. The host is a starburst galaxy with no evidence for dust and with strong Ly-alpha emission. The afterglow was located very close (~100 pc) to the center of the host suggesting that the progenitor was associated with a circumnuclear starburst.

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arXiv2005

e-VLBI observations of SN2001em - an off-axis GRB candidate

Studying transient phenomena with the Very Long Baseline Interferometry (VLBI) technique faces severe difficulties because the turnaround time of the experiments from the observations to the scientific result is rather long. The e-VLBI technique has made it possible to transfer the data from a number of European VLBI Network (EVN) telescopes to the central data processor at JIVE through optical fibres, and correlate them in real time. The main goal of this paper is to introduce this rapidly developing new technique, by presenting observational results from a recent experiment. We observed SN2001em, a Type Ib/c supernova with an e-VLBI array and the Multi-Element Radio Linked Interferometer Network (MERLIN) in the UK. The source is marginally detected in our observations. We cannot make definite conclusions whether it is resolved at 1.6 GHz or not. Our data show that SN2001em either started fading in the last couple of months, or its radio spectrum is inverted at low frequencies,indicating free-free or synchrotron self-absorption. This is quite unusual, but not unprecedented in radio SNe.

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arXiv2005

An expanding radio nebula produced by a giant flare from the magnetar SGR 1806-20

Soft gamma repeaters (SGRs) are "magnetars", a small class of slowly spinning neutron stars with extreme surface magnetic fields, ~10^15 gauss. On 2004 December 27, a giant flare was detected from the magnetar SGR 1806-20, the third such event ever recorded. This burst of energy was detected by a variety of instruments and even caused an ionospheric disturbance in the Earth's upper atmosphere recorded around the globe. Here we report the detection of a fading radio afterglow produced by this outburst, with a luminosity 500 times larger than the only other detection of a similar source. From day 6 to day 19 after the flare from SGR 1806-20, a resolved, linearly polarized, radio nebula was seen, expanding at approximately a quarter the speed of light. To create this nebula, at least 4x10^43 ergs of energy must have been emitted by the giant flare in the form of magnetic fields and relativistic particles. The combination of spatially resolved structure and rapid time evolution allows a study in unprecedented detail of a nearby analog to supernovae and gamma-ray bursts.

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