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Astron. Astrophys. 342, 47-48 (1999)

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3. Discussion

The X-ray spectrum of GRB 921013b as seen by WATCH on Granat (8-60 keV) can be very roughly approximated by a power law, N(E) [FORMULA]. If we extrapolate the spectrum to the EUVE 58-174 Å bandpass, we obtain [FORMULA] which is the flux simultaneous with the GRB assuming no interstellar or intrinsic absorption. The EUVE observation took place 11.0-hours after the high energy event. Assuming the presence of an X-ray afterglow with a decaying flux following [FORMULA] as seen in GRB 970228 (Costa et al. 1997), GRB 970828 (Murakami et al. 1997, Greiner et al. 1997) and GRB 980329 (int Zand et al. 1998), the unabsorbed flux that should have been detected by EUVE 11.0-hours later is [FORMULA] or [FORMULA] counts s-1. This is one order of magnitude below the 2-[FORMULA] upper limit for the flux after the first EUVE pass, [FORMULA].

By summing the images over several passes scaled by the inverse of the assumed decay [FORMULA], we can place a limit on the EUV flux simultaneous with the burst. We calculate this 2[FORMULA] limit to be [FORMULA] 680 counts s-1 or 5.1 [FORMULA] 10- 9 erg cm-2 s-1. This is well above the expected value of [FORMULA] 7 [FORMULA] 10-10 erg cm-2 s-1.

For astronomical sources, absorption due to the interstellar medium will reduce the observed flux from the source. If the burster were a nearby neutron star, absorption from the local cloud surrounding the Sun would contribute an absorbing hydrogen column of [FORMULA] cm-2. Using the effective cross sections of Rumph et al (1994), and assuming the local helium is 25% ionized (Dupuis et al. 1995), we find that this absorption has a negligible effect on our limits. If we assume that the source is more distant (either in the Galaxy or extragalactic), we must consider the absorption due to the galactic interstellar medium. The galactic HI column along this line of sight is [FORMULA] [FORMULA] cm-2 (Stark et al. 1992). The resulting absorbed EUV flux 11.0-hours after the burst is then [FORMULA] or [FORMULA] counts s-1. This value is well below the detection limit of the EUV measurement. We have not included here the effect of intrinsic absorption which we know is present in GRB 970828 (Groot et al. 1998) and GRB 980329 (Palazzi et al. 1998). Therefore any fading source lying in the Galaxy or at cosmological distances would have been beyond the reach of EUVE .

Let us assume that the object responsible for the burst is a galactic neutron star. What constraints can we place on the basis of the EUVE observation? We have calculated the expected fluxes at Earth from a neutron star with 1 [FORMULA] and 10 km radius, as a function of its absorption, distance and temperature, following Hurley et al. (1995) and Pizzichini et al. (1986). The results are shown in Fig. 1. It is obvious that if a neutron star is the counterpart, it would have had to be distant or heavily absorbed to escape detection. A lower limit of [FORMULA] 30 pc (for T [FORMULA] 2 [FORMULA] 105 K and [FORMULA] [FORMULA] 1018) can be derived. This corresponds to a height above the plane of [FORMULA] pc.

[FIGURE] Fig. 1. The obtained EUVE sensitivity (horizontal thick line ) compared with the fluxes (58-174 Å) for neutron stars as function of distance, temperature and hydrogen column.

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998