Abstract. We present K-band imaging observations of ten Gamma-Ray Burst (GRB) host galaxies for which an optical and/or radio afterglow associated with the GRB event was clearly identified. Data were obtained with the Very Large Telescope and New Technology Telescope at ESO (Chile), and with the Gemini-North telescope at Mauna Kea (Hawaii). Adding to our sample nine other GRB hosts with K-band photometry and determined redshifts published in the literature, we compare their observed and absolute K magnitudes as well as their R − K colours with those of other distant sources detected in various optical, near-infrared, mid-infrared and submillimeter deep surveys. We find that the GRB host galaxies, most of them lying at 0.5 < ∼ z < ∼ 1.5, exhibit very blue colours, comparable to those of the faint blue star-forming sources at high redshift. They are sub-luminous in the K-band, suggesting a low stellar mass content. We do not find any GRB hosts harbouring R-and Kband properties similar to those characterizing the luminous infrared/submillimeter sources and the extremely red starbursts. Should GRBs be regarded as an unbiased probe of star-forming activity, this lack of luminous and/or reddened objects among the GRB host sample might reveal that the detection of GRB optical afterglows is likely biased toward unobscured galaxies. It would moreover support the idea that a large fraction of the optically-dark GRBs occur within dust-enshrouded regions of star formation. On the other hand, our result might also simply reflect intrinsic properties of GRB host galaxies experiencing a first episode of very massive star formation and characterized by a rather weak underlying stellar population. Finally, we compute the absolute B magnitudes for the whole sample of GRB host galaxies with known redshifts and detected at optical wavelengths. We find that the latter appear statistically even less luminous than the faint blue sources which mostly contributed to the B-band light emitted at high redshift. This indicates that the formation of GRBs could be favoured in particular systems with very low luminosities and, therefore, low metallicities. Such an intrinsic bias toward metal-poor environments would be actually consistent with what can be expected from the currently-favoured scenario of the "collapsar". The forthcoming launch of the SWIFT mission at the end of 2003 will provide a dramatic increase of the number of GRB-selected sources. A detailed study of the chemical composition of the gas within this sample of galaxies will thus allow us to further analyse the potential effect of metallicity in the formation of GRB events.

No abstract

Only a few of the dozen or so stellar-mass black holes have been observed away from the plane of the Galaxy 1 . Those few could have been ejected from the plane as a result of a "kick" received during a supernova explosion, or they could be remnants of the population of massive stars formed in the early stages of evolution of the Galaxy. Determining their orbital motion should help to distinguish between these options. Here we report the transverse motion (in the plane of the sky) for the black hole Xray nova XTE J1118+480 (refs 2-5), from which we derive a large space velocity. This X-ray binary has an eccentric orbit around the Galactic Centre, like most objects in the halo of the Galaxy, such as ancient stars and globular clusters. The properties of the system suggest that its age is comparable to or greater than the age of the Galactic disk. Only an extraordinary "kick" from a supernova could have launched the black hole into an orbit like this from a birth place in the disk of the Galaxy.The high galactic latitude (l = 157.7 • , b = +62.3 • ) X-ray nova XTE J1118+480 was discovered 2 with the RXTE All-Sky Monitor on 2000 March 29. It exhibited slow outbursts that lasted ∼7 months with a peak X-ray luminosity of 4 x 10 35 (D/kpc) 2 erg s −1 and energy spectra typical of black hole binaries in low/hard state 6 . From observations of the ∼19 mag optical counterpart 3 in quiescence a mass function for the compact object f(M) ∼ 6.0±0.4 M ⊙ and an average distance of ∼ 1.85±0.36 kpc were determined 4,5 . For ∼100 days the source exhibited a steady and slowly variable unresolved radio counterpart 7,8 with persistent inverted radio spectrum, which is interpreted as optically thick emission from a compact, powerful synchrotron jet 8,9 , that could have a size ≤0.03 AU 9 . 1To measure the transverse motion on the plane of the sky of XTE J1118+480 we carried out observations of the radio counterpart with the Very Long Baseline Array (VLBA) at 15.4 GHz (λ2 cm) and 8.4 GHz (λ3.6 cm) on 2000 May 4 and July 24; afterwards it faded below detection. In both epochs the source was unresolved by the synthesized beams of 1×0.6 milli arc sec (mas) and 2×1 mas respectively, which correspond to physical dimensions smaller than ∼0.7 D/kpc AU, and a brightness temperature ≥10 9 K. Because of the high galactic latitude and low column of interstellar gas along the line of sight (N H ∼ 10 20 cm −2 ) 10 , the VLBA images are relatively unaffected by Galactic electron scattering, allowing us to determine in each epoch the position of the compact, unresolved radio source with relative errors ≤0.35 mas.The observations were done in cycles that included the target, the strong calibrator 3C273, a primary calibrator (Cal. 1) used as phase-reference source, and a second extragalactic calibrator (Cal.2). Their high elevation and good weather ensured that the phase connection was successful. The epoch, source, frequency, bandwidth, position, and flux for the VLBA observations of XTE J1118+480 and the two extragalactic sources used as references ...