arXiv subjects

S. Barthelmy

Publications and source records attributed to S. Barthelmy.

65 records · Page 4Linked to original sources

arXiv2005

The Discovery of the Optical and Near-IR Afterglows of the First Swift Gamma-Ray Bursts

We present optical and near-infrared searches for afterglow emission from the first four Swift bursts with accurate positions from the X-ray Telescope (XRT). Using telescopes at Las Campanas, Keck, and Palomar observatories we rapidly identified and followed up afterglows for three of the four bursts. The burst positions were also observed with the Very Large Array, but no radio afterglow emission was detected. The optical/NIR afterglows are fainter than about 75% of all afterglows detected to date, with GRB 050126 being the faintest, and were identified thanks to accurate and rapid positions from the XRT and rapid response with >1-m telescopes. This suggests that the fraction of dust-obscured bursts is small, <10% when combined with afterglows localized by the HETE-2 Soft X-ray Camera. The X-ray fluxes are typical of the known population, with the exception of GRB 050126 which has the faintest X-ray afterglow to date (normalized to t=10 hr), and was detected thanks to a response time of only 130 s after the burst. Finally, we find that all three optical/NIR afterglows are located <2 arcsec away from the nominal XRT positions, suggesting that the XRT is capable of delivering highly accurate positions, which will revolutionize afterglow studies.

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arXiv2004

The Trigger Algorithm for the Burst Alert Telescope on Swift

The Swift Burst Alert Telescope (BAT) is a huge (5200 cm2) coded aperture imager that will detect gamma-ray bursts in real time and provide a location that the Swift satellite will use to slew the optical and x-ray telescopes. The huge size of BAT is a challenge for the on-board triggering: a change as small as 1% is equivalent to a 1 sigma statistical variation in 1 second. There will be three types of triggers, two based on rates and one based on images. The first type of trigger is for short time scales (4 msec to 64 msec). These will be traditional triggers (single background) and we check about 25,000 combinations of time-energy-focal plane subregions per second. The second type of trigger will be similar to what is used on HETE: fits to multiple background regions to remove trends for time scales between 64 msec and 64 seconds. About 500 triggers will be checked per second. For these rate triggers, false triggers and variable non-GRB sources will be rejected by requiring a new source to be present in an image. The third type of trigger works on longer time scales (minutes), and will be based on routine images that are made of the field of view.

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arXiv2004

The BAT-Swift Science Software

The BAT instrument tells the Swift satellite where to point to make immediate follow-up observations of GRBs. The science software on board must efficiently process gamma-ray events coming in at up to 34 kHz, identify rate increases that could be due to GRBs while disregarding those from known sources, and produce images to accurately and rapidly locate new Gamma-ray sources.

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arXiv2004

Setting the Triggering Threshold on Swift

Swift has two main types of ``rate'' triggers: short and long. Short trigger time scales range from 4ms to 64ms, while long triggers are 64ms to ~ 16 seconds. While both short and long trigger have criteria with one background sample (traditional ``one-sided'' triggers), the long triggers can also have criteria with two background samples (``bracketed'' triggers) which remove trends in the background. Both long and short triggers can select energy ranges of 15-25, 15-50, 25-100 and 50-350 KeV. There are more than 180 short triggering criteria and approximately 500 long triggering criteria used to detect gamma ray bursts. To fully utilize these criteria, the thresholds must be set correctly. The optimum thresholds are determined by a tradeoff between avoiding false triggers and capturing as many bursts as possible. We use realistic simulated orbital variations, which are the prime cause of false triggers.

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arXiv2002

The Infocus Hard X-ray Telescope: Pixellated CZT Detector/Shield Performance and Flight Results

The CZT detector on the Infocus hard X-ray telescope is a pixellated solid-state device capable of imaging spectroscopy by measuring the position and energy of each incoming photon. The detector sits at the focal point of an 8m focal length multilayered grazing incidence X-ray mirror which has significant effective area between 20--40 keV. The detector has an energy resolution of 4.0keV at 32keV, and the Infocus telescope has an angular resolution of 2.2 arcminute and a field of view of about 10 arcminutes. Infocus flew on a balloon mission in July 2001 and observed Cygnus X-1. We present results from laboratory testing of the detector to measure the uniformity of response across the detector, to determine the spectral resolution, and to perform a simple noise decomposition. We also present a hard X-ray spectrum and image of Cygnus X-1, and measurements of the hard X-ray CZT background obtained with the SWIN detector on Infocus.

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arXiv2002

Detection of an optical transient following the 13 March 2000 short/hard gamma-ray burst

We imaged the error box of a gamma-ray burst of the short (0.5 s), hard type (GRB 000313), with the BOOTES-1 experiment in southern Spain, starting 4 min after the gamma-ray event, in the I-band. A bright optical transient (OT 000313) with I = 9.4 +/- 0.1 was found in the BOOTES-1 image, close to the error box (3-sigma) provided by BATSE. Late time VRIK'-band deep observations failed to reveal an underlying host galaxy. If the OT 000313 is related to the short, hard GRB 000313, this would be the first optical counterpart ever found for this kind of events (all counterparts to date have been found for bursts of the long, soft type). The fact that only prompt optical emission has been detected (but no afterglow emission at all, as supported by theoretical models) might explain why no optical counterparts have ever been found for short, hard GRBs.This fact suggests that most short bursts might occur in a low-density medium and favours the models that relate them to binary mergers in very low-density enviroments.

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arXiv2000

Rapid Optical Followup Observations of SGR Events with ROTSE-I

In order to observe nearly simultaneous emission from Gamma-ray Bursts (GRBs), the Robotic Optical Transient Search Experiment (ROTSE) receives triggers via the GRB Coordinates Network (GCN). Since beginning operations in March, 1998, ROTSE has also taken useful data for 10 SGR events: 8 from SGR 1900+14 and 2 from SGR 1806-20. We have searched for new or variable sources in the error regions of these SGRs and no optical counterparts were observed. Limits are in the range m_ROTSE ~ 12.5 - 15.5 during the period 20 seconds to 1 hour after the observed SGR events.

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arXiv1999

Discovery of the Optical Transient of the Gamma Ray Burst 990308

The optical transient of the faint Gamma Ray Burst 990308 was detected by the QUEST camera on the Venezuelan 1-m Schmidt telescope starting 3.28 hours after the burst. Our photometry gives $V = 18.32 \pm 0.07$, $R = 18.14 \pm 0.06$, $B = 18.65 \pm 0.23$, and $R = 18.22 \pm 0.05$ for times ranging from 3.28 to 3.47 hours after the burst. The colors correspond to a spectral slope of close to $f_ν \propto ν^{1/3}$. Within the standard synchrotron fireball model, this requires that the external medium be less dense than $10^{4} cm^{-3}$, the electrons contain $> 20%$ of the shock energy, and the magnetic field energy must be less than 24% of the energy in the electrons for normal interstellar or circumstellar densities. We also report upper limits of $V > 12.0$ at 132 s (with LOTIS), $V > 13.4$ from 132-1029s (with LOTIS), $V > 15.3$ at 28.2 min (with Super-LOTIS), and a 8.5 GHz flux of $< 114 μJy$ at 110 days (with the Very Large Array). WIYN 3.5-m and Keck 10-m telescopes reveal this location to be empty of any host galaxy to $R > 25.7$ and $K > 23.3$. The lack of a host galaxy likely implies that it is either substantially subluminous or more distant than a red shift of $\sim 1.2$.

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arXiv1999

Observation of contemporaneous optical radiation from a gamma-ray burst

The origin of gamma-ray bursts (GRBs) has been enigmatic since their discovery. The situation improved dramatically in 1997, when the rapid availability of precise coordinates for the bursts allowed the detection of faint optical and radio afterglows - optical spectra thus obtained have demonstrated conclusively that the bursts occur at cosmological distances. But, despite efforts by several groups, optical detection has not hitherto been achieved during the brief duration of a burst. Here we report the detection of bright optical emission from GRB990123 while the burst was still in progress. Our observations begin 22 seconds after the onset of the burst and show an increase in brightness by a factor of 14 during the first 25 seconds; the brightness then declines by a factor of 100, at which point (700 seconds after the burst onset) it falls below our detection threshold. The redshift of this burst, approximately 1.6, implies a peak optical luminosity of 5 times 10^{49} erg per second. Optical emission from gamma-ray bursts has been generally thought to take place at the shock fronts generated by interaction of the primary energy source with the surrounding medium, where the gamma-rays might also be produced. The lack of a significant change in the gamma-ray light curve when the optical emission develops suggests that the gamma-rays are not produced at the shock front, but closer to the site of the original explosion.

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arXiv1998

A giant, periodic flare from the soft gamma repeater SGR1900+14

Soft gamma repeaters are high-energy transient sources associated with neutron stars in young supernova remnants. They emit sporadic, short (~ 0.1 s) bursts with soft energy spectra during periods of intense activity. The event of March 5, 1979 was the most intense and the only clearly periodic one to date. Here we report on an even more intense burst on August 27, 1998, from a different soft gamma repeater, which displayed a hard energy spectrum at its peak, and was followed by a ~300 s long tail with a soft energy spectrum and a dramatic 5.16 s period. Its peak and time integrated energy fluxes at Earth are the largest yet observed from any cosmic source. This event was probably initiated by a massive disruption of the neutron star crust, followed by an outflow of energetic particles rotating with the period of the star. Comparison of these two bursts supports the idea that magnetic energy plays an important role, and that such giant flares, while rare, are not unique, and may occur at any time in the neutron star's activity cycle.

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arXiv1996

Search for TeV Counterparts of Gamma-Ray Bursts with the HEGRA Experiment

The HEGRA experiment is an air shower detector system for the study of neutral and charged cosmic rays in the energy range between 500 GeV to 10 PeV. Here we give an overview of how the HEGRA detector is used to search for TeV gamma-radiation associated with Gamma-Ray Bursts (GRBs) registered with the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory. Furthermore, results of an archival search for GRB radiation above 15 TeV carried out with the HEGRA air shower arrays are shown. We conclude with a summary of the search activities planned for the future.

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