arXiv subjects

S. A. Yost

Publications and source records attributed to S. A. Yost.

102 records · Page 6Linked to original sources

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

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

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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arXiv2000

A Search for Near-Infrared Emission From the Halo of NGC 5907 at Radii of 10 kpc to 30 kpc

We present a search for near-infrared (3.5-5 micron) emission from baryonic dark matter in the form of low-mass stars and/or brown dwarfs in the halo of the nearby edge-on spiral galaxy NGC 5907. The observations were made using a 256 by 256 InSb array with a pixel scale of 17" at the focus of a liquid-helium-cooled telescope carried above the Earth's atmosphere by a sounding rocket. In contrast to previous experiments which have detected a halo around NGC 5907 in the V, R, I, J and K bands at galactic radii 6kpc < r < 10kpc, our search finds no evidence for emission from a halo at 10kpc < r < 30kpc. Assuming a halo mass density scaling as r^(-2), which is consistent with the flat rotation curves that are observed out to radii of 32kpc, the lower limit of the mass-to-light ratio at 3.5-5 microns for the halo of NGC 5907 is 250 (2 sigma) in solar units. This is comparable to the lower limit we have found previously for NGC 4565 (Uemizu et al. 1998). Based on recent models, our non-detection implies that hydrogen- burning stars contribute < 15% of the mass of the dark halo of NGC 5907. Our results are consistent with the previous detection of extended emission at r < 10kpc if the latter is caused by a stellar population that has been ejected from the disk because of tidal interactions. We conclude that the dark halo of NGC 5907, which is evident from rotation curves that extend far beyond 10kpc, is not comprised of hydrogen burning stars.

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arXiv2000

The afterglows of gamma-ray bursts

Gamma-ray burst astronomy has undergone a revolution in the last three years, spurred by the discovery of fading long-wavelength counterparts. We now know that at least the long duration GRBs lie at cosmological distances with estimated electromagnetic energy release of 10**51 -- 10**53 erg, making these the brightest explosions in the Universe. In this article we review the current observational state, beginning with the statistics of X-ray, optical, and radio afterglow detections. We then discuss the insights these observations have given to the progenitor population, the energetics of the GRB events, and the physics of the afterglow emission. We focus particular attention on the evidence linking GRBs to the explosion of massive stars. Throughout, we identify remaining puzzles and uncertainties, and emphasize promising observational tools for addressing them. The imminent launch of HETE-2 and the increasingly sophisticated and coordinated ground-based and space-based observations have primed this field for fantastic growth.

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arXiv1999

Precision Calculation of Bhabha Scattering at LEP

For the small-angle Bhabha-scattering process, we consider the error budget for the calculation of the LEP/SLC luminosity in the Monte Carlo event generator BHLUMI 4.04, from the standpoint of new calculations of exact results for the respective O(alpha**2) photonic corrections in the context of the Yennie-Frautchi-Suura exponentiation. We find that an over-all precision tag for the currently available program BHLUMI 4.04 can be reduced from 0.11% to 0.061% at LEP1 and from 0.25% to 0.122% at LEP2. For the large-angle Bhabha process, we present the Monte Carlo program BHWIDE and compare its predictions with predictions of other Monte Carlo programs as well as semi-analytical calculations.

hep-ph↗

arXiv1999

New Results on the Theoretical Precision of the LEP/SLC Luminosity

We consider the error budget for the calculation of the LEP/SLC luminosity in the Monte Carlo event generator BHLUMI4.04 from the standpoint of new calculations of the exact result for the O(alpha) correction to the process e+ e- -> e+ e- + gamma in the low angle luminosity regime at SLC/LEP energies, for the double bremsstrahlung effect e+ e- -> e+ e- + gamma gamma in this regime, and for the size of the two-loop virtual correction to e+ e- -> e+ e- in this regime in context of Yennie-Frautschi-Suura exponentiation. We find that the error on the O(alpha**2) photonic correction can be reduced from the currently published value 0.1% to the value 0.027%. This leads to an over-all precision tag for the currently available program BHLUMI4.04 of 0.061%. This reduction of the precision of the calculation is important for the final LEP1 EW precision Z physics tests of the Standard Model.

hep-ph↗

arXiv1996

Bhabha Process at LEP - Theoretical Calculations

In this contribution we give a short overview of the situation in the precision calculation of the Bhabha process and we present a preliminary numerical result on the second-order sub-leading correction to the small angle Bhabha process.

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arXiv1996

Exact O(alpha) Corrections to the Single Hard Bremsstrahlung Process in Low Angle Bhabha Scattering in the SLC/LEP Energy Regime

We present the exact O(alpha) correction to the process e+ e- -> e+ e- + gamma in the low angle luminosity regime at SLC/LEP energies. We give explicit formulas for the completely differential cross section. As an important application, we illustrate the size of the respective corrections of O(alpha^2) to the SLC/LEP luminosity cross section. We show explicitly that our results have the correct infrared limit, as a cross-check. Some comments are made about the implementation of our results in the framework of a Monte Carlo event generator. This latter implementation will appear elsewhere.

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arXiv1994

Exact Results on e+ e- --> e+ e- + 2 Photons at SLC/LEP Energies

We use the spinor methods of the CALKUL collaboration, as realized by Xu, Zhang and Chang, to calculate the differential cross section for e+ e- --> e+ e- + 2 photons for c.m.s. energies in the SLC/LEP regime. An explicit complete formula for the respective cross section is obtained. The leading log approximation is used to check the formula. Applications of the formula to high precision luminosity calculations at SLC/LEP are discussed.

hep-ph↗

arXiv1994

Theoretical Expectations For High Mass Photon Pairs in L+ L- Gamma Gamma Events at LEP/SLC

Recently, the L3 collaboration has reported the observation of four events in the reactions e+ e- --> L+ L- + (2 Photons), L = e, mu, tau, with the invariant photon pair mass near 60 GeV in a data sample collected in the L3 detector corresponding to 950,000 produced Z0's. More recently, more data from the other LEP collaborations have become available. In this paper, we use the Monte Carlo genrator YFS3 and our recent exact results on e+ e- --> L+ L- + (2 Photons) to assess the QED expectations for such L3-type high mass photon pair events in e+ e- --> L+ L- + (n Photons) near the Z0 resonance.

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