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  • Space Radiation  (22)
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  • 1
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    The Optical Counterpart to Gamma-Ray Burst GRB 970228 Observed Using the Hubble Space Telescope (1997)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-23
    Beschreibung: Although more than 2,000 astronomical gamma-ray bursts (GRBS) have been detected, and numerous models proposed to explain their occurrence, they have remained enigmatic owing to the lack of an obvious counterpart at other wavelengths. The recent ground-based detection of a transient optical source in the vicinity of GRB970228 may therefore have provided a breakthrough. The optical counterpart appears to be embedded in an extended source which, if a galaxy, as has been suggested would lend weight to those models that place GRBs at cosmological distances. Here we report, observations using the Hubble Space Telescope of the transient counterpart and extended source 26 and 39 days after the initial gamma-ray outburst. We find that the counterpart has faded since the initial detection (and continues to fade), but the extended source exhibits no significant change in brightness between the two dates of the observations reported here. The size and apparent constancy of the extended source imply that it is extragalactic, but its faintness makes a definitive statement about its nature difficult. Nevertheless, the decay profile of the transient source is consistent with a popular impulsive-fireball model13, which assumes a merger between two neutron stars in a distant galaxy.
    Schlagwort(e): Space Radiation
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  • 2
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    The Effect of Magnetic Fields on Gamma-Ray Bursts Inferred from Multi-Wavelength Observations of the Bursts of 23 January 1999 (1999)
    In:  Other Sources
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    Publikationsdatum: 2011-08-23
    Beschreibung: Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature at which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of gamma-ray, optical, infrared, submillimeter, millimeter and radio observations of the burst ORB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.
    Schlagwort(e): Space Radiation
    Materialart: Nature; Volume 398; 394-399
    Format: text
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  • 3
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    GRB 990712: First Detection of Polarization Variability in a Gamma-ray Burst Afterglow (2000)
    In:  Other Sources
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    Publikationsdatum: 2019-07-17
    Beschreibung: We report the detection of significant polarization in the afterglow of GRB 990712 on three instances 0.44-1.45 days after the gamma-ray burst. This polarization is intrinsic to the afterglow. The degree of polarization is not constant, and smallest at the second measurement. The polarization angle does not vary significantly during these observations. We find that none of the existing models predict such polarization variations constant polarization angle, and discuss ways in which these models might be modified to accommodate the observed behavior of this afterglow.
    Schlagwort(e): Space Radiation
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  • 4
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    TOO Observations Soft Gamma Repeaters (1998)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: The goal of the project was to study the X-ray properties of the persistent and burst emission of Soft Gamma Repeaters (SGRs) during periods of burst activity. We monitored this activity with BATSE on the Compton Gamma-Ray Observatory, and made X-ray observations with the Rossi X-ray Timing Explorer (RXTE). SGR1806-20 became active in October 1996. We made observations with the PCA on the RXTE in November 1996. In the RXTE data we detected several hundred brief SGR events, which occurred in clear bunches, and persistent emission. From a Fouder analysis of the persistent emission (excluding time intervals with bursts) we found a period of 7.47 s. These pulsations are also present in RXTE data taken several weeks later (PI Dr. T. Strohmayer), which were combined with our data. Comparison with ASCA data taken in 1993 and 1995 shows that the period, which reflects the spin of a neutron star, increases on a time scale of 1500 years. These results show that SGR1 806-20 is a neutron star with a superstrong magnetic field (about 1"15) Gauss), thereby establishing, for the first time, the existence of magnetars.
    Schlagwort(e): Space Radiation
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  • 5
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    The Radio-to-X-Ray Spectrum of GRB 970508 on 1997 May 21.0 UT (1998)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: 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.
    Schlagwort(e): Space Radiation
    Materialart: Astrophysical Journal; 500; L97-L100
    Format: text
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  • 6
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    The Decay of Optical Emission from the gamma-Ray Burst GRB970228 (1997)
    In:  CASI
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    Publikationsdatum: 2019-07-13
    Beschreibung: The origin of gamma-ray bursts has been one of the great unsolved mysteries in high-energy astrophysics for almost 30 years. The recent discovery of fading sources at X-ray and optical wavelengths coincident with the location of the gamma-ray burst GRB970228 therefore provides an unprecedented opportunity to probe the nature of these high-energy events. The optical counterpart appears to be a transient point source embedded in a region of extended nebulosity, the latter having been tentatively identified as a high-redshift galaxy. This would seem to favour models that place gamma-ray bursts at cosmological distances, although a range of mechanisms for producing the bursts is still allowed. A crucial piece of information for distinguishing between such models is how the brightness of the optical counterpart evolves with time. Here we re-evaluate the existing photometry of the optical counterpart of GRB970228 to construct an optical light curve for the transient event. We find that between 21 hours and six days after the burst, the R-band brightness decreased by a factor of approximately 40, with any subsequent decrease in brightness occurring at a much slower rate. As the point source faded, it also became redder. The initial behaviour of the source appears to be consistent with the 'fireball' model, but the subsequent decrease in the rate of fading may prove harder to explain.
    Schlagwort(e): Space Radiation
    Materialart: NASA-TM-112889 , NAS 1.15:112889 , Letters to Nature; 387; 479-481
    Format: application/pdf
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  • 7
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    Spectral Energy Distributions and Light Curves of GRB 990123 and Its Afterglow (1999)
    In:  Other Sources
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    Publikationsdatum: 2019-07-17
    Beschreibung: 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.
    Schlagwort(e): Space Radiation
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  • 8
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    A Non-Triggered Burst Supplement to the BATSE Gamma-Ray Burst Catalogs (1998)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-17
    Beschreibung: 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.
    Schlagwort(e): Space Radiation
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  • 9
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    Radio and Optical Follow-Up Observations and Improved IPN Position of GRB 970111 (1997)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-17
    Beschreibung: We report on Westerbork 840 MHz, 1.4 and 5 GHz radio observations of the improved IPN-WFC error box of the gamma ray burst GRB 970111, between 26.4 hours and 120 days after the event onset. In the approximately 16 sq arcmin area defined by the IPN (BATSE and Ulysses) annulus and the published refined BeppoSAX Wide Field Camera (WFC) error box we detected no steady sources brighter than 0.56 mJy (4sigma), and no varying radio emission, down to 1.0 mJy (4sigma). We also report on B, V, R and I band observations of the error box with the 4.2 m William Herschel Telescope at La Palma. Subject headings: gamma rays: bursts - gamma rays: individual (GRB 9701 1 1)
    Schlagwort(e): Space Radiation
    Materialart: Astrophysical Journal
    Format: text
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  • 10
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    IAUC 6530: GRO J1744-28; C/1995 01 (1997)
    In:  Other Sources
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    Publikationsdatum: 2019-07-17
    Beschreibung: We have recorded a series of outbursts starting on 1996 Dec. 2 that are consistent with the position of GRO J1744-28, the bursting pulsar. The bursts are detected between 20 and 100 keV, and on Dec. 2 they came in variable rates ranging from every 300 to every 500 s, with a preliminary total number estimated between 80 and 1 00 events. After Dec. 3, the burst rate dropped to 15-20 bursts/day, where it has remained. The current average error radius of the burst locations is about 4 deg. The burst fluence is currently about 2 x 10E-7 erg cmE-2. This indicates that the source is in outburst again. Although the source location is currently very close to the sun, we encourage observations in other wavelengths whenever possible. Persistent, hard x-ray emission from GRO J1744-28 (IAUC 6272, 6284, 6285) is currently being detected through both pulsation and earth occultation. Pulsations with a period near 0.467 s became detectable on Dec. 17, rising to a 20- to 40-keV-rms-pulsed flux of 1.3(1) x 10E-9 erg cmE-1 sE-1 (170 mCrab) by 1997 Jan. 3. On Jan. 1.0, the intrinsic pulse frequency was 2.1408983(4) Hz, and the spin-up rate was 2.8(5) x l0E-12 HzsE-1. This assumes the following circular orbit parameters: P = 11.83665(14) days; epoch of longitude 90 deg = JD 2450126.9977(4) TDB; a sin i = 2.6371(5) light-s (determined using data from the 1995 Dec.-l996 Apr. outburst). Earth-occultation monitoring of flux from GRO J1744-28 shows its intensity increasing by a factor of about two during 1996 Dec. 25-1997 Jan. 6. Although source confusion prevents a precise flux estimate, subtracting a 25-percent background for unresolved galactic-center sources yields a persistent flux for Jan. 3-6 in the energy bands 20-30, 30-40, and 40-50 keV of 590, 470, and 290 mcrab, respectively."
    Schlagwort(e): Space Radiation
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