arXiv subjects

J. S. Bloom

Publications and source records attributed to J. S. Bloom.

At least 145 records · Page 8Linked to original sources

arXiv2002

The redshift determination of GRB 990506 and GRB 000418 with the Echellete Spectrograph Imager on Keck

Using the Echellete Spectrograph Imager (ESI) on the Keck II 10-m telescope we have measured the redshifts of the host galaxies of gamma-ray bursts GRB 990506 and GRB 000418, z=1.30658 +/- 0.00004 and 1.1181 +/- 0.0001, respectively. Thanks to the excellent spectral resolution of ESI we resolved the [O II] 3727 doublet in both cases. The measured redshift of GRB 990506 is the highest known for a dark burst GRB, though entirely consistent with the notion that dark and non-dark bursts have a common progenitor origin. The relative strengths of the [O II], He I, [Ne III], and H gamma emission lines suggest that the host of GRB 000418 is a starburst galaxy, rather than a LINER or Seyfert 2. Since the host of GRB 000418 has been detected at sub-millimeter wavelengths these spectroscopic observations suggest that the sub-millimeter emission is due to star-formation (as opposed to AGN) activity. The [O II]-derived unobscured star-formation rates are 13 and 55 M_solar/yr for the hosts of GRB 990506 and GRB 000418, respectively. In contrast, the star-formation rate of the host of GRB 000418 derived from sub-millimeter observations is twenty times larger.

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arXiv2002

The Faint Optical Afterglow and Host Galaxy of GRB 020124: Implications for the Nature of Dark Gamma-Ray Bursts

We present ground-based optical observations of GRB 020124 starting 1.6 hours after the burst, as well as subsequent Very Large Array (VLA) and Hubble Space Telescope (HST) observations. The optical afterglow of GRB 020124 is one of the faintest afterglows detected to date, and it exhibits a relatively rapid decay, $F_ν\propto t^{-1.60\pm 0.04}$, followed by further steepening. In addition, a weak radio source was found coincident with the optical afterglow. The HST observations reveal that a positionally coincident host galaxy must be the faintest host to date, R>29.5 mag. The afterglow observations can be explained by several models requiring little or no extinction within the host galaxy, A_V~0-0.9 mag. These observations have significant implications for the statistics of the so-called dark bursts (bursts for which no optical afterglow is detected), which are usually attributed to dust extinction within the host galaxy. The faintness and relatively rapid decay of the afterglow of GRB 020124, combined with the low inferred extinction indicate that some dark bursts are intrinsically dim and not dust obscured. Thus, the diversity in the underlying properties of optical afterglows must be observationally determined before substantive inferences can be drawn from the statistics of dark bursts.

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arXiv2002

GRB 010921: Strong Limits on an Underlying Supernova from HST

GRB 010921 was the first HETE-2 GRB to be localized via its afterglow emission. The low-redshift of the host galaxy, z=0.451, prompted us to undertake intensive multi-color observations with the Hubble Space Telescope with the goal of searching for an underlying supernova component. We do not detect any coincident supernova to a limit 1.34 mag fainter than SN 1998bw at 99.7% confidence, making this one of the most sensitive searches for an underlying SN. Analysis of the afterglow data allow us to infer that the GRB was situated behind a net extinction (Milky Way and the host galaxy) of A_V ~ 1.8 mag in the observer frame. Thus, had it not been for such heavy extinction our data would have allowed us to probe for an underlying SN with brightness approaching those of more typical Type Ib/c supernovae.

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arXiv2002

Time-Dependent Optical Spectroscopy of GRB 010222: Clues to the GRB Environment

We present sequential optical spectra of the afterglow of GRB 010222 obtained one day apart using the Low Resolution Imaging Spectrometer (LRIS) and the Echellette Spectrograph and Imager (ESI) on the Keck telescopes. Three low-ionization absorption systems are spectroscopically identified at z1=1.47688, z2=1.15628, and z3=0.92747. The higher resolution ESI spectrum reveals two distinct components in the highest redshift system at z1a=1.47590 and z1b=1.47688. We interpret the z1b=1.47688 system as an absorption feature of the disk of the host galaxy of GRB 010222. The best fitted power-law optical continuum and [Zn/Cr] ratio imply low dust content or a local gray dust component near the burst site. In addition, we do not detect strong signatures of vibrationally excited states of H_2. If the GRB took place in a superbubble or young stellar cluster, there are no outstanding signatures of an ionized absorber, either. Analysis of the spectral time dependence at low resolution shows no significant evidence for absorption-line variability. This lack of variability is confronted with time-dependent photoionization simulations designed to apply the observed flux from GRB 010222 to a variety of assumed atomic gas densities and cloud radii. The absence of time dependence in the absorption lines implies that high-density environments are disfavored. In particular, if the GRB environment was dust free, its density was unlikely to exceed nH=10^2 cm^-3. If depletion of metals onto dust is similar to Galactic values or less than solar abundances are present, then nH > 2 x 10^4 cm^-3 is probably ruled out in the immediate vicinity of the burst.

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arXiv2002

The Broadband Afterglow of GRB980329

We present radio observations of the afterglow of the bright gamma-ray burst GRB980329 made between one month and several years after the burst, a re-analysis of previously published submillimeter data, and late-time optical and near-infrared (NIR) observations of the host galaxy. From the absence of a spectral break in the optical/NIR colors of the host galaxy, we exclude the earlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our data with the numerous multi-wavelength observations of the early afterglow, fit a comprehensive afterglow model to the entire broadband dataset, and derive fundamental physical parameters of the blast-wave and its host environment. Models for which the ejecta expand isotropically require both a high circumburst density and extreme radiative losses from the shock. No low density model (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51} erg, with the ejecta narrowly collimated to an opening angle of a few degrees, driven into a surrounding medium with density ~ 20 cm^{-3}, provides a satisfactory fit to the lightcurves over a range of redshifts.

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arXiv2002

Detection of a supernova signature associated with GRB 011121

Using observations from an extensive monitoring campaign with the Hubble Space Telescope we present the detection of an intermediate-time flux excess which is redder in color relative to the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red ``bump,'' which exhibits a spectral roll-over at ~7200 Angstrom, is well described by a redshifted Type Ic supernova that occurred approximately at the same time as the gamma-ray burst event. The inferred luminosity is about half that of the bright supernova 1998bw. These results serve as compelling evidence for a massive star origin of long-duration gamma-ray bursts. Models that posit a supernova explosion weeks to months preceding the gamma-ray burst event are excluded by these observations. Finally, we discuss the relationship between spherical core-collapse supernovae and gamma-ray bursts.

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arXiv2002

GRB 011121: A Massive Star Progenitor

Of the cosmological gamma-ray bursts, GRB 011121 has the lowest redshift, z=0.36. More importantly, the multi-color excess in the afterglow detected in the Hubble Space Telescope (HST) light curves is compelling observational evidence for an underlying supernova. Here we present near-infrared and radio observations of the afterglow. We undertake a comprehensive modeling of these observations and those reported in the literature and find good evidence favoring a wind-fed circumburst medium. In detail, we infer the progenitor had a mass loss rate of Mdot ~ 10^-7 / v_w3 Mo/yr where v_w3 is the speed of the wind from the progenitor in units of 10^3 km/s. This mass loss rate is similar to that inferred for the progenitor of SN 1998bw which has been associated with GRB 980425. Our data, taken in conjunction with the HST results of Bloom et al. (2002), provide a consistent picture: the long duration GRB 011121 had a massive star progenitor which exploded as a supernova at about the same time as the GRB event.

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arXiv2002

GRB 010921: Discovery of the First HETE Afterglow

We report the discovery of the optical and radio afterglow of GRB 010921, the first gamma-ray burst afterglow to be found from a localization by the High Energy Transient Explorer (HETE) satellite. We present optical spectroscopy of the host galaxy which we find to be a dusty and apparently normal star-forming galaxy at z = 0.451. The unusually steep optical spectral slope of the afterglow can be explained by heavy extinction, A_V > 0.5 mag, along the line of sight to the GRB. Dust with similar A_V for the the host galaxy as a whole appears to be required by the measurement of a Balmer decrement in the spectrum of the host galaxy. Thanks to the low redshift, continued observations of the afterglow will enable the strongest constraints, to date, on the existence of a possible underlying supernova.

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arXiv2001

The Observed Offset Distribution of Gamma-Ray Bursts from Their Host Galaxies: A Robust Clue to the Nature of the Progenitors

We present a comprehensive study to measure the locations of gamma-ray bursts (GRBs) relative to their host galaxies. In total, we find the offsets of 20 long-duration GRBs from their apparent host galaxy centers utilizing ground-based images from Palomar and Keck and space-based images from the Hubble Space Telescope (HST). The median projected angular (physical) offset is 0.17 arcsec (1.3 kpc). The median offset normalized by the individual host half-light radii is 0.98 suggesting a strong connection of GRB locations with the UV light of their hosts. This provides strong observational evidence for the connection of GRBs to star formation. We compare the observed offset distribution with an exponential disk, a model for the location of collapsars. The statistical comparison shows good agreement, with the Kolmogorov-Smirnov probability that the observed offsets derive from the model distribution of P_KS = 0.45. We also compare the observed GRB offsets with the expected offset distribution of delayed merging remnant progenitors (black hole--neutron star and neutron star--neutron star binaries). We find that delayed merging remnant progenitors, insofar as the predicted offset distributions from population synthesis studies are representative, can be ruled out at the 2 x 10^{-3} level. This is arguably the strongest observational constraint yet against delayed merging remnants as the progenitors of long-duration GRBs. In the course of this study, we have also discovered the putative host galaxies of GRB 990510 and GRB 990308 in archival HST data. (abstract abridged)

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arXiv2001

Keck Spectroscopy and HST Imaging of GRB 000926: Probing a Host Galaxy at z = 2. 038

We present early-time Keck spectroscopic observations and late-time Hubble Space Telescope (HST) imaging of GRB 000926. The HST images show a small offset between the optical transient and the compact host galaxy. Combined with the large equivalent widths measured for metallic absorption lines by the Keck Echellette Spectrograph and Imager (ESI) and the Low-Resolution Imaging Spectrometer (LRIS), this indicates that the GRB exploded near the center of its host. The ESI spectroscopy reveals two absorption systems centered at z = 2.0379 \pm 0.0008 with a velocity separation of 168 km s^{-1}, which we interpret as being due to individual clouds in the host galaxy. The ratios of chromium to zinc equivalent widths indicates the host is depleted in dust relative to local values to a similar degree as damped Lyman alpha systems at the same redshift. Further, the two clouds appear to have similar relative metal abundance and dust to gas ratio. If one cloud is associated with the GRB site, this implies the explosion did not significantly alter the surrounding environment.

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arXiv2001

The Unusually Long Duration Gamma-ray Burst GRB 000911

Of all the well localized gamma-ray bursts, GRB 000911 has the longest duration (T_90 ~ 500 s), and ranks in the top 1% of BATSE bursts for fluence. Here, we report the discovery of the afterglow of this unique burst. In order to simultaneously fit our radio and optical observations, we are required to invoke a model involving an hard electron distribution, p ~ 1.5 and a jet-break time less than 1.5 day. A spectrum of the host galaxy taken 111 days after the burst reveals a single emission line, interpreted as [OII] at a redshift z = 1.0585, and a continuum break which we interpret as the Balmer limit at this redshift. Despite the long T_90, the afterglow of GRB 000911 is not unusual in any other way when compared to the set of afterglows studied to date. We conclude that the duration of the GRB plays little part in determining the physics of the afterglow.

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arXiv2001

GRB 010222: A Burst Within a Starburst

We present millimeter and submillimeter wavelength observations and near-infrared K-band imaging toward the bright gamma-ray burst GRB 010222. Over seven epochs the flux density of the source was constant with an average flux density 3.74+/-0.53 mJy at 350 GHz and 1.05+/-0.22 mJy at 250 GHz, giving a spectral index alpha=3.78+/-0.25. We rule out the possibility that this emission originated from the burst or its afterglow and we conclude that it is due to a dusty, high redshift starburst galaxy (SMM J14522+4301). We argue that the host galaxy of GRB 010222 is the most plausible counterpart of SMM J14522+4301, based in part on the centimeter detection of the host at the expected level. The optical/NIR properties of the host galaxy of GRB 010222 suggest that it is a blue, sub-L_*, similar to other GRB host galaxies. This contrasts with the enormous far-infrared luminosity of this galaxy based on our submillimeter detection (L_Bol~4X10^{12} L_odot). We suggest that this GRB host galaxy has a very high star formation rate, SFR 600 M_\odot/yr, most of which is unseen at optical wavelengths.

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arXiv2001

The GRB Host Galaxies and Redshifts

Observations of GRB host galaxies and their environments in general can provide valuable clues about the nature of progenitors. Bursts are associated with faint, ~ 25 mag, galaxies at cosmological redshifts, ~ 1. The host galaxies span a range of luminosities and morphologies, and appear to be broadly typical for the normal, evolving, actively star-forming galaxy populations at comparable redshifts and magnitudes, but may have somewhat elevated SFR per unit luminosity. There are also spectroscopic hints of massive star formation, from the ratios of [Ne III] and [O II] lines. The observed, unobscured star formation rates are typically a few M_sun/yr, but a considerable fraction of the total star formation in the hosts may be obscured by dust. A census of detected optical afterglows provides a powerful new handle on the obscured fraction of star formation in the universe; the current results suggest that at most a half of the massive star formation was hidden by dust.

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arXiv2001

The Afterglow and the Host Galaxy of the Dark Burst GRB 970828

GRB 970828 was the first well-localized gamma-ray burst (GRB) X-ray afterglow for which no optical afterglow was found despite a prompt, deep search down to R_lim ~ 24.5 mag. We report the discovery of a short-lived radio flare within the X-ray localization error circle of this burst. Such radio flares are seen in about 25% of GRB afterglows and their origin is not well understood. The precise radio position enabled us to identify the likely host galaxy of this burst, and to measure its redshift, z = 0.9578. The host appears to be an interacting/merging system. Under the assumption that the X-ray afterglow is mainly due to synchrotron mechanism we infer the optical afterglow flux. The observed upper limits to the optical flux are easily explained by invoking an intervening dusty cloud within the host galaxy. These observations support the idea that GRBs with no detectable optical afterglows or "dark GRBs" can be due to dust extinction within the host galaxies. The census of dark GRBs can then be used to constrain the fraction of the obscured star formation in the universe. We argue that the existing data already indicate that the obscured star-formation rate is no more than 1/2 of that seen at UV and optical wavelengths.

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arXiv2001

The Cosmic Gamma-Ray Bursts

Cosmic gamma-ray bursts are one of the great frontiers of astrophysics today. They are a playground of relativists and observers alike. They may teach us about the death of stars and the birth of black holes, the physics in extreme conditions, and help us probe star formation in the distant and obscured universe. In this review we summarise some of the remarkable progress in this field over the past few years. While the nature of the GRB progenitors is still unsettled, it now appears likely that at least some bursts originate in explosions of very massive stars, or at least occur in or near the regions of massive star formation. The physics of the burst afterglows is reasonably well understood, and has been tested and confirmed very well by the observations. Bursts are found to be beamed, but with a broad range of jet opening angles; the mean gamma-ray energies after the beaming corrections are ~ 10^51 erg. Bursts are associated with faint ~ 25 mag) galaxies at cosmological redshifts, with ~ 1. The host galaxies span a range of luminosities and morphologies, but appear to be broadly typical for the normal, actively star-forming galaxy populations at comparable redshifts and magnitudes. Some of the challenges for the future include: the nature of the short bursts and possibly other types of bursts and transients; use of GRBs to probe the obscured star formation in the universe, and possibly as probes of the very early universe; and their detection as sources of high-energy particles and gravitational waves.

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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

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

Beaming in Gamma-Ray Bursts: Evidence for a Standard Energy Reservoir

Gamma-ray bursts (GRBs) are the most brilliant objects in the Universe but efforts to estimate the total energy released in the explosion -- a crucial physical quantity -- have been stymied by their unknown geometry: spheres or cones. We report on a comprehensive analysis of GRB afterglows and derive their conical opening angles. We find that the gamma-ray energy release, corrected for geometry, is narrowly clustered around 5x10**50 erg. We draw three conclusions. First, the central engines of GRBs release energies that are comparable to ordinary supernovae, suggesting a connection. Second, the wide variation in fluence and luminosity of GRBs is due entirely to a distribution of opening angles. Third, only a small fraction of GRBs are visible to a given observer and the true GRB rate is at least a factor of 500 times larger than the observed rate.

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