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Astron. Astrophys. 332, L29-L32 (1998)

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4. Discussion and conclusions

The BeppoSAX follow-up observation of the error box of GRB970111 was the first prompt follow-up observation of a GRB ever performed by an X-ray satellite. Before BeppoSAX the time-scale of a possible X-ray emission from GRB remnants was completely unknown. This first basically non-detection, therefore, could only be interpreted as an upper limit to the time-scale of the decline of an X-ray afterglow or to its flux. Now, with the detection of the X-ray afterglows of GRB970228, GRB970402, GRB970508 and GRB971214, BeppoSAX has set up a new scenario in which GRB970111 seems misplaced. Also the detection of the X-ray afterglow of a GRB (GRB970828, Remillard et al. 1997; Murakami et al. 1997) by the RossiXTE and the ASCA satellites supports the general framework for the GRBs' afterglow built by BeppoSAX.

GRB970228, GRB970402 and GRB970828 showed a similar behavior, with a fading X-ray counterpart continuously decaying from the GRB main emission into the afterglow following an approximate [FORMULA] law. In the case of GRB970228, the spectral analysis (Frontera et al. 1997b) confirms the continuity between the latest GRB emission and the X-ray counterpart detected after few hours. This temporal behaviour could be explained in the framework of the fireball model (Cavallo & Rees 1978; Rees & Meszaros 1992) as a highly radiative expansion of a relativistic shell. GRB970508 has shown a X-ray counterpart whose decay is more complicated than the above three. The above model could still account for this different behavior, but it needs to invoke a non-uniform surrounding medium, with a density scaling as [FORMULA] (Vietri 1997).

Whether 1SAX J1528.1+1937 is related to GRB970111 or not, this gamma-ray burst had a much faster decay than observed for any of the others. In order to make this clear, we compare a hypothetic power-law decay of GRB970111 with the "typical" power-law decay of GRB970228 reported in Costa et al. (1997a). Therefore, in Fig. 3 we assume that the new X-ray source is associated with the GRB and impose a power-law decay of the afterglow starting from the WFC mean flux at a time centred on the GRB X-rays duration. The needed power-law index is -1.5. Alternatively, assuming that 1SAX J1528.1+1937 is not related to GRB970111 we can derive a lower limit to the power-law index by using the upper limit of the MECS flux in the region of sky defined by the error box, to obtain a value very similar to the 1.5 value given above.

[FIGURE] Fig. 3. X-ray (2-10 keV) decay law of the candidate counterpart of GRB970111, compared to GRB970228. The dot-dashed and the solid horizontal lines are the mean X-ray flux for GRB970228 and GRB970111, respectively. The inclined dashed line is the decay law suggested by Costa et al. (1997a) for GRB970228. The inclined solid line shows the power-law index, 1.5, needed for connecting the WFC GRB970111 mean flux and the 1SAXJ 1528.1+1937 flux

Trying to extract GRB970111 from the group as an intrinsically different event, we note that its gamma-ray fluence is about more than three times larger than the largest of the other three. On the other hand, even if this GRB is of the "No High Eenergy" type (that is, it shows only weak emission above 300 keV, Pendleton et al. 1997), the ratio between the X-ray (2-10 keV) and gamma-ray (40-700 keV) fluences is about 4%, to be compared to 20% (2-10 keV) for GRB970228 (Frontera et al. 1997b), 5% (2-10 keV) for GRB970402 (Nicastro et al. 1997) and 40% (2-26 keV) for GRB970508 (Piro et al. 1997b). GRB970111 appears therefore as the one (together with the April event) with the less efficient low X-rays mechanism for energy release. Furthermore, no optical source was found in the WFC error box changing its intensity more than 0.5 magnitudes at a level of B=23 and R=22.6 from about 19 hours to about one month later (Castro-Tirado et al. 1997; Gorosabel et al. 1998). A radio search at 1.4 GHz (Frail et al. 1997) and at millimetric wavelength (Smith et al. 1997) did not find a counterpart to 1SAXJ1528.1+1937. These results support the idea that the optical, radio and millimetric channels are unefficient as well. Since GRB970111 was one of the brightest events ever detected in gamma-rays, one may conclude that its gamma-ray channel was efficient enough to dissipate most of the energy generated in the burst.

Alternative interpretations of the lack of X/optical/radio afterglow of the GRB970111 may be either a very rapidly evolving afterglow, with a decay law faster than observed in the other BeppoSAX GRB afterglows, or the absence of an afterglow source. The former hypothesis would be in agreement with the model by Tavani (1997) of a decay behavior represented by a power law with index [FORMULA] due to the observation in a fixed energy band (2-10 keV) of a synchrotron emission spectrum with a rapidly evolving critical frequency. Alternatively, the latter situation could be due, as an example, to the scenario in which the event that caused the GRB occurred in a region in which the interstellar medium density is low enough (perhaps the external regions of a host galaxy) to justify the absence of an external shock, possibly responsible for the afterglow emission in the other cases (Katz & Piran 1997).

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998