3. Results and discussion
Table 2. IR sources detected in the GRB field (CAM 6 pfov).
The 1.5 pfov image (Figs. 3 and 4), with much better resolution than the 6 pfov image, shows that there is at least one source close to object #31, labelled A. This source is a bright galaxy, the brightest extragalactic object within the X-ray error box (Pedersen et al. 1997b). Another nearby source (labelled B) could be scattered stray light (Table 3) and a further ISO measurement would be needed to confirm or refute that possibility.
Table 3. IR sources detected by ISO in the
The PHT 174 µm map (Fig. 5) shows no indication for the presence of a point source at the expected GRB position. A strong extended structure can be seen with an amplitude of about 3 MJy/sr which is probably due to infrared galactic cirrus. From the measured noise per pixel we derive an 5- upper limit of 350 mJy at 174 µm.
The first ISO observation was carried out during the second BeppoSAX pointing (4.63-5.21 April), when no X-ray source was detected, implying a decrease by a factor of three or more in the X-ray flux, with respect to the first pointing (Piro et al. 1997).
With the exception of one object, none of the 40 sources inside the GRB error box displays significant variability in the 8-days interval between the two succesive ISO pointings, above a 5- level which is 140 µJy in the inner parts of the raster and 300 µJy in the outer parts (Table 2). The above-mentioned IR source (# 20), the brightest one in the field of view, is the variable star BL Cir (Hoffmeister 1965), which lies just outside the error box of SAX J1450.1-6920. On the basis of its optical spectrum (a late G - early K type, Pedersen et al. 1997a) and its faintness, the object has been suggested to be a previously unknown R CrB star (Harrison et al. 1997). The strong IR excess observed in the ISO data would rather indicate that the object might have undergone a strong episode of dust formation, resembling a Mira-type star. It is very unlikely that this single object is responsible of the high-energy event, as no theoretical GRB model would account for the phenomenon, but it could be worthwhile to monitor this object from now on.
Among the variety of GRB models, a considerable fraction deals with accretion of matter onto a compact object through an accretion disk, which might emit significantly in the IR. The emission at IR wavelengths could be enhanced in the case that the source heats up the surrounding dust, producing large amounts of IR radiation after the event (McBreen, Plunkett and Metcalfe 1995). One of the few dozens of theoretical models involving GRBs at cosmological distance is the fireball model (Meszaros and Rees 1993a,b), in which the release of 1051 erg within a small region and in a very short time leads to the formation of a forward blast wave moving ahead of a fireball, sweeping up the interstellar matter, and producing an afterglow at frequencies gradually declining from X-rays to visible and radio wavelengths (Paczynski and Rhoads 1993, Meszaros and Rees 1997). This type of behaviour has recently been observed in several bursts. As the fireball continues to radiate at longer wavelengths which increase with time (Katz 1994), IR emission should eventually be detected, but we have failed to detect transient IR emission from GRB 970402 with ISO.
© European Southern Observatory (ESO) 1998
Online publication: January 8, 1998