R. L. Aptekar

Parts I and II

Data are presented on the temporal structure, fluxes, energy spectra and coordinates of the sources of gamma-ray bursts detected in the KONUS experiment on Venera 11 and Venera 12 space probes in the period September 1978 to May 1979. The statistical distributions of gamma bursts in duration, intensity, and peak power, as well as the distribution of the burst sources over the celestial sphere presented are based on the updated KONUS information obtained until February 1980.

A giant periodic flare from the soft γ-ray repeater SGR1900+14

Soft γ-ray repeaters are transient sources of high-energy photons; they emit sporadic and short (about 0.1 s) bursts of ‘soft’ γ-rays during periods of activity, which are often broken by long stretches of quiescence. These objects are associated with neutron stars in young supernova remnants. The event of 5 March 1979 was the most intense burst to date, and the only one that showed a clear periodicity in the signal. Here we report the detection, on 27 August 1998, of an even more intense burst from a different soft γ-ray repeater. This event was characterized by ‘hard’ γ-rays at its peak, followed by a tail 300 s long with a soft spectrum and a clear periodicity of 5.16 s. The burst was probably initiated by a massive disruption of the crust of the neutron star, followed by an outflow of energetic particles rotating with the period of the star. A comparison of the events of 27 August 1998 and 5 March 1979 supports the idea that magnetic energy plays an important role in the genesis of such events. Although these giant flares are rare, they are not unique events and may occur at any time in a neutron stars activity cycle.

Multiwavelength observations of GRB 050820A : an exceptionally energetic event followed from start to finish

We present observations of the unusually bright and long γ-ray burst GRB 050820A, one of the best sampled broadband data sets in the Swift era. The γ-ray light curve is marked by a soft precursor pulse some 200 s before the main event; the lack of any intervening emission suggests that it is due to a physical mechanism distinct from the GRB itself. The large time lag between the precursor and the main emission enabled simultaneous observations in the γ-ray, X-ray, and optical bandpasses, something only achieved for a handful of events to date. While the contemporaneous X-rays are the low-energy tail of the prompt emission, the optical does not directly track the γ-ray flux. Instead, the early-time optical data appear consistent with the forward shock synchrotron peak passing through the optical and are therefore likely the beginning of the afterglow. On hour timescales after the burst, the X-ray and optical light curves are inconsistent with an adiabatic expansion of the shock into the surrounding region, but rather indicate that there is a period of energy injection. Observations at late times allow us to constrain the collimation angle of the relativistic outflow to 6°.8 ≾ θ ≾ 9°.3. Our estimates of both the kinetic energy of the afterglow (EKE = 5.2^(+7.9)_(4.1) × 10^(51) ergs) and the prompt γ-ray energy release (Eγ = 7.5^(+6.7)-(2.4) × 10^(51) ergs) make GRB 050820A one of the most energetic events for which such values could be determined.

GRB 060313: A New Paradigm for Short-Hard Bursts?

We report the simultaneous observations of the prompt emission in the gamma-ray and hard X-ray bands by the Swift BATand the Konus-Wind instruments of the short-hard burst, GRB 060313. The observations reveal multiple peaks in both the gamma-ray and hard X-ray bands suggesting a highly variable outflow from the central explosion. We also describe the early-time observations of the X-ray and UV/optical afterglows by the Swift XRT and UVOT instruments. The combination of the X-ray and UV/optical observations provides the most comprehensive light curves to date of a short-hard burst at such an early epoch. The afterglows exhibit complex structure with different decay indices and flaring. This behavior can be explained by the combination of a structured jet, radiative loss of energy, and decreasing microphysics parameters occurring in a circumburst medium with densities varying by a factor of approximately two on a length scale of 10 17 cm. These density variations are normally associated with the environment of a massive star and inhomogeneities in its windy medium. However, the mean density of the observed medium (n � 10 � 4 cm 3 ) is much less than that expected for a massive star. Although the collapse of a massive star as theoriginofGRB060313isunlikely,themergerofacompactbinaryalsoposesproblemsforexplainingthebehavior of this burst. Two possible suggestions for explaining this scenario are that some short bursts may arise from a mechanism that does not invoke the conventional compact binary model, or that soft late-time central engine activity is producing UV/optical but no X-ray flaring.

A Giant Flare from a Soft Gamma Repeater in the Andromeda Galaxy (M31)

The light curve, energy spectra, energetics, and IPN localization of an exceedingly intense, short-duration, hard-spectrum burst, GRB 070201, obtained from Konus-Wind, INTEGRAL (SPI-ACS), and MESSENGER data are presented. The total fluence of the burst and the peak flux are -->S = 2.00+ 0.10−0.26 × 10−5 erg cm−2 and -->Fmax = 1.61+ 0.29−0.50 × 10−3 erg cm−2 s−1. The IPN error box has an area of 446 arcmin2 and covers the peripheral part of the M31 galaxy. Assuming that the source of the burst is indeed in M31 at a distance of 0.78 Mpc, the measured values of the fluence S and maximum flux -->Fmax correspond to a total energy of -->Q = 1.5 × 1045 erg and a maximum luminosity -->L = 1.2 × 1047 erg s−1. These data are in good agreement with the corresponding characteristics of the previously observed giant flares from other soft gamma repeaters. The evidence for the identification of this event as a giant flare from a soft gamma repeater in the M31 galaxy is presented.

Giant flare in SGR 1806-20 and its Compton reflection from the Moon

We analyze the data obtained when the Konus-Wind gamma-ray spectrometer detected a giant flare in SGR 1806-20 on December 27, 2004. The flare is similar in appearance to the two known flares in SGR 0526-66 and SGR 1900+14 while exceeding them significantly in intensity. The enormous X-ray and gamma-ray flux in the narrow initial pulse of the flare leads to almost instantaneous deep saturation of the gamma-ray detectors, ruling out the possibility of directly measuring the intensity, time profile, and energy spectrum of the initial pulse. In this situation, the detection of an attenuated signal of inverse Compton scattering of the initial pulse emission by the Moon with the Helicon gamma-ray spectrometer onboard the Coronas-F satellite was an extremely favorable circumstance. Analysis of this signal has yielded the most reliable temporal, energy, and spectral characteristics of the pulse. The temporal and spectral characteristics of the pulsating flare tail have been determined from Konus-Wind data. Its soft spectra have been found to contain also a hard power-law component extending to 10 MeV. A weak afterglow of SGR 1806-20 decaying over several hours is traceable up to 1 MeV. We also consider the overall picture of activity of SGR 1806-20 in the emission of recurrent bursts before and after the giant flare.We analyze the data obtained when the Konus-Wind gamma-ray spectrometer detected a giant flare in SGR 1806-20 on December 27, 2004. The flare is similar in appearance to the two known flares in SGR 0526-66 and SGR 1900+14 while exceeding them significantly in intensity. The enormous X-ray and gamma-ray flux in the narrow initial pulse of the flare leads to almost instantaneous deep saturation of the gamma-ray detectors, ruling out the possibility of directly measuring the intensity, time profile, and energy spectrum of the initial pulse. In this situation, the detection of an attenuated signal of Compton back-scattering of the initial pulse emission by the Moon with the Helicon gamma-ray spectrometer onboard the Coronas-F satellite was an extremely favorable circumstance. Analysis of this signal has yielded the most reliable temporal, energy, and spectral characteristics of the pulse. The temporal and spectral characteristics of the pulsating flare tail have been determined from Konus-Wind data. Its soft spectra have been found to contain also a hard power-law component extending to 10 MeV. A weak afterglow of SGR 1806-20 decaying over several hours is traceable up to 1 MeV. We also consider the overall picture of activity of SGR 1806-20 in the emission of recurrent bursts before and after the giant flare.

On the possibility of identifying the short hard burst GRB 051103 with a giant flare from a soft gamma repeater in the M81 group of galaxies

The light curve, energy characteristics, and localization of the short hard burst GRB 051103 are considered. Evidence for identifying this event with a giant flare from a soft gamma repeater in the nearby M81 group of interacting galaxies is discussed.

Unusual Burst Emission from the New Soft Gamma Repeater SGR 1627–41

In June-July 1998 the Konus-Wind burst spectrometer observed a series of bursts from the new soft gamma repeater SGR 1627-41. Time histories and energy spectra of the bursts have been studied, revealing fluences and peak fluxes in the ranges of 3 ·10−7 7.5 ·10−6 erg cm−2 and 10−5 10−4 erg cm−2 s−1 respectively. One event, 18 June 6153.5s UT stands out dramatically from this series. Its fluence is ∼ 7 · 10−4 erg cm−2 and peak flux ∼ 2 · 10−2 erg cm−2 s−1. These values from a source at a distance of 5.8 kpc yield an energy output of ∼ 3 · 10 erg and maximum luminosity of ∼ 8 · 10 erg s−1, similar to the values for the famous March 5, 1979 and August 27, 1998 events. In terms of energy, this event is another giant outburst seen in a third SGR! However, this very energetic burst differs significantly from the other giant outbursts. It exhibits no separate initial pulse with a fast rise time, no extended tail, and no pulsations. It is rather similar to ordinary repeated bursts but is a few hundred times stronger in intensity. According to the magnetar model by Thompson and Duncan (1995) such a burst may be initiated by a strong starquake when a crust fracture propagates over the whole surface of a neutron star.In 1998 June-July, the Konus-Wind burst spectrometer observed a series of bursts from the new soft gamma repeater SGR 1627-41. Time histories and energy spectra of the bursts have been studied, revealing fluences and peak fluxes in the ranges 3 × 10-7 to 7.5 × 10-6 ergs cm-2 and 10-5 to 10-4 ergs cm-2 s-1, respectively. One event, 18 June 6153.5 s UT, stands out dramatically from this series. Its fluence is ~7 × 10-4 ergs cm-2, and its peak flux is ~2 × 10-2 ergs cm-2 s-1. These values from a source at a distance of 5.8 kpc yield an energy output of ~3 × 1042 ergs and a maximum luminosity of ~8 × 1043 ergs s-1 for isotropic emission, similar to the values for the famous 1979 March 5 and 1998 August 27 events. In terms of energy, this event is another giant outburst seen in a third soft gamma repeater! However, this very energetic burst differs significantly from the other giant outbursts. It exhibits no separate initial pulse with a fast rise time, no extended tail, and no pulsations. It is rather similar to ordinary repeated bursts, but is a few hundred times stronger in intensity. According to the magnetar model by Thompson & Duncan, such a burst may be initiated by a strong starquake when a crust fracture propagates over the whole surface of a neutron star.

Konus Catalog of Soft Gamma Repeater Activity: 1978 to 2000

Observational data on the bursting activity of all five known Soft Gamma Repeaters are presented. This information was obtained with Konus gamma-ray burst experiments on board Venera 11-14, Wind, and Kosmos-2326 spacecraft in the period from 1978 to 2000. These data on appearance rates, time histories, and energy spectra of repeated soft bursts obtained with similar instruments and collected together in a comparable form should be useful for further studies of SGRs.Observational data on the bursting activity of all five known soft gamma repeaters (SGRs) are presented. This information was obtained with Konus gamma-ray burst experiments on board Venera 11-14, Wind, and Kosmos 2326 in the period from 1978 to 2000. These data on rates, time histories, and energy spectra of repeated soft bursts, obtained with similar instruments and collected together in a comparable form, should be useful for further studies of SGRs.

Observations of Giant Outbursts from Cygnus X-1

We present interplanetary network localization, spectral, and time history information for seven episodes of exceptionally intense gamma-ray emission from Cyg X-1. The outbursts occurred between 1995 and 2003, with durations up to ~28,000 s. The observed 15-300 keV peak fluxes and fluences reached 3 × 10-7 ergs cm-2 s-1 and 8 × 10-4 ergs cm-2, respectively. By combining the triangulations of these outbursts we derive an ~1700 square arcminute (3 σ) error ellipse that contains Cyg X-1 and no other known high-energy sources. The outbursts reported here occurred both when Cyg X-1 was in the hard state as well as in the soft one and at various orbital phases. The spectral data indicate that these outbursts display the same parameters as those of the underlying hard and soft states, suggesting that they represent another manifestation of these states.