Testing Protoplanetary Disk Alignment In Young Binaries

We present K-band (2.2 mum) imaging polarimetry that resolves 19 T Tauri binary and multiple systems in the Taurus-Auriga and Scorpius-Ophiuchus star-forming regions. We observed systems with projected separations 1. 5 - 7. 2 (similar to200 - 1000 AU) in order to determine the relative orientation of the circumstellar disks in each binary system. Scattered light from these disks is polarized, allowing us to deduce the position angle of the disk on the sky from the position angle of polarization even though our observations do not resolve the disks themselves. We detected measurable polarization ( typically 0.5% - 2%, with typical uncertainty 0.1%) from both stars in 14 of the systems observed. In eight of the nine binary systems, the two stars\u27 polarization position angles are within 30degrees of each other, inconsistent with random orientations. In contrast, the five triple and quadruple systems appear to have random disk orientations when comparing the polarization position angles of the widest pair in the system; the close pairs are unresolved in all but one system. Our observations suggest that disks in wide ( 200 - 1000 AU) binaries are aligned with each other within less than or similar to20degrees but not perfectly coplanar. However, we cannot conclusively rule out random relative disk orientations if the observed polarizations are significantly contaminated by interstellar polarization. Even in the presence of interstellar polarization our observations securely exclude coplanar disks. These results provide constraints on possible binary formation mechanisms if the observed orientations are primordial. On the other hand, models of disk-binary interactions indicate that the disks may have had time to decrease their relative inclinations since formation. If the common orientation of the disks in these binaries is a tracer of the binary orbital plane, then our results also have significance for the stability of planetary orbits, suggesting that planetary systems in wide binaries should be stable over 10(9) yr timescales

Previously Claimed(/Unclaimed) X-ray Emission Lines in High Resolution Afterglow Spectra

We review the significance determination for emission lines in the Chandra HETGS spectrum for GRB020813, and we report on a search for additional lines in high resolution Chandra spectra. No previously unclaimed features are found. We also discuss the significance of lines sets reportedly discovered using XMM data for GRB011211 and GRB030227. We find that these features are likely of modest, though not negligible, significance.Comment: 4 pages, 1 figures, to appear in Santa Fe GRB Conference Proceedings, 200

Chandra Observations of the Optically Dark GRB030528

The X-ray-rich GRB030528 was detected by the HETE satellite and its localization was rapidly disseminated. However, early optical observations failed to detect a counterpart source. In a 2-epoch ToO observation with Chandra, we discovered a fading X-ray source likely counterpart to GRB030528. The source brightness was typical of X-ray afterglows observed at similar epochs. Other observers detected an IR source at a location consistent with the X-ray source. The X-ray spectrum is not consistent with a large absorbing column.Comment: 4 pages, 1 figures, to appear in Santa Fe GRB Conference Proceedings, 200

Optical and X-ray Observations of the Afterglow to XRF030723

The X-ray-flash XRF030723 was detected by the HETE satellite and rapidly disseminated, allowing for an optical transient to be detected ~1 day after the burst. We discuss observations in the optical with Magellan, which confirmed the fade of the optical transient. In a 2-epoch ToO observation with Chandra, we discovered a fading X-ray source spatially coincident with the optical transient. We present spectral fits to the X-ray data. We also discuss the possibility that the source underwent a rebrightening in the X-rays, as was observed in the optical. We find that the significance of a possible rebrightening is very low (~1 sigma).Comment: 4 pages, 2 figures, to appear in Santa Fe GRB Conference Proceedings, 200

An Optical Study of BG Geminorum: An Ellipsoidal Binary with an Unseen Primar Star

We describe optical photometric and spectroscopic observations of the bright variable BG Geminorum. Optical photometry shows a pronounced ellipsoidal variation of the K0 I secondary, with amplitudes of ~0.5 mag at VRI and a period of 91.645 days. A deep primary eclipse is visible for wavelengths < 4400A; a shallower secondary eclipse is present at longer wavelengths. Eclipse timings and the radial velocity curve of the K0 secondary star indicate an interacting binary where a lobe-filling secondary, M_2 ~ 0.5 Msun, transfers material into a extended disk around a massive primary, M_1 ~ 4.5 Msun. The primary star is either an early B-type star or a black hole. If it did contain a black hole, BG Gem would be the longest period black hole binary known by a factor of 10, as well as the only eclipsing black hole binary system.Comment: 27 pages, includes 8 figures and 5 tables, accepted to A

The X-ray Position and Optical Counterpart of the Accretion-Powered Millisecond Pulsar XTE J1814-338

We report the precise optical and X-ray localization of the 3.2 ms accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray Observatory as well as optical observations conducted during the 2003 June discovery outburst. Optical imaging of the field during the outburst of this soft X-ray transient reveals an R = 18 star at the X-ray position. This star is absent (R > 20) from an archival 1989 image of the field and brightened during the 2003 outburst, and we therefore identify it as the optical counterpart of XTE J1814-338. The best source position derived from optical astrometry is R.A. = 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of the pulsar in outburst is best fit by an absorbed power-law (with photon index = 1.41 +- 0.06) plus blackbody (with kT = 0.95 +- 0.13 keV) model, where the blackbody component contributes approximately 10% of the source flux. The optical broad-band spectrum shows evidence for an excess of infrared emission with respect to an X-ray heated accretion disk model, suggesting a significant contribution from the secondary or from a synchrotron-emitting region. A follow-up observation performed when XTE J1814-338 was in quiescence reveals no counterpart to a limiting magnitude of R = 23.3. This suggests that the secondary is an M3 V or later-type star, and therefore very unlikely to be responsible for the soft excess, making synchroton emission a more reasonable candidate.Comment: Accepted for publication in ApJ. 6 pages; 3 figure

Scientific Highlights of the HETE-2 Mission

The HETE-2 mission has been highly productive. It has observed more than 250 GRBs so far. It is currently localizing 25 - 30 GRBs per year, and has localized 43 GRBs to date. Twenty-one of these localizations have led to the detection of X-ray, optical, or radio afterglows, and as of now, 11 of the bursts with afterglows have known redshifts. HETE-2 has confirmed the connection between GRBs and Type Ic supernovae, a singular achievement and certainly one of the scientific highlights of the mission so far. It has provided evidence that the isotropic-equivalent energies and luminosities of GRBs are correlated with redshift, implying that GRBs and their progenitors evolve strongly with redshift. Both of these results have profound implications for the nature of GRB progenitors and for the use of GRBs as a probe of cosmology and the early universe. HETE-2 has placed severe constraints on any X-ray or optical afterglow of a short GRB. It is also solving the mystery of "optically dark' GRBs, and revealing the nature of X-ray flashes.Comment: 10 pages, 9 figures, to appear in proc. "The Restless High-Energy Universe", Royal Tropical Institute, Amsterdam; revised text, added ref

HETE Observations of the Gamma-Ray Burst GRB030329: Evidence for an Underlying Soft X-ray Component

An exceptionally intense gamma-ray burst, GRB030329, was detected and localized by the instruments on board the High Energy Transient Explorer satellite (HETE) at 11:37:14 UT on 29 March 2003. The burst consisted of two \~10s pulses of roughly equal brightness and an X-ray tail lasting >100s. The energy fluence in the 30-400 keV energy band was 1.08e-4 erg/cm2, making GRB030329 one of the brightest GRBs ever detected. Communication of a 2 arcmin error box 73 minutes after the burst allowed the rapid detection of a counterpart in the optical, X-ray, radio and the ensuing discovery of a supernova with most unusual characteristics. Analyses of the burst lightcurves reveal the presence of a distinct, bright, soft X-ray component underlying the main GRB: the 2-10 keV fluence of this component is ~7e-6 erg/cm2. The main pulses of GRB030329 were preceded by two soft, faint, non-thermal bumps. We present details of the HETE observations of GRB030329.Comment: 22 pages, 5 figures, to be published in ApJ 617, no. 2 (10 December 2004). Referee comments have been incorporated; results of improved spectral analysis are include

Optical afterglows of gamma-ray bursts: a bimodal distribution?"

The luminosities of the optical afterglows of Gamma Ray Bursts, 12 hours (rest frame time) after the trigger, show a surprising clustering, with a minority of events being at a significant smaller luminosity. If real, this dichotomy would be a crucial clue to understand the nature of optically dark afterglows, i.e. bursts that are detected in the X-ray band, but not in the optical. We investigate this issue by studying bursts of the pre-Swift era, both detected and undetected in the optical. The limiting magnitudes of the undetected ones are used to construct the probability that a generic bursts is observed down to a given magnitude limit. Then, by simulating a large number of bursts with pre-assigned characteristics, we can compare the properties of the observed optical luminosity distribution with the simulated one. Our results suggest that the hints of bimodality present in the observed distribution reflects a real bimodality: either the optical luminosity distributions of bursts is intrinsically bimodal, or there exists a population of bursts with a quite significant grey absorption, i.e. wavelength independent extinction. This population of intrinsically weak or grey-absorbed events can be associated to dark bursts.Comment: 10 pages, 5 figures, accepted for publication in MNRA