GRB follow-up team

The most important achievements

2005

Observations

*       GRB 050408    One of the most important achievements of SAO in 2005.

*       GRB 050509b

*       GRB 050824

*       PSR 0656+14 and Geminga

Theory

*       The cosmic gamma-ray bursts lasting more than 2 seconds (long GRBs, 2-100 sec) are now thought to be related to explosions of massive supernovae (SNe). They are idenitified in optics with very distant (host) galaxies at cosmological distances (more than 1 GPs) with red shifts about 1 (and up to z ~10). Rusults of our investigations (astro-ph/0506544) were used in modelling of energy distribution in spectra of one of such galaxies (GRB 021004, z=2.3).
      A gamma-ray burst can be accompanied with an optical afterglow lasting several days in which typical spectral and phorometric features of massive supernovae usually of types Ib or Ic are observed, as was in the case of GRB 030329 (astro-ph/0505535). In 2005 we studied a host galaxy of this (famous) GRB 030329 - a galaxy at z=0.17 with low metallicity and active star formation . Spectra and spectral energy distibution were obtained (astro-ph/0508541, astro-ph/0507488, a paper was accepted for publishing in A&A). The figure shows spectral energy distribution for a host galaxy of GRB 030329 from optical to IR.

*       In 2005 we together with researchers of the Space Reasearch Institute of RAS, Astro Space Center of RAS and Astronomical Institute of Saint-Petersburg State University studies effects of collimation of high-energy photons and other possible observational manifestations of angular and spectral distribution of photons in gamma-ray burst sources. The matter is about an alternative to relativistic fireball, if all "long" GRBs are really reltated with normal/nonpeculiar massive supernovae. Within the framework of assumptions of our model we considered effects of radiation pressure and how jets originate due to even small asymmetry of powerful radiation field in the burst source itself. Possible mechanisms of GRB origin in regions of size 10{8} cm (a compact model of gamma-ray bursts) are discussed. Observational consequencies of such "compact" energy release for GRBs are considered. (4 communications are published and submitted for publishing. A paper was accepted for publishing in "Astrofizika" journal.)

2004

Joint observational requests, in which an active participation of SAO in follow-up observations with the new specialized space platform SWIFT is supposed, are prepared and partially realized. We observed with the BTA the optical objects related with gamma-ray bursts: GRB040924 (observations of the optical transient in V, Rc and Ic filters), GRB041006 (observations of the optical transient in B, V, Rc and Ic filters), GRB041218 (the spectrum was obtained in 6 hours after the burst), GRB041219 (the spectrum of the transient in B, V, Rc filters was obtained in 2 days after the burst). New data for the GRB030329 host galaxy in V and Rc filters were obtained with the BTA.

All known results of the earliest spectroscopic and photometrical observations of gamma-ray bursts were reviewed and compared to all known earliest spectra of some massive core-collapse supernovae (SN). The summarizing of our (SAO) and other spectroscopic and photometrical data showed that the properties of early spectra/colors of optical transients of gamma-ray bursts and massive SNe are explained by a shock wave moving through stellar wind generated by the pre- supernova/pre-gamma-ray-burst. The behavior of light curves, colors and gamma-ray burst spectra (which is very similar to a ultra-violet jump observed for some SNe) is a result of dense compact envelopes near massive star-progenitors of SNe and gamma-ray bursts. The importance is emphasized of observations of namely early SNe and gamma-ray bursts spectra for understanding a crucial question in the physical mechanism of gamma-ray bursts: are ALL “long” gamma-ray bursts related indeed with massive SNe?

The analysis of results of the 6-meter telescope observations (+ data of observations with other instruments) shows a close relation between gamma-ray bursts and explosions of massive stars (with formation of a compact object of type of a neutron star) as a result of such an explosion. The basic principles of a compact model of the gamma-ray burst source are formulated.

2003

The earliest spectra were obtained with the BTA of an optical transient (OT) related to the gamma-ray burst GRB 030329. Typical broad details, which are already in these early spectra, point to a direct relation with a supernova (SN) burst. In the first hours the OT GRB spectrum can be a mixture of a GBR afterglow spectrum and a early UV spectrum of a massive Ib/c type SN. It could be a crucial argument in favor of an idea that a cosmic gamma-ray burst is an explosion onset of distant "massive" SNs or a collapse of massive stellar nuclei in the end of their evolution.

In 2001-2002 the effect of OT GRB 970508 reddening in several weeks after GRB was interpreted as an effect confirming the relation between long-duration GRBs and the evolution of massive stars and SN explosions, whereas nonmonotonies in the OT GRB 970508 light curve in one-three days could be a consequence of the following evolution scenario for a GRB source: "a massive star --> a WR star --> a pre-SN = a pre-GRB --> GRB and an explosion of "massive" Ib/c type SNs".

A technique of observations with a new focal reducer SCORPIO was developed. BVRI photometry of the GRB 000926 host galaxy (B=26.13) was carried out with the BTA. All objects down to a BTA-record depth of B = 27.6, V = 26.8, R = 26.8, I = 25.6 were measured photometrically in its field of dimension of ~3 arc min. By BTA/SCORPIO BVRI fluxes the photometric red shifts z were obtained for all objects and the Hubble B-diagram (B value vs. z) was plotted up to z~3.6.

Observational manifestations of different type absorption laws in GRB galaxies were studied. The modeling of energy distribution over their spectra was carried out; luminosities, ages and inner absorptions were determined; the impact of the latter on estimates of star-forming rates or SNe rate was accounted for. A question about "dark" (optically invisible) GRBs whose sources can be in highly dusty regions of galaxies was studied. A conclusion was formulated that the GRB galaxies do not differ in their spectra from other "field" galaxies, i.e. from normal galaxies of the same z. It means that we can determine an average rate of GRB observations directly from the count of these galaxies: ~5*10^{-8} GRBs per year in the Galaxy.

2002

An interpretation of the GRB 970508 reddening effect in several weeks after the burst (similar to 7 other GRBs with z<1) as an effect that directly confirms the relation between long gamma-ray bursts and the evolution of massive stars and supernova bursts. OT GRB 970508 non-homogeneities of kind of the second burst in one-three days after GRB can be a direct consequence of an evolution scenario for the gamma-ray burst source: "a massive star --> a Wolf-Rayet star --> a pre-supernova = pre-GRB --> GRB and the supernova of Ib/c type explosion".

Photometric red shifts for all extended objects with S/N>3 and "Hubble B diagrams" (relations between the stellar magnitude in B filter and z) were constructed for the objects with the red shifts about 3 in the field of the gamma-ray burst GRB 000926 host galaxy in four (BVRI) fluxes measured with the BTA/SCORPIO. Both the host galaxies (with spectroscopic z) and field galaxies obey well the Hubble general dependence for distant objects. Observational manifestations of different types of internal extinction laws in the gamma-ray burst host galaxies were studied. By the results of photometric z estimations in the field of GRB 000926 the galaxies-candidates were selected in the spectra of which an absorption line at the wavelength of 2175\AA (the absorption line of graphite dust) can present. A review of data on optical photometry and spectroscopy of all gamma-ray burst host galaxies known by the end of 2002 (including spectra with continuum of GRB optical transients) was also made. The results are presented in the form of the Hubble R diagram.

The field of OT GRB 021004 was investigated at the BTA with the SCORPIO device on the 29th, 30th of November and the 5th of December. An extended object (a blue host galaxy of gamma-ray burst with B=24.55, V=24.35, Rc=24.36, Ic=23.79) was detected which coincides with the position of an Optical transient (OT GRB) of this burst observed in October 2002.

2001

According to energy distribution in spectra obtained in BTA observations, the gamma-ray burst host galaxies do not differ from ordinary “field” galaxies of similar stellar magnitudes and red shifts. It allows us to determine an average year rate of gamma-ray bursts directly from observations.

2000

By data, obtained in BTA observations, we measured the red shift Z=0.7063 ± 0.0017 of a distant galaxy of 24^th st. magn., in which there occurred a burst of a massive star accompanied by the gamma-ray burst GRB 991208. The evolution of a corresponding variable optical source was watched.

In cooperation with Space Science Institute (USA) and Astrophysical Institute of Andalucia (Spain).

1998

During a year the program on search for host galaxies of gamma-ray bursts has been carried out at the BTA. Blue compact sources were discovered in the area of optical transients GRB 970508 and GRB 980703. The identification of GRB 980519 with the today’s most distant host galaxy was fulfilled.

1997

As a result of BTA simultaneous 4-band observations we obtained the most detailed light curve of an optical stellar-like source corresponding to the gamma-ray burst of May, 8, 1997. The slope of continuous spectrum was measured in maximum and after it. The variability of colors has been watched up to 200 days after the gamma-ray burst. The brightness fall rate and the color indexes were changing. The effect of a sharp rate deceleration is noticeable in infrared (~8000A) in 36 days after the burst. These new facts affect essentially the modern notion of physical nature of gamma-ray bursts. Perhaps we observe a formation of a nebula related to the outburst in the moment of gamma-ray burst caused by the activity of a compact object of type of a neutron star.

1996

The BVRI CCD photometry of the pulsar PSR 0656+14 was carried out with BTA. By VLA radio source location a stellar-like object was identified. The flux in R and I bands were first determined what allowed us (taking into account the UV observations with HST) to speak confidently about essentially non-thermal nature of radiation in UV spectral optical part up to ~8000A. The optical spectrum can be presented as a sum of non-thermal spectrum of atmosphere (surface) of a neutron star and a non-thermal component (power-law spectrum with the index of about -0.85).

(In cooperation with Astro-Space Center of PhIAS and Penn State University, USA).

1994

Deep CCD images of the localization areas of bright gamma-ray bursts GRB 790418 and GRB 790613 were first obtained. In both cases a faint blue stellar-like object (V=24.5m, B-V < 0m) was discovered in the gamma-ray bursts localization areas. The observed brightness and colors allows us to suppose that it can be objects of type of neutron stars at distance of about 40 pc.

(See SAO’s Reports)