2.8. Quiescent Counterparts and the historical "No Host" Problem

One of the main obstacles in resolving the GRB mystery was the lack of identified counterparts in other wavelengths. This has motivated numerous attempts to discover GRB counterparts (for a review see [117, 118]). This is a difficult task - it was not known what to expect and where and when to look for it.

The search for counterparts is traditionally divided to efforts to find a flaring (burst), a fading or a quiescent counterpart. Fading counterparts - afterglow - have been recently discovered by BeppoSAX and as expected this discovery has revolutionized GRB studies. This allowed also the discovery of host galaxies in several cases, which will be discussed in the following section 2.9. Soft X-ray flaring (simultaneous with the GRB) was discovered in several bursts but it is an ambiguous question whether this should be considered as a part of the GRB itself or is it a separate component. Flaring has not been discovered in other wavelengths yet. Quiescent counter parts were not discovered either.

Most cosmological models suggest that GRBs are in a host galaxy. If so then deep searches within the small error boxes of some GRBs localized by the IPN system should reveal the host galaxy. until the discovery of GRB afterglow these searches have yielded only upper limits on the magnitudes of possible hosts. This has lead to what is called the "No Host" Problem. Schaefer et al. conducted searches in the near and far infrared [119] using IRAS, in radio using the VLA [120] and in archival optical photographs [121] and have found only upper limits and no clear counterpart candidates. Similar results from multiple wavelength observations have been obtained by Hurley et al. [67]. Vrba, Hartmann & Jennings [122] have monitored the error boxes of seven bursts for five year. They did not find any unusual objects. As for the "no host" problem this authors, as well as Luginbuhl et al. [123] and Larson, McLean & Becklin [124] concluded, using the standard galaxy luminosity function, that there are enough dim galaxies in the corresponding GRB error boxes which could be the hosts of cosmological burst and therefore, there is no "no host" problem.

More recently Larson & McLean [125] monitored in the infrared nine of the smallest error boxes of burst localized by the IPN with a typical error boxes of eight arc-min². They found in all error boxes at least one bright galaxy with K 15.5. However, the error boxes are too large to discern between the host galaxy and unrelated background galaxies. Schaefer et al. [126], searched the error boxes of five GRBs using the HST. Four of these are smaller boxes with a size of ~ 1 arc-min². They searched but did not find any unusual objects with UV excess, variability, parallax or proper motion. They have found only faint galaxies. For the four small error boxes the luminosity upper limits of the host galaxy are 10-100 times smaller than the luminosity of an L_* galaxy. Band & Hartmann [127] concluded that the error boxes of Larsen & McLean [125] are too large to discriminate between the presence or the absence of host galaxies. However, they find that the absence of host galaxies in the Schaefer et al. [126] data is significant, at the 2 ^. 10^-6 level. Suggesting that there are no bright hosts.

This situation has drastically changed and the "no host" problem has disappeared with afterglow observations. These observations have allowed for an accurate position determination and to identification of host galaxies for several GRBs. Most of these host galaxies are dim with magnitude 24.4 < R < 25.8. This support the conclusions of the earlier studies that GRBs are not associated with bright galaxies and definitely not with cores of such galaxies (ruling out for example AGN type models). These observations are consistent with GRBs rate being either a constant or being proportional to the star formation rate [128]. According to this analysis it is not surprising that most hosts are detected at R ~ 25. However, though these two models are consistent with the current data both predict the existence of host galaxies brighter than 24 mag, which were not observed so far. One could say now that the "no host" problem has been replaced by the "no bright host" problem. But this may not be a promlem but rather an indication on the nature of the sources.

The three GRBs with measured cosmological redshifts lie in host galaxies with a strong evidence for star formation. These galaxies display prominent emission lines from line associated with star-formation. In all three cases the strength of those lines is high for galaxies of comparable magnitude and redshift [16, 129, 130, 131, 128]. The host of GRB980703, for example, show a star forming rate of ~ 10 M yr^-1 or higher with a lower limit of 7 M yr^-1 [129]. For most GRBs with afterglow the host galaxy was detected but no emission or absorption lines were found and no redshift was measured. This result is consistent with the hypothesis that all GRBs are associated with star-forming galaxies. For those hosts that are at redshift 1.3 < z < 2.5 the corresponding emission lines are not observed as for this redshift range no strong lines are found in the optical spectroscopic window [131].

The simplest conclusion of the above observations is that all GRBs are associated with star forming regions. Still one has to keep in mind that those GRBs on which this conclusion was based had a strong optical afterglow, which not all GRBs show. It is possible that the conditions associated with star forming regions (such as high interstellar matter density - or the existance of molecular clouds) are essential for the appearance of strong optical afterglow and not for the appearance of the GRB itself.