arXiv subjects

K. Hurley

Publications and source records attributed to K. Hurley.

At least 145 records · Page 8Linked to original sources

arXiv2001

GRB 000301C: a possible short/intermediate duration burst connected to a DLA system

We discuss two main aspects of the GRB 000301C afterglow (Fynbo et al. 2000, Jensen et al. 2000); its short duration and its possible connection with a Damped Ly-alpha Absorber (DLA). GRB 000301C falls in the short class of bursts, though it is consistent with belonging to the proposed intermediate class or the extreme short end of the distribution of long-duration GRBs. Based on two VLT spectra we estimate the HI column density to be Log(N(HI))=21.2+/-0.5. This is the first direct indication of a connection between GRB host galaxies and Damped Ly-alpha Absorbers.

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arXiv2001

Multi-wavelength observations of the Soft Gamma Repeater SGR 1900+14 during its April 2001 activation

The soft-gamma repeater SGR 1900+14 became active on 18 April 2001 after about two years of quiescence; it had remained at a very low state of activity since the fall of 1998, when it exhibited extraordinary flaring. We have observed the source in the gamma and X rays with Ulysses and Chandra, and in the radio with MERLIN. We report here the confirmation of a two component X-ray spectrum (power law + blackbody), indicating emission from the neutron star surface. We have determined that there is a dust halo surrounding the source that extends up to >~ 100'' from the center of SGR 1900+14, due to scattering in the interstellar medium.

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arXiv2001

Broadband Observations of the Afterglow of GRB 000926: Observing the Effect of Inverse Compton Scattering and Evidence for a High-Density Environment

GRB 000926 has one of the best-studied afterglows to-date, with multiple X-ray observations, as well as extensive multi-frequency optical and radio coverage. Broadband afterglow observations, spanning from X-ray to radio frequencies, provide a probe of the density structure of the circumburst medium, as well as of the ejecta energetics, geometry, and the physical parameters of the relativistic blastwave resulting from the explosion. We present an analysis of {\em Chandra X-ray Observatory} observations of this event, along with {\em Hubble Space Telescope} and radio monitoring. We combine these data with ground-based optical and IR observations and fit the synthesized afterglow lightcurve using models where collimated ejecta expand into a surrounding medium. We find that we can explain the broadband lightcurve with reasonable physical parameters only if the cooling is dominated by inverse Compton scattering. Excess X-ray emission in the broadband spectrum indicates that we are directly observing a contribution from inverse Compton scattering. It is the first time this has been observed in a GRB afterglow, and it implies that the GRB exploded in a reasonably dense (n~30 cm^{-3}) medium, consistent with a diffuse interstellar cloud environment.

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arXiv2001

The X-ray afterglow of GRB000926 observed by BeppoSAX and Chandra: a mildly collimated fireball in a dense medium ?

We present X-ray observations of the afterglow of GRB000926 performed around and after the break observed in the optical light curve two days after the burst. The steep X-ray light curve observed around the break confirms the presence of this feature in X-rays. However, the spectral and temporal properties are not consistent with a standard jet scenario based on synchrotron emission, requiring a more complicated model. We find that X-ray and optical data are compatible with a moderately collimated fireball (with opening angle $θ\approx25 °$) expanding in a dense medium ($n\approx 4\times 10^4$ cm$^{-3}$). This produces two breaks in the light curve. The first, at $t\approx$ 2 days, is due to jet behavior. The second, around 5 days, is attributed to the transition of the fireball to a non-relativistic expansion. This transition predicts a{\it flattening} of the light curve, that explains the late X-ray measurement in excess above the extrapolation of the simple jet scenario, and is also consistent with optical data.

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arXiv2001

The optical afterglow and host galaxy of GRB000926

We present the discovery of the Optical Transient (OT) of the long-duration gamma-ray burst GRB000926. The optical transient was detected independently with the Nordic Optical Telescope and at Calar Alto 22.2 hours after the burst. At this time the magnitude of the transient was R = 19.36. The transient faded with a decay slope of about 1.7 during the first two days after which the slope increased abruptly (within a few hours) to about 2.4. The light-curve started to flatten off after about a week indicating the presence of an underlying extended object. This object was detected in a deep image obtained one month after the GRB at R=23.87+-0.15 and consists of several compact knots within about 5 arcsec. One of the knots is spatially coincident with the position of the OT and hence most likely belongs to the host galaxy. Higher resolution imaging is needed to resolve whether all the compact knots belong to the host galaxy or to several independent objects. In a separate paper we present a discussion of the optical spectrum of the OT, and its inferred redshift (Moller et al. in prep.).

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arXiv2001

HST/STIS observations of GRB000301C: CCD imaging and NUV MAMA spectroscopy

We present HST/STIS observations of the optical counterpart (OT) of the gamma-ray burster GRB 000301C obtained on 2000 March 6, five days after the burst. CCD clear aperture imaging reveals a R ~ 21.50+/-0.15 source with no apparent host galaxy. An 8000 s, 1150 < lambda/A < 3300 NUV-MAMA prism spectrum shows a relatively flat continuum (in f_lambda) between 2800 and 3300 A, with a mean flux 8.7 (+0.8,-1.6)+/- 2.6 10^(-18) ergs/s/cm^2/A, and a sharp break centered at 2797+/-25 A. We interpret it as HI Lyman break at z = 2.067+/-0.025 indicating the presence of a cloud with a HI column density log(HI) > 18 on the line-of-sight to the OT. This value is conservatively a lower limit to the GRB redshift. However, the facts that large N(HI) system are usually considered as progenitors of present day galaxies and that other OTs are found associated with star forming galaxies strongly suggest that it is the GRB redshift. In any case, this represents the largest direct redshift determination of a gamma-ray burster to date. Our data are compatible with an OT spectrum represented by a power-law with an intrinsic index α= 1.2((f_nu \propto nu^-alpha) and no extinction in the host galaxy or with alpha = 0.5 and extinction by a SMC-like dust in the OT rest-frame with A_V = 0.15. The large N(HI) and the lack of detected host is similar to the situation for damped Ly-alpha absorbers at z > 2.

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arXiv2001

The afterglow of the short/intermediate-duration gamma-ray burst GRB 000301C: A jet at z=2.04

We present Ulysses and NEAR data from the detection of the short or intermediate duration (2 s) gamma-ray burst GRB000301C (2000 March 1.41 UT). The gamma-ray burst (GRB) was localised by the Inter Planetary Network (IPN) and RXTE to an area of 50 arcmin^2. A fading optical counterpart was subsequently discovered with the Nordic Optical Telescope (NOT) about 42h after the burst. The GRB lies at the border between the long-soft and the short-hard classes of GRBs. If GRB000301C belongs to the latter class, this would be the first detection of an afterglow to a short-hard burst. We present UBRI and JHK photometry from the time of the discovery until 11 days after the burst. Finally, we present spectroscopic observations of the optical afterglow obtained with the ESO VLT Antu telescope 4 and 5 days after the burst. The optical light curve is consistent with being achromatic from 2 to 11 days after the burst and exhibits a break. A broken power-law fit yields a shallow pre-break decay power-law slope of a_1=-0.72+-0.06, a break time of t_b=4.39+-0.26 days after the burst, and a post-break slope of a_2=-2.29+-0.17, which is best explained by a sideways expanding jet in an ambient medium of constant mean density. In the optical spectrum we find absorption features that are consistent with FeII, CIV, CII, SiII and Ly-a at a redshift of 2.0404+-0.0008. We find evidence for a curved shape of the spectral energy distribution of the observed afterglow. It is best fitted with a power-law spectral distribution with index b ~ -0.7 reddened by an SMC-like extinction law with A_V~0.1 mag. Based on the Ly-a absorption line we estimate the HI column density to be log(N(HI))=21.2+-0.5. This is the first direct indication of a connection between GRB host galaxies and Damped Ly-a Absorbers.

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arXiv2001

GRB 000418: A Hidden Jet Revealed?

We report on optical, near-infrared and centimeter radio observations of GRB000418 which allow us to follow the evolution of the afterglow from 2 to 200 days after the gamma-ray burst. In modeling these broad-band data, we find that an isotropic explosion in a constant density medium is unable to simultaneously fit both the radio and optical data. However, a jet-like outflow with an opening angle of 10-20 degress provides a good description of the data. The evidence in favor of a jet interpretation is based on the behavior of the radio light curves, since the expected jet break is masked at optical wavelengths by the light of the host galaxy. We also find evidence for extinction, presumably arising from within the host galaxy, with A(V)=0.4 mag, and host flux densities of F_R=1.1 uJy and F_K=1.7 uJy. These values supercede previous work on this burst due to the availability of a broad-band data set allowing a global fitting approach. A model in which the GRB explodes into a wind-stratified circumburst medium cannot be ruled out by these data. However, in examining a sample of other bursts (e.g. GRB990510, GRB000301C) we favor the jet interpretation for GRB000418.

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arXiv2001

Detection of the optical afterglow of GRB 000630: Implications for dark bursts

We present the discovery of the optical transient of the long-duration gamma-ray burst GRB000630. The optical transient was detected with the Nordic Optical Telescope 21.1 hours after the burst. At the time of discovery the magnitude of the transient was R = 23.04+-0.08. The transient displayed a power-law decline characterized by a decay slope of alpha = -1.035+-0.097. A deep image obtained 25 days after the burst shows no indication of a contribution from a supernova or a host galaxy at the position of the transient. The closest detected galaxy is a R=24.68+-0.15 galaxy 2.0 arcsec north of the transient. The magnitudes of the optical afterglows of GRB980329, GRB980613 and GRB000630 were all R>=23 less than 24 hours from the burst epoch. We discuss the implications of this for our understanding of GRBs without detected optical transients. We conclude that i) based on the gamma-ray properties of the current sample we cannot conclude that GRBs with no detected OTs belong to another class of GRBs than GRBs with detected OTs and ii) the majority (>75%) of GRBs for which searches for optical afterglow have been unsuccessful are consistent with no detection if they were similar to bursts like GRB000630 at optical wavelengths.

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arXiv2001

Multi-Color Observations of the GRB000926 Afterglow

We present multi-color light-curves of the optical afterglow of GRB 000926. Beginning ~1.5 days after the burst, the light-curves of this GRB steepen measurably. The existence of such achromatic breaks are usually taken to be an important observational signature that the ejecta are not expanding isotropically, but rather have a collimated jet-like geometry. If we interpret the data in this context, we derive an opening angle of 5 degrees, which reduces the energy release compared to an isotropic model by a factor of ~275, to 1.7 x 10^51 erg. To fit the data with a simple jet model requires extinction along the line of sight. The derived A_V is in the range 0.11 - 0.82 mag, depending on the adopted extinction law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expected extinction from our Galaxy, we attribute this to the GRB host. We note that this extinction is typical of a galactic disk, and therefore the event likely took place in the disk of its host.

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arXiv2000

Observation of X-ray lines from a Gamma-Ray Burst (GRB991216): Evidence of Moving Ejecta from the Progenitor

We report on the discovery of two emission features observed in the X-ray spectrum of the afterglow of the gamma-ray burst (GRB) of 16 Dec. 1999 by the Chandra X-Ray Observatory. These features are identified with the Ly$_α$ line and the narrow recombination continuum by hydrogenic ions of iron at a redshift $z=1.00\pm0.02$, providing an unambiguous measurement of the distance of a GRB. Line width and intensity imply that the progenitor of the GRB was a massive star system that ejected, before the GRB event, $\approx 0.01 \Ms$ of iron at a velocity $\approx 0.1 c$, probably by a supernova explosion.

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arXiv2000

The Giant Flare of 1998 August 27 from SGR 1900+14: I. An Interpretive Study of BeppoSAX and Ulysses Observations

The giant flare of 1998 August 27 from SGR 1900+14 was extraordinary in many ways: it was the most intense flux of gamma rays ever detected from a source outside our solar system; it was longer than any previously detected burst from a soft gamma repeater (SGR) in our Galaxy by more than an order of magnitude; and it showed a remarkable four-peaked, periodic pattern in hard X-rays with the same rotation period that was found modulating soft X-rays from the star in quiescence. The event was detected by several gamma-ray experiments in space, including the Ulysses gamma-ray burst detector and the BeppoSAX Gamma Ray Burst Monitor. These instruments operate in different energy ranges, and comparisons of their measurements reveal complex patterns of spectral evolution as the intensity varies. In this paper, we present a joint analysis of the BeppoSAX and Ulysses data and discuss some implications of these results for the SGRs. We also present newly-analyzed Venera/SIGNE and ISEE-3 data on the 1979 March 5 giant flare from an SGR in the Large Magellanic Cloud (SGR 0526-66), and compare them with the August 27 event. Our results are consistent with the hypothesis that giant flares are due to catastrophic magnetic instabilities in highly magnetized neutron stars, or "magnetars".

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arXiv2000

VLT identification of the optical afterglow of the gamma-ray burst GRB 000131 at z=4.50

We report the discovery of the gamma-ray burst GRB 000131 and its optical afterglow. The optical identification was made with the VLT 84 hours after the burst following a BATSE detection and an Inter Planetary Network localization. GRB 000131 was a bright, long-duration GRB, with an apparent precursor signal 62 s prior to trigger. The afterglow was detected in ESO VLT, NTT, and DK1.54m follow-up observations. Broad-band and spectroscopic observations of the spectral energy distribution reveals a sharp break at optical wavelengths which is interpreted as a Ly-alpha absorption edge at 6700 A. This places GRB 000131 at a redshift of 4.500 +/- 0.015. The inferred isotropic energy release in gamma rays alone was approximately 10^54 erg (depending on the assumed cosmology). The rapid power-law decay of the afterglow (index alpha=2.25, similar to bursts with a prior break in the lightcurve), however, indicates collimated outflow, which relaxes the energy requirements by a factor of < 200. The afterglow of GRB 000131 is the first to be identified with an 8-m class telescope.

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arXiv2000

The very red afterglow of GRB 000418 - further evidence for dust extinction in a GRB host galaxy

We report near-infrared and optical follow-up observations of the afterglow of the Gamma-Ray Burst 000418 starting 2.5 days after the occurrence of the burst and extending over nearly seven weeks. GRB 000418 represents the second case for which the afterglow was initially identified by observations in the near-infrared. During the first 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow flattens with respect to a simple power-law decay. Attributing this to an underlying host galaxy we find its magnitude to be R=23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R-K=4 mag. The observations can be explained by an adiabatic, spherical fireball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions.

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arXiv2000

X-ray afterglow detection of the short gamma-ray burst 991014

GRB 991014 is one of the shortest gamma-ray bursts detected so far with the Wide Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. The duration is 9.6 sec in 2-28 keV and 3.2 sec in 40 to 700 keV (as measured between the times when 5 and 95% of the burst photons have been accumulated). We refine the InterPlanetary Network annulus of the burst, present the detection of the X-ray afterglow of GRB 991014 within this refined annulus, and discuss X-ray and gamma-ray observations of the prompt and afterglow emission. Except for the briefness of the prompt event, no other unusual aspects were found in the prompt and afterglow observations as compared to such measurements in previous gamma-ray bursts.

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arXiv2000

A Jet Model for the Afterglow Emission from GRB000301C

We present broad-band radio observations of the afterglow of GRB000301C, spanning from 1.4 to 350 GHz for the period of 3 to 83 days after the burst. This radio data, in addition to measurements at the optical bands, suggest that the afterglow arises from a collimated outflow, i.e. a jet. To test this hypothesis in a self-consistent manner, we employ a global fit and find that a model of a jet, expanding into a constant density medium (ISM+jet), provides the best fit to the data. A model of the burst occurring in a wind-shaped circumburst medium (wind-only model) can be ruled out, and a wind+jet model provides a much poorer fit of the optical/IR data than the ISM+jet model. In addition, we present the first clear indication that the reported fluctuations in the optical/IR are achromatic with similar amplitudes in all bands, and possibly extend into the radio regime. Using the parameters derived from the global fit, in particular a jet break time, t_{jet}=7.5 days, we infer a jet opening angle of θ=0.2, and consequently the estimate of the emitted energy in the GRB itself is reduced by a factor of 50 relative to the isotropic value, giving E=1.1 \times 10^{51} ergs.

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arXiv2000

Interplanetary Network Localization of GRB991208 and the Discovery of its Afterglow

The extremely energetic (~10^-4 erg/cm^2) gamma-ray burst (GRB) of 1999 December 8 was triangulated to a ~14 sq. arcmin. error box ~1.8 d after its arrival at Earth with the 3rd interplanetary network (IPN), consisting of the Ulysses, Near Earth Asteroid Rendezvous (NEAR), and WIND spacecraft. Radio observations with the Very Large Array ~2.7 d after the burst revealed a bright fading counterpart whose position is consistent with that of an optical transient source whose redshift is z=0.707. We present the time history, peak flux, fluence, and refined 1.3 sq. arcmin. error box of this event, and discuss its energetics. This is the first time that a counterpart has been found for a GRB localized only by the IPN.

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arXiv2000

The Ulysses Supplement to the GRANAT/WATCH Catalog of Cosmic Gamma-Ray Bursts

We present 3rd interplanetary network (IPN) localization data for 56 gamma-ray bursts in the GRANAT/ WATCH catalog which occurred between 1990 November and 1994 September. These localizations are obtained by triangulation using various combinations of spacecraft in the IPN, which consisted of Ulysses, BATSE, Pioneer Venus Orbiter (PVO), Mars Observer (MO), WATCH, and PHEBUS. The intersections of the triangulation annuli with the WATCH error circles produce error boxes with areas as small as 16 sq. arcmin., reducing the sizes of the error circles by up to a factor of 800.

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