Allyn Dullighan

TESTING PROTOPLANETARY DISK ALIGNMENT IN YOUNG BINARIES

We present K-band (2.2 � m) 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 1B5–7B 2( � 200–1000 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’ polarization position angles are within 30 � 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 P20 � 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. Subject headings: circumstellar matter — planetary systems: formation — planetary systems: protoplanetary disks — polarization — stars: formation — stars: pre–main-sequence

Scientific highlights of the HETE-2 mission

Abstract The High Energy Explorer Satellite 2 (HETE-2) mission has been highly productive. It has observed more than 250 γ-ray bursts (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 redshift determinations. HETE-2 has also observed more than 45 bursts from soft γ-ray repeaters, and more than 700 X-ray bursts. 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 may be correlated with redshift; such a correlation would imply 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 has made it possible to explore the previously unknown behavior of optical afterglows at very early times, and has opened up the era of high-resolution spectroscopy of GRB optical afterglows. It is also solving the mystery of “optically dark” GRBs, and revealing the nature of X-ray flashes.

High-energy observations of XRF 030723: Evidence for an off-axis gamma-ray burst?

We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-ray Monitor (WXM) and French Gamma Telescope observations of XRF 030723 along with observations of the XRF afterglow made using the 6.5 m Magellan Clay telescope and Chandra. The observed peak energy E of the νFν burst spectrum is found to lie within (or below) the WXM 2-25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best-fit value is E = 8.4 keV. The ratio of X-ray to γ-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy Eiso and Epk satisfy the relation discovered by Amati et al. (2002), a redshift of z = 0.38 can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that the X-ray fade may actually trace the optical fade. If this is the case, the late-time rebrightening observed in the optical cannot be due to a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium that is due to a progenitor wind.

Optical and X‐ray Observations of the Afterglow to XRF 030723

The X‐ray‐flash XRF 030723 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 (∼ 1σ).

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.

IR and Optical Observations of GRB 030115

We present an upper limit on the brightness of the afterglow of the long GRB 030115 measured from Infrared (IR) images taken with the Magellan Classic Cam instrument of Ks > 22 at 6.2 days after the burst. We also present measurments of the host galaxy of GRB 030115 from archival optical and IR HST images taken with the Advanced Camera for Surveys and the Near Infrared Camera and Multi Object Spectrometer 25+ days after the burst. GRB 030115 is classified as an Optically Dark Burst, as its afterglow was found in the J, H, and K IR bands after a null result was reported in the optical. It is the first HETE GRB to have its afterglow found initially in the IR.

Chandra Observations of the Optically Dark GRB 030528

The X‐ray‐rich GRB 030528 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 the likely counterpart to GRB 030528. 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.

The Growing SXC Burst Catalog: A Transient for Each Detection

The first year of full SXC operations has resulted in the detection of over a dozen GRBs and XRFs, and the SXC GRB catalog continues to grow at a rate of approximately 1/month. The small error circles, rapid position dissemination, and most importantly, aggressive follow up observations have led to a remarkable trend: nearly all SXC detections have an associated optical/IR/radio/X‐ray counterpart. The systematic errors dominating the SXC astrometry are being reduced, and recent recalibration efforts will result in smaller error radii sent out to the GCN. Flight localizations calculated tens of seconds after the burst onset will also enable large telescopes with small fields of view to engage in rapid, deep searches. These two factors will aid in building a unique database of GRBs with associated transients.

A Study of the Background in the HETE Equatorial Orbit

After a year of operation, we have become well acquainted with HETE’s in‐orbit background. In particular, as it is near solar maximum, we have observed through intense periods of geomagnetic disturbance. We describe the resulting deviations from our mean backgrounds. Aside from these periods, the background has been quite stable over this first year. We compare the averaged background in the WXM and Fregate instruments for the months of March and September, 2001.