Engineering topics

Kouveliotou, C.

Publications and source records attributed to Kouveliotou, C..

At least 109 records · Page 6

NASA NTRS1999

Spectral Energy Distributions and Light Curves of GRB 990123 and its Afterglow

Investigations of the 'fireball' model currently believed to explain the prompt gamma-ray and afterglow emissions of gamma-ray bursts. On January 23 a gamma-ray burst (GRB) occurred for which for the first time prompt optical emission was detected. We here report the results of gamma-ray, optical/infrared, sub-mm, mm and radio observations of this burst and its afterglow, which indicate that the prompt and afterglow emissions from GRB 990123 are associated with three distinct regions in the fireball. The afterglow synchrotron spectrum one day after the burst has a much lower peak frequency than those of previous bursts; this explains the short-lived nature of the radio emission, which is not expected to reappear. We suggest that such differences reflect variations in the magnetic-field strengths in the afterglow emitting regions.

Galama, T. J.↗

NASA NTRS1999

Spectral Energy Distributions and Light Curves of GRB 990123 and Its Afterglow

Gamma-ray bursts (GRBs) are thought to result from the interaction of an extremely relativistic outflow interacting with a small amount of material surrounding the site of the explosion. Multi-wavelength observations covering the gamma-ray to radio wavebands allow investigations of this "fireball" model. On 23 January 1999 optical emission was detected while the gamma-ray burst was still underway. Here we report the results of gamma-ray, optical/infra-red, sub-mm, mm and radio observations of this burst and its afterflow, which indicate that the prompt and afterflow emissions from GRB 990123 are associated with three distinct regions in the fireball. The afterglow one day after the burst has a much lower peak frequency than those of previous bursts; this explains the short-lived nature of the radio emission, which is not expected to reapear. We suggest that such differences reflect variations in the magnetic-field strengths in the afterglow emitting regions.

Galama, T. J.↗

NASA NTRS1999

Statistical Properties of SGR 1806-20 Bursts

We find that the fluence distribution of bursts observed with each instrument are well described by power laws with indices 1.43, 1.67 and 1.76 respectively. The distribution of time intervals between successive bursts from SGR 1806-20 is described by a lognormal function with peak at 97 s. There is no correlation between burst intensity and either the waiting times till the next burst or the time elapsed since the previous burst, We find a correlation between the duration and fluence of the bursts, but with substantial scatter. In all these statistical properties, SGR bursts resemble self-organized critical systems, such as earthquakes and solar flares. Our results thus support the hypothesis that the energy source for SGR bursts is crustquakes due to the magnetic field of the neutron star, rather than any accretion- or nuclear-power.

Gogus, E.↗

NASA NTRS1998

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/Ulysses burst locations. We find no associations with any of the precise BATSE/Ulysses locations, which allows us to conclude that the fraction of high-fluence gamma-ray bursts associated with known supernovae is small (<0.2%). For the more numerous weaker bursts, the corresponding limiting fraction of 2.5% is far less constraining due to the imprecise locations of these events. This fraction (2.5%) of bursts corresponds to approximately 30% of the recent supernovae used as a comparison data set. Thus, although we find no significant evidence to support a burst/supernova association, the possibility cannot be excluded for weak bursts.

Kippen, R. M.↗

NASA NTRS1998

The 1.4 GHz Light Curve of GRB 970508

We report on Westerbork 1.4 GHz radio observations of the radio counterpart to gamma-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 rho of the spectrum of injected electrons (dN/d(gamma(sub e) proportional to (gamma(sub e)exp -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.

Galama, T. J.↗

NASA NTRS1998

The Radio-to-X-Ray Spectrum of GRB 970508 on 1997 May 21.0 UT

We have reconstructed the spectrum of the afterglow of GRB 970508 on 1997 May 21.0 UT (12.1 days after the gamma-ray burst) on the basis of observations spanning the X-ray-to-radio range. The low-frequency power-law index of the spectrum, alpha = 0.44 +/- 0.07 (F(sub nu) proportional to nu(exp alpha)), is in agreement with the expected value alpha = 1/3 for optically thin synchrotron radiation. The 1.4 GHz emission is self-absorbed. We infer constraints on the break frequencies nu(sub c) and nu(sub m) on 1997 May 21.0 UT from a spectral transition from F(sub nu) approx. nu(exp -0.6) to F(sub nu) approx. nu(exp -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.

Galama, T. J.↗

NASA NTRS1998

The 1.4 GHz Light Curve of GRB 970508

We report on Westerbork 1.4 GHz radio observations of the radio counterpart to gamma-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/dgamma(sub e) proportional to gamma(sup -p, sub e) 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 synchrotrons radiation and in which a significant fraction of The electrons cool fast.

Galama, T.↗

NASA NTRS1998

An X-ray Pulsar with a Superstrong Magnetic Field in the Soft Gamma-Ray Repeater SGR1806-20

Soft gamma-ray repeaters (SGRs) emit multiple, brief (approximately O.1 s) intense outbursts of low-energy gamma-rays. They are extremely rare; three are known in our galaxy and one in the Large Magellanic Cloud. Two SGRs are associated with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806-20, with a period of 7.47 s and a spindown rate of 2.6 x 10(exp -3) s/yr. We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are approximately 1500 years and 8 x 10(exp 14) gauss, respectively. Our observations demonstrate the existence of 'magnetars', neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions that SGR bursts are caused by neutron-star 'crust-quakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millenium, a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars.

Kouveliotou, C.↗

NASA NTRS1998

Properties of the Second Outburst of the Bursting Pulsar (GRO J1744-28) as Observed with BATSE

One year after its discovery, the Bursting Pulsar (GRO J1744-28) went into outburst again, displaying the hard X-ray bursts and pulsations that make this source unique. We report on Burst and Transient Source Experiment (BATSE) observations of both the persistent and burst emission for this second outburst and draw comparisons to the first. The second outburst was smaller than the first in both duration and peak luminosity. The persistent flux, burst peak flux and burst fluence were all reduced in amplitude by a factor approximately 1.7. Despite these differences, the average burst occurrence rate and average burst durations were roughly the same through each outburst. Similar to the first outburst, no spectral evolution was found within bursts and the parameter alpha was very small at the start of the outburst (alpha = 2.1 +/- 1.7 on 1996 December 2). Although no spectral evolution was found within individual bursts, we find evidence for a small (20%) variation of the spectral temperature during the course of the second outburst.

Woods, P.↗

NASA NTRS1998

The X-Ray, Optical and Infrared Counterpart to GRB 980703

We report on our X-ray, optical and infrared follow-up observations of GRB980703. A previously unknown X-ray source in the GRB error box displays a powerlaw decline with index alpha= -1.0,5(exp +0.24 sub -0.29), fairly consistent with the average optical/infrared decay index alpha = -1.61 plus or minus 0.12. We invoke host galaxy extinction to match the observed spectral slope with the slope expected from 'fireball' models. The hypothesis that between 1998 July 4.4 and 8.4 UT no spectral breaks are present in the infrared to X-ray spectral range is in good agreement with the observations. For these two epochs we obtain an extinction of A(sub v) = 1.50 +/- 0.11 (July 4.4) and A(sub v) = 0.85 +/- 0.26 (July 8.4). From the X-ray data we estimate the optical extinction to be A(sub v) = 20.2(+12.3/-7.3), inconsistent with the former value. Optical spectra we took confirm the redshift of z=0.966 found by Djorgovski et al. We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field is much lower in the case of GRB 980703.

Vreeswijk, P. M.↗

NASA NTRS1998

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 the Burst and Transient Source Experiment (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 3, 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 about 1. During the first 70 days of the X-ray outburst, the PDSs of GRO J0422+32 show a significant QPO Peak near about 0.2 Hz, superposed on the power-law tail. The break frequency of the PDSs obtained during the primary X-ray outburst of GRO J0422+32 occurs at 0.041+/-0.006 Hz; during the secondary outburst the break is at 0.081q0.01.5 Hz. The power density at the break ranged between 44 and 89% Hz(exp -1/2) 20-100 keV). 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.

VanderHooft, F.↗

NASA NTRS1998

Hard X-Ray Lags in GRO J1719-24

We have used the Fourier cross spectra of GRO J1719-24. as obtained with BATSE, to estimate the phase lags between the X-ray flux variations in the 20-,50 and 50-100 keV energy bands as a function of Fourier frequency in the interval 0.002-0.488 Hz. Our analysis covers the entire approximately 80 day X-ray outburst of this black-hole candidate, following the first X-ray detection on 1993 September 25. The X-ray variations in the 50-100 keV band, lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.072 +/- 0.010 rad in the frequency interval 0.02-0.20 Hz. The time lags of GRO J1719-24 decrease with frequency as a power law, with index 1.04 +/- 0.13 for frequencies >/= 0.01 Hz. These results are discussed together with those obtained in recent similar studies of the black-hole candidates Cyg X-1 and GRO J0422+32.

VanderHooft, Frank↗

NASA NTRS1998

Rapid Search for a Near-Infrared Counterpart of GRB 960924

We report the results of a rapid search for a near-infrared counterpart of the very strong gamma-ray burst 960924. The survey was performed in JHK-prime only 3.5 days after the burst. It was centered at the most probable burster position and covers about 1/3 of the BACODINE/IPN/Huntsville error box represented by the intersection of the 1-sigma, BATSE error circle and the 3-sigma, IPN annulus. Second epoch observations were carried out in the near-infrared about 100 days and 500 days after the burst. No Near Infrared (NIR) transient source was detected within the surveyed IPN annulus to magnitude limits J is approximately 17.5 and K is approximately 15-5.

Klose, S.↗

NASA NTRS1998

A Non-Triggered Burst Supplement to the BATSE Gamma-Ray Burst Catalogs

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory detects gamma-ray bursts (GRBs) with a real-time burst detection (or "trigger") system running onboard the spacecraft. Under some circumstances, however, a GRB may not activate the onboard burst trigger. For example, the burst may be too faint to exceed the onboard detection threshold, or it may occur while the onboard burst trigger is disabled for technical reasons. This paper is a catalog of such "non-triggered" GRBs that were detected in a search of the archival continuous data from BATSE. It lists 873 non-triggered bursts that were recorded between 1991 December 9.0 and 1997 December 17.0. For each burst, the catalog gives an estimated source direction, duration, peak flux, and fluence. Similar data are presented for 50 additional bursts of unknown origin that were detected in the 25-50 keV range; these events may represent the low-energy "tail" of the GRB spectral distribution. This catalog increases the number of GRBs detected with BATSE by 48% during the time period covered by the search.

Kommers, J.↗

NASA NTRS1998

SGR 1900+14

SGR 1900+14 soft gamma-ray repeater has entered an extraordinary new phase of activity with a previously unobserved frequency and intensity of bursts. Activity began on May 26 and has continued at least through May 30. Triangulation with Ulysses and BATSE gives a preliminary annulus whose center is at R.A. = 2lh58ml8s, Decl. = -11ol8'.4 (equinox 2000.0), and whose radius is 47.269 +/- 0.019 deg. This annulus includes the network synthesis error box (Hurley et al. 1994, Ap.J. 431, L31), the ROSAT source proposed as a possible quiescent x-ray counterpart to the soft repeater, and the peculiar double M star system coincident with the ROSAT source. As all these locations are mutually consistent, we conclude that the ROSAT source and M star system are indeed the counterparts to the soft gamma-ray repeater, and urge that observations at other wavelengths be undertaken immediately, while the source remains active. An image may be found at http://ssl.berkeley.edu/ipn3/sgrl9OO+l4/. On May 30, BATSE triggered on an extremely intense outburst from this source that comprised at least 38 bursts of varying intensity and duration; the series of bursts started at May 30.37759 UT and lasted for about 350 s. The peak flux of each of the three major bursts in the series is 1.1 x lOE-4 erg cmE-2 sE-1 (between 25 and 300 keV, integrated for 128 ms). Assuming a distance of about 12 kpc for SGR 1900+14, this corresponds to a luminosity of about 2 X 10(exp 42) ergs / second for each burst. This bunching of emission is very similar to the behavior of SGR 1806-20 as detected with the Rossi X-ray Timing explorer in 1996. Overall, between May 26 and June 1, we have detected over 50 bursts from SGR 1900+4. The frequency and intensity of emissions are unlike any previously seen behavior from this source. Follow-up observations at infrared and radio wavelengths, while the source is still active are encouraged.

Hurley, K.↗

NASA NTRS1997

ROSAT X-Ray Observation of the Second Error Box for SGR 1900+14

The positions of the two error boxes for the soft gamma repeater (SGR) 1900+14 were determined by the "network synthesis" method, which employs observations by the Ulysses gamma-ray burst and CGRO BATSE instruments. The location of the first error box has been observed at optical, infrared, and X-ray wavelengths, resulting in the discovery of a ROSAT X-ray point source and a curious double infrared source. We have recently used the ROSAT HRI to observe the second error box to complete the counterpart search. A total of six X-ray sources were identified within the field of view. None of them falls within the network synthesis error box, and a 3 sigma upper limit to any X-ray counterpart was estimated to be 6.35 x 10(exp -14) ergs/sq cm/s. The closest source is approximately 3 min. away, and has an estimated unabsorbed flux of 1.5 x 10(exp -12) ergs/sq cm/s. Unlike the first error box, there is no supernova remnant near the second error box. The closest one, G43.9+1.6, lies approximately 2.dg6 away. For these reasons, we believe that the first error box is more likely to be the correct one.

Li, P.↗

NASA NTRS1997

The Identification of Two Different Spectral Types of Pulses in Gamma-Ray Bursts

It is shown in this study that two different types of spectral emission are generally produced in gamma-ray bursts. A subset of bursts is identified that exhibits a marked lack of fluence above 300 keV, and these bursts are shown to have luminosities about an order of magnitude lower than bursts with significant fluence above 300 keV. The bursts lacking emission above 300 keV exhibit an effectively homogeneous intensity distribution. In addition, it is shown that both types of emission are common in many bursts, demonstrating that a single source object is capable of generating both of them. These results strongly favor a gamma-ray burst source object that produces two different types of emission with varying degrees of superposition. The impact of this behavior is strong enough that it affects the properties of the burst intensity distribution, as well as the burst spectral characteristics.

Pendleton, G. N.↗

NASA NTRS1997

Real-Time Optical Flux Limits from Gamma-Ray Bursts Measured by the Gamma-Ray Optical Counterpart Search Experiment

The Gamma-Ray Optical Counterpart Search Experiment presents new experimental upper limits on the optical flux from gamma-ray bursts (GRBs). Our experiment consisted of a fully automated very wide-field opto-electronic detection system that imaged locations of GRBs within a few seconds of receiving trigger signals provided by BATSE's real-time burst coordinate distribution network. The experiment acquired 3800 observing hours, recording 22 gamma-ray burst triggers within approx. 30 s of the start of the burst event. Some of these bursts were imaged while gamma-ray radiation was being detected by BATSE. We identified no optical counterparts associated with gamma-ray bursts among these events at the mV approx. 7.0-8.3 sensitivity level. We find the ratio of the upper limit to the V-band optical flux, F nu, to the gamma-ray fluence, Phi gamma, from these data to be 1 x 10(exp-18) less than F nu Phi gamma less than 2 x 10(exp -16).

Park, H. S.↗