We report on the discovery of two emission features observed in the x-ray spectrum of the afterglow of the gamma-ray burst (GRB) of 16 December 1999 by the Chandra X-ray Observatory. These features are identified with the Ly α line and the narrow recombination continuum by hydrogenic ions of iron at a redshift z = 1.00 ± 0.02, providing an unambiguous measurement of the distance of a GRB. Line width and intensity imply that the progenitor of the GRB was a massive star system that ejected, before the GRB event, a quantity of iron ∼0.01 of the mass of the sun at a velocity ∼0.1 of the speed of light, probably by a supernova explosion.

We report on Chandra observations of the central region of M31. By combining eight Chandra ACIS-I observations taken between 1999 and 2001, we have identified 204 X-ray sources within the central ∼ 17 ′ × 17 ′ region of M31, with a detection limit of ∼ 2 × 10 35 erg s −1 . Of these 204 sources, 22 are identified with globular clusters, 2 with supernova remnants, 9 with planetary nebula, and 9 as supersoft sources. By comparing individual images, about 50% of the sources are variable on time scales of months. We also found 13 transients, with light curves showing a variety of shapes. We also extracted the energy spectra of the 20 brightest sources; they can be well fit by a single power-law with a mean photon index of 1.8. The spectral shapes of 12 sources are shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at 1.7 × 10 37 erg s −1 ). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star-formation history might be different in different regions.Subject headings: galaxies: individual (M31) -X-rays: galaxies -X-rays: stars supersoft source populations (Di Stefano et al., in preparation) In this paper, we adopt a distance of 780 kpc (Stanek & Garnavich 1998;Macri et al. 2001) and assume a hydrogen column density equal to the Galactic value of ∼ 7 × 10 20 cm −2 (Dickey & Lockman 1990) unless otherwise specified. The quoted errors throughout this paper are at 1-σ confidence, unless otherwise specified. Observations and Data ReductionM31 was observed with Chandra regularly as part of the AO-1 and AO-2 GTO program during 1999-2001. The program was designed to search for transients. The observations consist of a series of HRC snapshots (∼ 1 ks) that cover the entire galaxy. If a transient is discovered in the HRC mosaic, then a follow-up ACIS image (∼ 5 ks) of the transient is obtained; otherwise an ACIS image of the nucleus is obtained. In this paper we focus only on the ACIS-I (I0, I1, I2 and I3) data obtained for the central 16.9 ′ × 16.9 ′ region of M31. These data consist of 8 separate observations, with exposure times ranging from 4 to 8.8 ks. The details of the observations are given in Table 1. The nuclear region of M31 was placed near the aim-point of the ACIS-I array. The focalplane temperature was −110 • C during the first four observations, and −120 • C for the others. The observations were made at various spacecraft roll angles; consequently, the total region covered by the observations is slightly larger than 16.9 ′ × 16.9 ′ , and sources near the outer edge of ACIS are not observable in all eight exposures.All data were telemetered in Faint mode and were collected with a frame transfer time of 3.2 s. In order to reduce the instrumental background, only data with ASCA grades of 0, 2, 3, 4, and 6 were included. We selected data free from bad columns, hot...

Spectroscopic observations of the fast X-ray transient and superluminal jet source SAX J1819.3[2525 (V4641 Sgr) reveal a best-Ðtting period of days and a semiamplitude of P spect \ 2

We consider the X-ray luminosity difference between neutron star and black hole soft X-ray transients (NS and BH SXTs) in quiescence. The current observational data suggest that BH SXTs are significantly fainter than NS SXTs. The luminosities of quiescent BH SXTs are consistent with the predictions of binary evolution models for the mass transfer rate if (1) accretion occurs via an ADAF in these systems and (2) the accreting compact objects have event horizons. The luminosities of quiescent NS SXTs are not consistent with the predictions of ADAF models when combined with binary evolution models, unless most of the mass accreted in the ADAF is prevented from reaching the neutron star surface. We consider the possibility that mass accretion is reduced in quiescent NS SXTs because of an efficient propeller and develop a model of the propeller effect that accounts for the observed luminosities. We argue that modest winds from ADAFs are consistent with the observations while strong winds are probably not.Comment: LateX, 37 pages, 7 figures; Accepted for publication in The Astrophysical Journa

Previously we claimed that Black Hole X-ray Novae (BHXN) in quiescence are much less luminous than equivalent Neutron Star X-ray Novae (NSXN). This claim was based on the quiescent detection of a single short period BHXN (A0620-00, P orb =7.8 hrs) and two longer period BHXN (GRO J1655-40, P orb =62.9 hrs; V404 Cyg, P orb =155.3 hrs), along with sensitive upper limits. We announce the detection of two more short period BHXN (GRO J0422+32, P orb =5.1 hrs; GS 2000+25, P orb =8.3 hrs), an upper limit for a third which is improved by two orders of magnitude (4U 1543-47, P orb =27.0 hrs) and a new, much lower quiescent measurement of GRO J1655-40. Taken together, these new Chandra measurements confirm that the quiescent X-ray luminosities of BHXN are significantly lower than those of NSXN. We argue that this provides strong evidence for the existence of event horizons in BHXN.