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Daniel E. Reichart

Publications and source records attributed to Daniel E. Reichart.

At least 91 records · Page 5Linked to original sources
arXiv2002

Evidence for Circumburst Extinction of Gamma-Ray Bursts with Dark Optical Afterglows and Evidence for a Molecular Cloud Origin of Gamma-Ray Bursts

First, we show that the gamma-ray bursts with dark optical afterglows (DOAs) cannot be explained by a failure to image deeply enough quickly enough, and argue that circumburst extinction is the most likely solution. If so, many DOAs will be ``revived'' with rapid follow up and NIR searches in the HETE-2 and Swift eras. Next, we consider the effects of dust sublimation and fragmentation, and show that DOAs occur in clouds of size R > 10L_{49}^{1/2} pc and mass M > 3x10^5L_{49} M_{sun}, where L is the luminosity of the optical flash. Stability considerations show that such clouds cannot be diffuse, but must be molecular. Consequently, we compute the expected column density distribution of bursts that occur in Galactic-like molecular clouds, and show that the column density measurements from X-ray spectra of afterglows, DOAs and otherwise, satisfy this expectation in the source frame.

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arXiv2001

Gamma-Ray Bursts as a Probe of Cosmology

We show that, if the long GRBs are produced by the collapse of massive stars, GRBs and their afterglows may provide a powerful probe of cosmology and the early universe.

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arXiv2001

Evidence for a Molecular Cloud Origin for Gamma-Ray Bursts: Implications for the Nature of Star Formation in the Universe

It appears that the majority of rapidly-, well-localized gamma-ray bursts with undetected, or dark, optical afterglows, or `dark bursts' for short, occur in clouds of size R > 10L_{49}^{1/2} pc and mass M > 3x10^5L_{49} M_{sun}, where L is the isotropic-equivalent peak luminosity of the optical flash. We show that clouds of this size and mass cannot be modeled as a gas that is bound by pressure equilibrium with a warm or hot phase of the interstellar medium (i.e., a diffuse cloud): Such a cloud would be unstable to gravitational collapse, resulting in the collapse and fragmentation of the cloud until a burst of star formation re-establishes pressure equilibrium within the fragments, and the fragments are bound by self-gravity (i.e., a molecular cloud). Consequently, dark bursts probably occur in molecular clouds, in which case dark bursts are probably a byproduct of this burst of star formation if the molecular cloud formed recently, and/or the result of lingering or latter generation star formation if the molecular cloud formed some time ago. We then show that if bursts occur in Galactic-like molecular clouds, the column densities of which might be universal, the number of dark bursts can be comparable to the number of bursts with detected optical afterglows: This is what is observed, which suggests that the bursts with detected optical afterglows might also occur in molecular clouds. We confirm this by modeling and constraining the distribution of column densities, measured from absorption of the X-ray afterglow, of the bursts with detected optical afterglows: We find that this distribution is consistent with the expectation for bursts that occur in molecular clouds, and is not consistent with the expectation for bursts that occur in diffuse clouds. More...

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arXiv2001

Gamma-Ray Bursts as a Probe of Cosmology

We show that, if the long GRBs are produced by the collapse of massive stars, GRBs and their afterglows may provide a powerful probe of cosmology and the early universe.

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arXiv2001

Turning to the Dark Side: Evidence for Circumburst Extinction of Gamma-Ray Bursts with Dark Optical Afterglows

We present evidence that the majority of rapidly-, well-localized gamma-ray bursts with undetected, or dark, optical afterglows, or `dark bursts' for short, are most likely the result of extinction by dust in the circumburst medium. First, we show that the dark bursts cannot be explained by a failure to image deeply enough quickly enough: We fit a variety of brightness distribution models to the optical data, and find that approx. 57(+13,-11)% of all bursts, and approx. 82(+22,-17)% of dark bursts, have afterglows that are fainter than R = 24 mag 18 hours after the burst. Secondly, we show that dark bursts tend to be X-ray and radio faint. Thirdly, we show that these correlations, and more specifically the optical vs. X-ray and optical vs. radio distributions, can be explained within the framework of the relativistic fireball model if the dark bursts are extinguished by circumburst dust, and the density of the circumburst medium spans many orders of magnitude, from densities that are typical of the Galactic disk to densities that are typical of dense clouds. Finally, we show that Galactic extinction, host galaxy extinction unrelated to the circumburst medium, and the following high redshift effects - Lyman limit absorption in the source frame, absorption by the Ly-alpha forest, absorption by excited molecular hydrogen in the circumburst medium, and source-frame extinction by the FUV component of the extinction curve - might contribute to the number of dark bursts, but only in small numbers.

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arXiv2001

Dust Sublimation and Fragmentation in the Circumburst Medium: Evidence for a Large, Massive Cloud Origin for Gamma-Ray Bursts with Dark Optical Afterglows

The distances to which the optical flash destroys dust via sublimation, and the burst and afterglow change the size distribution of the dust via fragmentation, are functions of grain size. Furthermore, the sublimation distance is a decreasing function of grain size, while the fragmentation distance is a decreasing function of grain size for large grains and an increasing function of grain size for small grains. We investigate how these very different, but somewhat complementary, processes change the optical depth of the circumburst medium. To this end, we adopt a canonical distribution of graphite and silicate grain sizes, and a simple fragmentation model, and we compute the post-burst/optical flash/afterglow optical depth of a circumburst cloud of constant density n and size R as a function of burst and afterglow isotropic-equivalent X-ray energy E and spectral index alpha, and optical flash isotropic-equivalent peak luminosity L: This improves upon previous analyses that consider circumburst dust of a uniform grain size. We find that circumburst clouds do not significantly extinguish (tau < 0.3) the optical afterglow if R < 10L_{49}^{1/2} pc, fairly independent of n, E, and alpha, or if N_H < 5x10^{20} cm^{-2}. On the other hand, we find that circumburst clouds do significantly extinguish (tau > 3) the optical afterglow if R > 10L_{49}^{1/2} pc and N_H > 5x10^{21} cm^{-2}, creating a so-called `dark burst'. The majority of bursts are dark, and as circumburst extinction is probably the primary cause of this, this implies that most dark bursts occur in clouds of this size and mass M > 3x10^5L_{49} M_{sun}. Clouds of this size and mass are typical of giant molecular clouds, and are active regions of star formation.

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arXiv2001

Determining the Gamma-Ray Burst Rate as a Function of Redshift

We exploit the 14 gamma-ray bursts (GRBs) with known redshifts z and the 7 GRBs for which there are constraints on z to determine the GRB rate R_{GRB}(z), using a method based on Bayesian inference. We find that, despite the qualitative differences between the observed GRB rate and estimates of the SFR in the universe, current data are consistent with R_{GRB}(z) being proportional to the SFR.

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arXiv2001

Sloan Digital Sky Survey Multicolor Observations of GRB010222

The discovery of an optical counterpart to GRB010222 (detected by BeppoSAX; Piro 2001) was announced 4.4 hrs after the burst by Henden (2001a). The Sloan Digital Sky Survey's 0.5m photometric telescope (PT) and 2.5m survey telescope were used to observe the afterglow of GRB010222 starting 4.8 hours after the GRB. The 0.5m PT observed the afterglow in five, 300 sec g' band exposures over the course of half an hour, measuring a temporal decay rate in this short period of F_nu \propto t^{-1.0+/-0.5}. The 2.5m camera imaged the counterpart nearly simultaneously in five filters (u' g' r' i' z'), with r' = 18.74+/-0.02 at 12:10 UT. These multicolor observations, corrected for reddening and the afterglow's temporal decay, are well fit by the power-law F_nu \propto nu^{-0.90+/-0.03} with the exception of the u' band UV flux which is 20% below this slope. We examine possible interpretations of this spectral shape, including source extinction in a star forming region.

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arXiv2001

Light Curves and Spectra of Dust Echoes From Gamma-Ray Bursts and their Afterglows: Continued Evidence that GRB 970228 is Associated with a Supernova

The relative brightening and reddening of the optical afterglows of GRB 970228 and GRB 980326 around 20 - 30 days after these bursts have been attributed to supernovae, which are red (blueward of their spectral peak), and which peak in time after about 20(1+z) days. However, this direct evidence for a GRB/SN connection has recently been challenged. It has been suggested that the late afterglows of these bursts can be explained by dust echoes, of which we consider two cases: (1) the scattering of light from the afterglow (the forward shock), the optical flash (the reverse shock), and/or optical light from the burst itself by a shell of dust at a radius R from the progenitor, and (2) the absorption and thermal re-emission of this light by this shell of dust. In this paper, we model and compute dust echo light curves and spectra for both cases. Although the late-time afterglow of GRB 980326 was not sufficiently well sampled to rule out a dust echo description, we find that the late-time afterglow of GRB 970228 cannot be explained by a dust echo.

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arXiv2001

Dust Extinction Curves and Ly-alpha Forest Flux Deficits for Use in Modeling GRB Afterglows and All Other Extragalactic Point Sources

Since gamma-ray burst afterglows were first detected in 1997, the relativistic fireball model has emerged as the leading theoretical explanation of the afterglows. In this paper, we present a very general, Bayesian inference formalism with which this, or any other, afterglow model can be tested, and with which the parameter values of acceptable models can be constrained, given the available photometry. However, before model comparison or parameter estimation can be attempted, one must also consider the physical processes that affect the afterglow as it propagates along the line of sight from the burst source to the observer. Namely, how does extinction by dust, both in the host galaxy and in our galaxy, and absorption by the Ly-alpha forest and by H I in the host galaxy, change the intrinsic spectrum of the afterglow? Consequently, we also present in this paper a very general, eight-parameter dust extinction curve model, and a two-parameter model of the Ly-alpha forest flux deficit versus redshift distribution. Using fitted extinction curves from Milky Way and Magellanic Cloud lines of sight, and measurements of Ly-alpha forest flux deficits from quasar absorption line systems, we construct a Bayesian prior probability distribution that weights this additional, but necessary, parameter space such that the volume of the solution space is reduced significantly, a priori. Finally, we discuss the broad applicability of these results to the modeling of light from all other extragalactic point sources, such as Type Ia supernovae.

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arXiv2001

A Possible Cepheid-Like Luminosity Estimator for the Long Gamma-Ray Bursts

We present a possible Cepheid-like luminosity estimator for the long gamma-ray bursts based on the variability of their light curves. To construct the luminosity estimator, we use CGRO/BATSE data for 13 bursts, Wind/KONUS data for 5 bursts, Ulysses/GRB data for 1 burst, and NEAR/XGRS data for 1 burst. Spectroscopic redshifts, peak fluxes, and high resolution light curves are available for 11 of these bursts; partial information is available for the remaining 9 bursts. We find that the isotropic-equivalent luminosities L of these bursts positively correlate with a rigorously-constructed measure V of the variability of their light curves. We fit a model to these data that accommodates both intrinsic scatter (statistical variance) and extrinsic scatter (sample variance). If one excludes GRB 980425 from the fit on the grounds that its association with SN 1998bw at a redshift of z = 0.0085 is not secure, the luminosity estimator spans approx. 2.5 orders of magnitude in L, and the slope of the correlation between L and V is positive with a probability of 1 - 1.4 x 10^-4 (3.8 sigma). Although GRB 980425 is excluded from this fit, its L and V values are consistent with the fitted model, which suggests that GRB 980425 may well be associated with SN 1998bw, and that GRB 980425 and the cosmological bursts may share a common physical origin. If one includes GRB 980425 in the fit, the luminosity estimator spans approx. 6.3 orders of magnitude in L, and the slope of the correlation is positive with a probability of 1 - 9.3 x 10^-7 (4.9 sigma). Independently of whether or not GRB 980425 should be included in the fit, its light curve is unique in that it is much less variable than the other approx. 17 light curves in our sample for which the signal-to-noise is reasonably good.

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arXiv2000

The Detectability of Gamma-Ray Bursts and Their Afterglows at Very High Redshifts

There is increasingly strong evidence that gamma-ray bursts (GRBs) are associated with star-forming galaxies, and occur near or in the star-forming regions of these galaxies. These associations provide indirect evidence that at least the long GRBs detected by BeppoSAX are a result of the collapse of massive stars. The recent evidence that the light curves and the spectra of the afterglows of GRB 970228 and GRB 980326 appear to contain a supernova component, in addition to a relativistic shock wave component, provide more direct clues that this is the case. Here we establish that GRBs and their afterglows are both detectable out to very high redshifts (z > 5).

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arXiv2000

The Fading of Supernova Remnant Cassiopeia A from 38 MHz to 16.5 GHz from 1949 to 1999 with New Observations at 1405 MHz

We report 1405 MHz measurements of the flux density of the approximately 320 year old supernova remnant Cassiopeia A, relative to the flux density of Cygnus A, made between 1995 and 1999. When compared to measurements made between 1957 and 1976, we find that the rate at which Cassiopeia A has been fading at this and nearby frequencies has changed from approximately 0.9 % yr^-1 in the 1960s to approximately 0.6 - 0.7 % yr^-1 now. Furthermore, we have collected from the literature measurements of this fading rate at lower (38 - 300 MHz) and higher (7.8 - 16.5 GHz) frequencies. We show that the fading rate has dropped by a factor of approximately 3 over the past 50 years at the lower frequencies, while remaining relatively constant at the higher frequencies, which is in agreement with the findings of others. Our findings at 1405 MHz, in conjunction with a measurement of the fading rate at the nearby frequency of 927 MHz by Vinyajkin (1997), show an intermediate behavior at intermediate frequencies. We also find that Cassiopeia A, as of approximately 1990, was fading at about the same rate, approximately 0.6 - 0.7 % yr^-1, at all of these frequencies. Future measurements are required to determine whether the fading rate will continue to decrease at the lower frequencies, or whether Cassiopeia A will now fade at a relatively constant rate at all of these frequencies.

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arXiv1999

The GRB/SN Connection: An Improved Spectral Flux Distribution for the Supernova Candidate Associated with GRB 970228

We better determine the spectral flux distribution of the supernova candidate associated with GRB 970228 by modeling the spectral flux distribution of the host galaxy of this burst, fitting this model to measurements of the host galaxy, and using the fitted model to better subtract out the contribution of the host galaxy to measurements of the afterglow of this burst.

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arXiv1999

The GRB/SN Connection: An Improved Spectral Flux Distribution for the SN-Like Component to the Afterglow of GRB 970228, the Non-Detection of a SN-Like Component to the Afterglow of GRB 990510, and GRBs as Beacons to Locate SNe at Redshifts z = 4 - 5

We better determine the spectral flux distribution of the supernova candidate associated with GRB 970228 by modeling the spectral flux distribution of the host galaxy of this burst, fitting this model to measurements of the host galaxy, and using the fitted model to better subtract out the contribution of the host galaxy to measurements of the afterglow of this burst. Furthermore, we discuss why the non-detection of a SN1998bw-like component to the afterglow of GRB 990510 does not necessarily imply that a SN is not associated with this burst. Finally, we discuss how bursts can be used as beacons to locate SNe out to redshifts of z = 4 - 5.

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