S.-K. King

Repetitive patterns in rapid optical variations in the nearby black-hole binary V404 Cygni.

How black holes accrete surrounding matter is a fundamental yet unsolved question in astrophysics. It is generally believed that matter is absorbed into black holes via accretion disks, the state of which depends primarily on the mass-accretion rate. When this rate approaches the critical rate (the Eddington limit), thermal instability is supposed to occur in the inner disk, causing repetitive patterns of large-amplitude X-ray variability (oscillations) on timescales of minutes to hours. In fact, such oscillations have been observed only in sources with a high mass-accretion rate, such as GRS 1915+105 (refs 2, 3). These large-amplitude, relatively slow timescale, phenomena are thought to have physical origins distinct from those of X-ray or optical variations with small amplitudes and fast timescales (less than about 10 seconds) often observed in other black-hole binaries—for example, XTE J1118+480 (ref. 4) and GX 339−4 (ref. 5). Here we report an extensive multi-colour optical photometric data set of V404 Cygni, an X-ray transient source containing a black hole of nine solar masses (and a companion star) at a distance of 2.4 kiloparsecs (ref. 8). Our data show that optical oscillations on timescales of 100 seconds to 2.5 hours can occur at mass-accretion rates more than ten times lower than previously thought. This suggests that the accretion rate is not the critical parameter for inducing inner-disk instabilities. Instead, we propose that a long orbital period is a key condition for these large-amplitude oscillations, because the outer part of the large disk in binaries with long orbital periods will have surface densities too low to maintain sustained mass accretion to the inner part of the disk. The lack of sustained accretion—not the actual rate—would then be the critical factor causing large-amplitude oscillations in long-period systems.

The Taos Project:upper bounds on the population of small kuiper belt objects and tests of models of formation and evolution of the outer solar system

We have analyzed the first 3.75 years of data from the Taiwanese American Occultation Survey (TAOS). TAOS monitors bright stars to search for occultations by Kuiper Belt objects (KBOs). This data set comprises 5 × 10^5 star hours of multi-telescope photometric data taken at 4 or 5 Hz. No events consistent with KBO occultations were found in this data set. We compute the number of events expected for the Kuiper Belt formation and evolution models of Pan & Sari, Kenyon & Bromley, Benavidez & Campo Bagatin, and Fraser. A comparison with the upper limits we derive from our data constrains the parameter space of these models. This is the first detailed comparison of models of the KBO size distribution with data from an occultation survey. Our results suggest that the KBO population is composed of objects with low internal strength and that planetary migration played a role in the shaping of the size distribution.

Occultation of X-rays from Scorpius X-1 by small trans-neptunian objects

Since the discovery of the trans-neptunian objects (TNOs) in 1992, nearly one thousand new members have been added to our Solar System, several of which are as big as—or even larger than—Pluto. The properties of the population of TNOs, such as the size distribution and the total number, are valuable information for understanding the formation of the Solar System, but direct observation is only possible for larger objects with diameters above several tens of kilometres. Smaller objects, which are expected to be more abundant, might be found when they occult background stars, but hitherto there have been no definite detections. Here we report the discovery of such occultation events at millisecond timescales in the X-ray light curve of Scorpius X-1. The estimated sizes of these occulting TNOs are ≤100 m. Their abundance is in line with an extrapolation of the distribution of sizes of larger TNOs.

Detectability of Occultations of Stars by Objects in the Kuiper Belt and Oort Cloud

The serendipitous detection of stellar occultations by outer solar system objects is a powerful method for ascertaining the small end (r 15 km) of the size distribution of Kuiper Belt objects and may potentially allow the exploration of objects as far out as the Oort Cloud. The design and implementation of an occultation survey is aided by a detailed understanding of how diffraction and observational parameters affect the detection of occultation events. In this study, stellar occultations are simulated, accounting for diffraction effects, finite source sizes, finite bandwidths, stellar spectra, sampling, and signal-to-noise ratios. Finally, the possibility of detecting small outer solar system objects from the Kuiper Belt all the way out to the Oort Cloud is explored for three photometric systems: a proposed space telescope, Whipple, the Taiwanese-American Occultation Survey, and the MMT.

Millisecond dip events in the 2007 RXTE/PCA data of Sco X−1 and the trans-Neptunian object size distribution

Millisecond dips in the RXTE/PCA archival data of Sco X−1 taken from 1996 to 2002 were reported recently. Those dips were found to be most likely caused by instrumental dead time but may also contain some true astronomical events, which were interpreted as the occultation of X-rays from Sco X−1 by trans-Neptunian objects (TNOs) of 100 m size. Here, we report the results of search for millisecond dip events with the new RXTE/PCA data of Sco X−1 taken in the year 2007. Adopting the same selection criteria as that in the previous study, we found only three dip events in 72-ks data, much fewer than the 107 events found in the 560-ks data taken from 1996 to 2002 reported earlier. The new data provide more detailed information of individual ‘very large events’ (VLEs), which is not available in the old archival data. Although the number of VLEs does not obviously increase during the occurrence of dip events, all the three dip events are coincident in time with VLEs that have no flags set for any of the propane or the six main xenon anodes. It is a strong indication of instrumental effects. No significant dips which might be real occultation by 60‐100 m TNOs were observed. With only 72-ks data, however, the previously proposed possibility that about 10 percent of the dip events might not be instrumental still cannot be strictly excluded. Using the absence of those anomalous VLEs as the criterion for identifying non-instrumental dip events, we found, at a lower confidence level, four dip events of duration 8‐10 ms in the 72-ks data. Upper limits to the size distribution of TNOs at the small size end are suggested.

The Taiwanese-American Occultation Survey: The Multi-Telescope Robotic Observatory

The Taiwanese-American Occultation Survey (TAOS) operates four fully automatic telescopes to search for occultations of stars by Kuiper Belt Objects. It is a versatile facility that is also useful for the study of initial optical GRB afterglows. This paper provides a detailed description of the TAOS multi-telescope system, control software, and high-speed imaging.

Millisecond dips in the RXTE/PCA light curve of Sco X-1 and trans-Neptunian object occultation

Millisecond dips in the RXTE/Proportional Counter Array (PCA) light curve of Sco X-1 were reported recently, which were interpreted as the occultation of X-rays from Sco X-1 caused by trans-Neptunian objects (TNO) of 100-m size. Inconclusive signatures of possible instrumental effects in many of these dip events related to high-energy cosmic rays were later found and the TNO interpretation became shaky. Here, we report more detailed analysis aiming at distinguishing true occultation events from those related to cosmic rays. Based on some indicative criteria derived from housekeeping data and two-channel spectral information, we suggest that about 10 per cent of the dips are probable events of occultation. The total number of TNOs of size from 60 to 100 m is estimated to be about 10 15 accordingly. Limited by the

TAOS: The Taiwanese–American Occultation Survey

The Taiwanese–American Occultation Survey (TAOS) seeks to determinethe number and size spectrum for small (∼3 km) bodies in the KuiperBelt. This will be accomplished by searching for the brief occultations of brightstars (R∼14) by these objects. We have designed and built a specialpurpose photometric monitoring system for this purpose. TAOS comprisesfour 50 cm telescopes, each equipped with a 2048×2048 pixel CCDcamera, in a compact array located in the central highlands of Taiwan. TAOSwill monitor up to 3,000 stars at 5 Hz. The system will go into scientificoperation at the end of 003.

Upper Limits on the Number of Small Bodies in Sedna-Like Orbits by the TAOS Project

We present the results of a search for occultation events by objects at distances between 100 and 1000 AU in light curves from the Taiwanese-American Occultation Survey. We searched for consecutive, shallow flux reductions in the stellar light curves obtained by our survey between 2005 February 7 and 2006 December 31 with a total of ~4.5 × 10^9 three-telescope simultaneous photometric measurements. No events were detected, allowing us to set upper limits on the number density as a function of size and distance of objects in Sedna-like orbits, using simple models.

First Results From The Taiwanese-American Occultation Survey (TAOS)

Results from the first 2 years of data from the Taiwanese-American Occultation Survey (TAOS) are presented. Stars have been monitored photometrically at 4 or 5 Hz to search for occultations by small (~3 km) Kuiper Belt objects (KBOs). No statistically significant events were found, allowing us to present an upper bound to the size distribution of KBOs with diameters 0.5 km < D < 28 km.