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

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3. BeppoSAX detection of 1SAX J1528.1+1937

The earliest ([FORMULA]) error box of GRB970111 was imaged with the narrow field X-ray instruments (NFI) onboard BeppoSAX: the Low Energy Concentrator Spectrometer (LECS, 0.1-10 keV, Parmar et al. 1997) and the three Medium Energy Concentrator Spectrometers (MECS, 2-10 keV, Boella et al. 1997b). This Target of Opportunity observation was started 59,400 s after the GRBM trigger time, from 12 January 02:14 to 13 January 06:01 UT, for a total net exposure time of 52,139 s with the MECS and 11,594 s with the LECS (the latter being operated only during satellite night-time).

At the time when the NFI observation was performed the WFC improved error box was not available and therefore any source included in the error box region was a potential counterpart for the GRB970111. Two relatively bright X-ray sources were detected in the [FORMULA] error box by the BeppoSAX/NFI (Butler et al. 1997), resolved into three sources in the ROSAT All Sky Survey (Voges et al. 1997). One of them, RXJ152845+1944.5, was also inside the early intersection of the WFC error box and the IPN error strip (Hurley et al. 1997). A peculiar radio source was detected with the VLA (Frail et al. 1997) in a position coincident with this X-ray source. The final WFC error box, however, excludes this source as possible counterpart to GRB970111. The latest WFC error box only includes an unknown X-ray source, 1SAX J1528.1+1937. The false colour image obtained from the MECS is shown in Fig. 2. The image shows the WFC error box, intersected by the IPN annulus, together with the MECS source error box.

[FIGURE] Fig. 2. False colour image of the MECS observation of the GRB970111. The WFC error box is shown as an irregular circle containing the MECS error circle of the X-ray source 1SAX J1528.1+1937. The latter lies outside the region of the WFC error box intersected by the IPN annulus. On the top left the source RXJ152845+1944.5 is clearly detected

The data analysis of the MECS image, performed with Ximage (Giommi et al. 1991), gives the position of the new source at RA= [FORMULA] and Decl.= [FORMULA], with a [FORMULA] error radius (90% confidence level). The probability that this detection is due to a background fluctuation is of the order of [FORMULA]. The count rate of 1SAX J1528.1+1937 is [FORMULA]  counts s-1 in the MECS (2-10 keV) and [FORMULA]  counts s-1 in the LECS (0.1-2 keV). Taking into account the vignetting correction for the off-axis position, and assuming a Crab-like energy spectrum, the above count rates correspond to fluxes of [FORMULA]  erg cm-2  s-1 in the 2-10 keV energy range and [FORMULA]  erg cm-2  s-1 in the 0.1-2 keV energy range.

The MECS error box of the new source 1SAX J1528.1+1937 is almost entirely contained within the WFC error box of GRB970111. Considering the 99% confidence IPN annulus the source 1SAX J1528.1+1937 is at a position only marginally consistent with GRB970111. Therefore, if we use the reduced WFC-IPN error box, the upper limit to the 2-10 keV flux of [FORMULA]  erg cm-2  s-1 (3 [FORMULA]).

In the context of the possible association of 1SAX J1528.1+1937 with GRB970111 it is interesting to note that dividing the NFI observation into three time intervals consisting of the first 10 ks, the following 15 ks, and the last 26 ks of exposure time, gives the count rates listed in Table 1. Even if the count rate is rather low, considering the combination of the temporal and positional coincidences and the indication of a decaying behavior, then the possibility of a random occurrence of 1SAX J1528.1+1937 in the error box of GRB970111 is lower than the [FORMULA] 3% derived from the source statistics of the ASCA GIS (Cagnoni et al. 1997).


Table 1. 2-10 keV flux variation of 1SAX J1528.1+1937. The start of the elapsed time is the GRB970111 trigger time

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998