arXiv subjects

C. Kouveliotou

Publications and source records attributed to C. Kouveliotou.

At least 253 records · Page 14Linked to original sources

arXiv1998

Reactivation and Precise IPN Localization of the Soft Gamma Repeater SGR1900+14

In 1998 May, the soft gamma repeater SGR1900+14 emerged from several years of quiescence and emitted a series of intense bursts, one with a time history unlike any previously observed from this source. Triangulation using Ulysses, BATSE, and KONUS data give a 1.6 square arcminute error box near the galactic supernova remnant G42.8+0.6. This error box contains a quiescent soft X-ray source which is probably a neutron star associated with the soft repeater.

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arXiv1998

ASCA Discovery of an X-ray Pulsar in the Error Box of SGR1900+14

We present a 2 - 10 keV ASCA observation of the field around the soft gamma repeater SGR1900+14. One quiescent X-ray source was detected in this observation, and it was in the SGR error box. In 2 - 10 keV X-rays, its spectrum may be fit by a power law with index -2.2, and its unabsorbed flux is 9.6 x 10^-12 erg cm^-2 s^-1. We also find a clear 5.16 s period. The properties of the three well-studied soft gamma repeaters are remarkably similar to one another, and provide evidence that all of them are associated with young, strongly magnetized neutron stars in supernova remnants.

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arXiv1998

SCUBA sub-millimeter observations of gamma-ray bursters. I. GRB 970508, 971214, 980326, 980329, 980519, 980703

We discuss the first results of our ongoing program of Target of Opportunity observations of gamma-ray bursts (GRBs) using the SCUBA instrument on the James Clerk Maxwell Telescope. We present the results for GRB 970508, 971214, 980326, 980329, 980519, and 980703. Our most important result to date is the detection of a fading counterpart to GRB 980329 at 850 microns. Although it proved to be difficult to find the infrared counterpart to this burst, the sub-millimeter flux was relatively bright. This indicates that intrinsically the brightness of this counterpart was very similar to GRB 970508. The radio through sub-millimeter spectrum of GRB 980329 is well fit by a power law with index alpha = +0.9. However, we cannot exclude a nu^(1/3) power law attenuated by synchrotron self-absorption. An alpha = +1 VLA-SCUBA power law spectrum is definitely ruled out for GRB 980703, and possibly also for GRB 980519. We cannot rule out that part of the sub-millimeter flux from GRB 980329 comes from a dusty star-forming galaxy at high redshift, such as the ones recently discovered by SCUBA. Any quiescent dust contribution will be much larger at sub-millimeter than at radio wavelengths. Both a high redshift and large dust extinction would help explain the reddening of the counterpart to GRB 980329, and a redshift of z = 5 has been suggested. The large intensity of this burst might then indicate that beaming is important.

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arXiv1998

Hard X-ray variability of the black-hole candidate GRO J0422+32 during its 1992 outburst

We have studied the hard X-ray variability of the soft X-ray transient GRO J0422+32 with BATSE in the 20-100 keV energy band. Our analysis covers 180 days following the first X-ray detection of the source on 1992 August 5, fully covering its primary and secondary X-ray outburst. We computed power density spectra (PDSs) in the 20-50, 50-100, and 20-100 keV energy bands, in the frequency interval 0.002-0.488 Hz. The PDSs of GRO J0422+32 are approximately flat up to a break frequency, and decay as a power law above, with index ~1. The canonical anticorrelation between the break frequency and the power density at the break, observed in Cyg X-1 and other BHCs in the low state, is not observed in the PDSs of GRO J0422+32. We compare our results with those of similar variability studies of Cyg X-1. The relation between the spectral slope and the amplitude of the X-ray variations of GRO J0422+32 is similar to that of Cyg X-1; however, the relation between the hard X-ray flux and the amplitude of its variation is opposite to what has been found in Cyg X-1. Phase lags between the X-ray flux variations of GRO J0422+32 at high and low photon energies, could only be derived during the first 30 days of its outburst. During this period, the variations in the 50-100 keV lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.039(3) rad in the frequency interval 0.02-0.20 Hz.

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arXiv1998

A `Hypernova' model for SN 1998bw associated with gamma-ray burst of 25 April 1998

The discovery of the peculiar supernova (SN) 1998bw and its possible association with the gamma-ray burst (GRB) 980425$^{1,2,3}$ provide new clues to the understanding of the explosion mechanism of very massive stars and to the origin of some classes of gamma-ray bursts. Its spectra indicate that SN~1998bw is a type Ic supernova$^{3,4}$, but its peak luminosity is unusually high compared with typical type Ic supernovae$^3$. Here we report our findings that the optical spectra and the light curve of SN 1998bw can be well reproduced by an extremely energetic explosion of a massive carbon+oxygen (C+O) star. The kinetic energy is as large as $\sim 2-5 \times 10^{52}$ ergs, more than ten times the previously known energy of supernovae. For this reason, the explosion may be called a `hypernova'. Such a C+O star is the stripped core of a very massive star that has lost its H and He envelopes. The extremely large energy, suggesting the existence of a new mechanism of massive star explosion, can cause a relativistic shock that may be linked to the gamma-ray burst.

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arXiv1998

A Search for Non-triggered Gamma Ray Bursts in the BATSE Data Base

We describe a search of archival data from the Burst and Transient Source Experiment (BATSE). The purpose of the search is to find astronomically interesting transients that did not activate the burst detection (or ``trigger'') system onboard the spacecraft. Our search is sensitive to events with peak fluxes (on the 1.024 s time scale) that are lower by a factor of 2 than can be detected with the onboard burst trigger. In a search of 345 days of archival data, we detected 91 events in the 50--300 keV range that resemble classical gamma ray bursts but that did not activate the onboard burst trigger. We also detected 110 low-energy (25--50 keV) events of unknown origins which may include activity from SGR 1806-20 and bursts and flares from X-ray binaries. This paper gives the occurrence times, estimated source directions, durations, peak fluxes, and fluences for the 91 gamma ray burst candidates. The direction and intensity distributions of these bursts imply that the biases inherent in the onboard trigger mechanism have not significantly affected the completeness of the published BATSE gamma ray burst catalogs.

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arXiv1998

On the Association of Gamma-Ray Bursts with Supernovae

The recent discovery of a supernova (SN 1998bw) seemingly associated with GRB~980425 adds a new twist to the decades-old debate over the origin of gamma-ray bursts. To investigate the possibility that some (or all) bursts are associated with supernovae, we performed a systematic search for temporal/angular correlations using catalogs of BATSE and BATSE/{\it Ulysses} burst locations. We find no associations with any of the precise BATSE/{\it Ulysses} locations, which allows us to conclude that the fraction of high-fluence gamma-ray bursts from known supernovae is small ($<$0.2%). For the more numerous weaker bursts, the corresponding limiting fraction of 1.5% is less constraining due to the imprecise locations of these events. This limit ($1.5% \simeq 18$ bursts) allows that a large fraction of the recent supernovae used as a comparison data set (18 supernovae $\simeq 20%$) could have associated gamma-ray bursts. Thus, although we find no significant evidence to support a burst/supernova association, the possibility cannot be excluded for weak bursts.

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arXiv1998

Discovery of the peculiar supernova 1998bw in the error box of GRB980425

The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRBs) and the measurements of the distances to some of them have established that these events come from Gpc distances and are the most powerful photon emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We here report the discovery of an optical transient, in the BeppoSAX Wide Field Camera error box of GRB980425, which occurred within about a day of the gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is emphasized by its extraordinary radio properties which require that the radio emitter expand at relativistical speed. Since SN1998bw is very different from all previously observed afterglows of GRBs, our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties.

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arXiv1998

Power Law Decays in the Optical Counterparts of GRB 970228 and GRB970508

We report on R_c and K band observations of the optical counterpart to GRB 970508 with the Fred Lawrence Whipple Observatory (FLWO) 1.2m telescope. Eleven R_c-band observations were obtained on 1997 May 12, and three on May14. The counterpart clearly faded between the two nights. On May 12 there was no evidence for variability (<9%) on 10-70 minute time scales based on 11 R_c-band observations. On May 19 a one hour observation set a limit on the K magnitude of K>18.2. Comparison of these data points with those obtained by other authors shows that the decay of the optical counterpart can be well fitted by a power law of the form f ~ t**-alpha where alpha = 1.22 +/- 0.03 with occasional fluctuations superposed. We note that the decay of the optical counterpart to another burst, GRB 970228, can also be well fitted with a power law with exponent alpha = 1.0/+0.2/-0.5 with occasional fluctuations superposed. These two decay light curves are remarkably similar in form to that predicted by cosmic-fireball models.

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arXiv1998

The 1.4 GHz light curve of GRB 970508

We report on Westerbork 1.4 GHz radio observations of the radio counterpart to $γ$-ray burst GRB~970508, between 0.80 and 138 days after this event. The 1.4 GHz light curve shows a transition from optically thick to thin emission between 39 and 54 days after the event. We derive the slope $p$ of the spectrum of injected electrons ($dN/dγ_{e}\proptoγ_{e}^{-p}$) in two independent ways which yield values very close to $p=2.2$. This is in agreement with a relativistic dynamically near-adiabatic blast wave model whose emission is dominated by synchrotron radiation and in which a significant fraction of the electrons cool fast.

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arXiv1998

The radio to X-ray spectrum of GRB 970508 on May 21.0 UT

We have reconstructed the spectrum of the afterglow of GRB 970508, on May 21.0 UT (12.1 days after the GRB), on the basis of observations spanning the X-ray to radio range. The low-frequency power law index of the spectrum, $α=0.44\pm0.07$ ($F_ν\proptoν^α$), is in agreement with the expected value $α=1/3$ for optically thin synchrotron radiation. The 1.4 GHz emission is self-absorbed. We infer constraints on the break frequecies $ν_{c}$ and $ν_{m}$ on May 21.0 UT from a spectral transition from $F_ν \sim ν^{-0.6}$ to $F_ν \sim ν^{-1.1}$ in the optical passband around 1.4 days. A model of an adiabatically expanding blast wave, emitting synchrotron radiation, in which a significant fraction of the electrons cool rapidly provides a successful and consistent description of the afterglow observations over nine decades in frequency, ranging in time from trigger until several months later.

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arXiv1998

RXTE Observations of the SGR 1806-20 Steady Emission

We discuss observations of the quiescent emission from the soft gamma repeater SGR 1806-20 by the Rossi X-ray Timing Explorer. We find that the 2-20 keV RXTE data is consistent with a constant spectral shape during both active bursting periods and periods of relative quiescence, and is best described by a nonthermal (power law) spectral shape. Using archival ASCA data we find that the quiescent spectrum of SGR 1806-20 is well fit over the energy range 1-30 keV by a power law of photon index 2.31 +/- 0.04, with thermal bremsstrahlung and Raymond-Smith models producing much worse fits to the data.

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arXiv1998

Optical follow-up of GRB 970508

We report on the results of optical follow-up observations of the counterpart of GRB 970508, starting 7 hours after the event. Multi-color U, B, V, R$_{c}$ and I$_{c}$ band observations were obtained during the first three consecutive nights. The counterpart was monitored regularly in R$_{c}$ until $\sim$ 4 months after the burst. The light curve after the maximum follows a decline that can be fitted with a power law with exponent $α$ = --1.141 $\pm$ 0.014. Deviations from a smooth power law decay are moderate (r.m.s. = 0.15 magnitude). We find no flattening of the light curve at late times. The optical afterglow fluence is a significant fraction, $\sim$ 5%, of the GRB fluence. The optical energy distribution can be well represented by a power law, the slope of which changed at the time of the maximum (the spectrum became redder).

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arXiv1997

Hard X-ray Lags in Cygnus X-1

We have used the Fourier cross spectra of Cyg X-1, as obtained with BATSE during a period of almost 2000 days, to estimate the phase (or time) lags between X-ray flux variations in the 20-50 keV and the 50-100 keV bands as a function of Fourier frequency, nu. We find that these lag spectra do not depend on source state, as measured by the fractional rms variations of the X-ray flux. For frequencies well below the Nyquist frequency the variations in the 50-100 keV band lag those in the 20-50 keV band by a time interval proportional to nu^{-0.8}. Binning effects cause the observed hard X-ray time lags to decrease to zero at the Nyquist frequency. Some previous results on time lags were affected by these binning effects. At photon energies above 10 keV the time lags are approximately proportional to the logarithm of the ratio the energies of the passbands used.

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arXiv1997

First Year Results From LOTIS

LOTIS (Livermore Optical Transient Imaging System) is a gamma-ray burst optical counterpart search experiment located near Lawrence Livermore National Laboratory in California. The system is linked to the GCN (GRB Coordinates Network) real-time coordinate distribution network and can respond to a burst trigger in 6-15 seconds. LOTIS has a total field-of-view of 17.4 deg x 17.4 deg with a completeness sensitivity of m_V ~ 11 for a 10 second integration time. Since operations began in October 1996, LOTIS has responded to over 30 GCN/BATSE GRB triggers. Seven of these triggers are considered good events subject to the criteria of clear weather conditions, < 60 s response time, and > 50% coverage of the final BATSE 3sigma error circle. We discuss results from the first year of LOTIS operations with an emphasis on the observations and analysis of GRB 971006 (BATSE trigger 6414).

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arXiv1997

Super-LOTIS: A High-Sensitive Optical Counterpart Search Experiment

We are constructing a 0.6 meter telescope system to search for early time gamma-ray burst(GRB) optical counterparts. Super-LOTIS (Super-Livermore Optical Transient Imaging System) is an automated telescope system that has a 0.8 x 0.8 deg field-of-view, is sensitive to Mv ~ 19 and responds to a burst trigger within 5 min. This telescope will record images of the gamma-ray burst coordinates that is given by the GCN (GRB Coordinate Network). A measurement of GRB light curves at early times will greatly enhance our understanding of GRB physics.

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arXiv1997

A Search for Optical Afterglow from GRB970828

We report on the results of R band observations of the error box of the gamma-ray burst of August 28, 1997, made between 4 hours and 8 days after this burst occurred. No counterpart was found varying by more than 0.2 magnitudes down to R = 23.8. We discuss the consequences of this non-detection for relativistic blast wave models of gamma-ray bursts, and the possible effect of redshift on the relation between optical absorption and the low-energy cut off in the X-ray afterglow spectrum.

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arXiv1997

HEXTE Observations of SGR 1806-20 During Outburst

We discuss observations of the soft gamma repeater SGR 1806-20 during the RXTE Target of Opportunity observations made in November 1996. During the ~50 ksec RXTE observation, HEXTE (15-250 keV) detected 17 bursts from the source, with fluxes ranging from 3 x 10^{-9} to 2.2 x 10^{-7} ergs cm^{-2} s^{-1} (20-100 keV). We obtained spectra for the brighter HEXTE by fitting thermal bremsstrahlung and power law functions over the energy range 17 - 200 keV. The best-fit temperatures and photon indices range from 30 - 55 keV and 2.2 - 2.7, respectively. The weighted average temperature of the HEXTE bursts was 41.8 +/- 1.7 keV, which is consistent with previous SGR 1806-20 burst spectra. The persistent emission from SGR 1806-20 was not detected with HEXTE.

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