arXiv subjects

A. J. Castro-Tirado

Publications and source records attributed to A. J. Castro-Tirado.

239 records · Page 14Linked to original sources

arXiv1998

BeppoSAX follow-up search for the X-ray afterglow of GRB970111

The BeppoSAX satellite has recently opened a new way towards the solution of the long standing gamma-ray bursts' (GRBs) enigma, providing accurate coordinates few hours after the event thus allowing for multiwavelength follow-up observational campaigns. The BeppoSAX Narrow Field Instruments observed the region of sky containing GRB970111 16 hours after the burst. In contrast to other GRBs observed by BeppoSAX no bright afterglow was unambiguously observed. A faint source (1SAXJ1528.1+1937) is detected in a position consistent with the BeppoSAX Wide Field Camera position, but unconsistent with the IPN annulus. Whether 1SAXJ1528.1+1937 is associated with GRB970111 or not, the X-ray intensity of the afterglow is significantly lower than expected, based on the properties of the other BeppoSAX GRB afterglows. Given that GRB970111 is one of the brightest GRBs observed, this implies that there is no obvious relation between the GRB gamma-ray peak flux and the intensity of the X-ray afterglow.

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arXiv1997

Hubble Space Telescope Imaging of the Field of GRB970508

We report on Hubble Space Telescope observations of the optical transient discovered in the error box of GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph and Near-Infrared Camera and Multi-Object Spectrometer. The observations reveal a point-like source with R = 23.1 +- 0.2 and H = 20.6 +- 0.3, in agreement with the power-law temporal decay seen in previous ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508, although Mg I absorption and [O II] emission seen in Keck spectra at a redshift of z = 0.835 suggest the presence of a dense, star-forming medium. The HST observations set very conservative upper limits of R ~24.5 and H ~22.2 on the brightness of any underlying extended source. If this subtends a substantial fraction of an arcsecond, then the R band limit is ~25.5. Subsequent photometry suggests a flattening of the light curve at later epochs. Assuming the OT decline follows a pure power-law and ascribing the flattening to the presence of an underlying component of constant flux, we find that this must have R = 25.4, consistent with the upper limits determined by HST. At z = 0.8, this would correspond to an absolute magnitude in the U band of ~-18, similar to that of the Large Magellanic Cloud. We propose a scenario in which the host galaxy of the GRB is of Magellanic type, possibly being a ``satellite'' of one of the bright galaxies located at few arcseconds from the optical transient.

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arXiv1997

Evidence for Diverse Optical Emission from Gamma-Ray Burst Sources

Optical Transients from gamma-ray burst sources, in addition to offering a distance determination, convey important information on the physics of the emission mechanism, and perhaps also about the underlying energy source. As the gamma-ray phenomenon is extremely diverse, with time scales spanning several orders of magnitude, some diversity in optical counterpart signatures appears plausible. We have studied the Optical Transient, which accompanied the gamma-ray burst of May 8, 1997 (GRB 970508). Observations conducted at the 2.5-m Nordic Optical Telescope (NOT) and the 2.2-m telescope at the German-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3 hours 5 minutes to 96 days after the high energy event. This brackets all other published observations, including radio. When analyzed in conjunction with optical data from other observatories, evidence emerges for a composite light curve. The first interval, from 3 to 8 hours after the event was characterized by a constant, or slowly declining brightness. At a later moment the brightness started increasing rapidly, and reached a maximum approximately 40 hours after the GRB. From that moment the GRB brightness decayed approximately as a power-law of index -1.21. The last observation, after 96 days, m_R = 24.28+-0.10, is brighter than the extrapolated power-law, and hints that a constant component, m_R = 25.50+-0.40 is present. The OT is unresolved (FWHM 0.83") at the faintest magnitude level. The brightness of the optical transient, its duration and the general shape of the light curve sets this source apart from the single other optical transient known, that of the February 28, 1997 event.

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arXiv1997

Early detection of the Optical Transient following the Gamma-Ray Burst GRB 970228

The optical counterpart of the Gamma-Ray Burst GRB 970228, discovered by Groot et al. (IAU Circ. 6584), is also detected in the B and R frames obtained about 4 hours earlier at the Bologna Observatory. Our observations indicate a very likely rise of the optical emission within these 4 hours. The R luminosity of the transient at maximum was about 15 times that of an underlying extended object. Follow-up data show that the maximum optical emission was delayed of not less than 0.7 days with respect to the gamma-ray peak and that no new big flares were seen after the main one. The optical transient became significantly redder once it has reverted to quiescence.

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arXiv1996

Infrared Observations and Energetic Outburst of GRS 1915+105

Multiple near-infrared wavelengths observations, carried out since 1993 on the galactic superluminal source of relativistic ejections GRS 1915+105, have yielded three important results. 1) The infrared counterpart of GRS 1915+105 exhibits various variations in the 1.2 - 2.2 micrometer band: the strongest are of nearly 1 magnitude in a few hours and of nearly 2 magnitudes over longer intervals of time. 2) The infrared properties of GRS 1915+105 are strikingly similar to those of SS 433, and unlike those of any other known stellar source in the Galaxy. The absolute magnitudes, colors, and time variabilities of these two sources of relativistic ejections suggest that GRS 1915+105, like SS 433, consists of a collapsed object (neutron star or black hole) with a thick accretion disk in a high-mass-luminous binary system. 3) During an intense and long-term X-ray outburst of GRS 1915+105 in 1995 August, where a pair of radio-emitting clouds emerged from the compact core in opposite directions at relativistic speeds, we observed the time-delayed reverberation of this radio flare/ejection event in the infrared wavelengths. The observed spectrum of the enhanced infrared emission suggests the appearance of a warm dust component.

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