K. E. McGowan

Northern Sky Variability Survey: Public Data Release*

The Northern Sky Variability Survey (NSVS) is a temporal record of the sky over the optical magnitude range from 8 to 15.5. It was conducted in the course of the first-generation Robotic Optical Transient Search Experiment (ROTSE-I) using a robotic system of four comounted unfiltered telephoto lenses equipped with CCD cameras. The survey was conducted from Los Alamos, New Mexico, and primarily covers the entire northern sky. Some data in southern fields between declinations 0° and -38° are also available, although with fewer epochs and noticeably lesser quality. The NSVS contains light curves for approximately 14 million objects. With a 1 yr baseline and typically 100–500 measurements per object, the NSVS is the most extensive record of stellar variability across the bright sky available today. In a median field, bright unsaturated stars attain a point-to-point photometric scatter of ~0.02 mag and position errors within 2. At Galactic latitudes |b| < 20°, the data quality is limited by severe blending due to the ~14 pixel size. We present basic characteristics of the data set and describe data collection, analysis, and distribution. All NSVS photometric measurements are available for on-line public access from the Sky Database for Objects in Time-Domain (SkyDOT) at Los Alamos National Laboratory. Copies of the full survey photometry may also be requested on tape.

The ROTSE‐III Robotic Telescope System

The observation of a prompt optical flash from GRB 990123 convincingly demonstrated the value of autonomous robotic telescope systems. Pursuing a program of rapid follow-up observations of gamma- ray bursts, the Robotic Optical Transient Search Experiment (ROTSE) has developed a next-generation instrument, ROTSE-III, that will continue the search for fast optical transients. The entire system was designed as an economical robotic facility to be installed at remote sites throughout the world. There are seven major system components: optics, optical tube assembly, CCD camera, telescope mount, enclosure, environmental sensing and protection, and data acquisition. Each is described in turn in the hope that the techniques developed here will be useful in similar contexts elsewhere.

XMM-NEWTON OBSERVATIONS OF PSR B1706-44

We report on the XMM-Newton observations of the young 102 ms pulsar PSR B1706-44. We have found that both a blackbody plus power law and a magnetized atmospheric model plus power law provide an excellent fit to the European Photon Imaging Camera (EPIC) spectra. The two scenarios are therefore indistinguishable on a statistical basis, although we are inclined to prefer the latter on physical grounds. In this case, assuming a source distance of ~2.3 kpc, the size of the region responsible for the thermal emission is R ? 13 km, compatible with the surface of a neutron star. A comparison of the surface temperature of PSR B1706-44 obtained from this fit with cooling curves favors a medium-mass neutron star with M ~ 1.45 or ~ 1.59 M?, depending on two different models of proton superfluidity in the interior. The large collecting area of XMM-Newton allows us to resolve a substructure in the broad soft X-ray modulation detected by Chandra, revealing the presence of two separate peaks with pulsed fractions of 7% ? 4% and 15% ? 3%, respectively.

The RAPTOR experiment: a system for monitoring the optical sky in real time

The Rapid Telescopes for Optical Response (RAPTOR) experiment is a spatially distributed system of autonomous robotic telescopes that is designed to monitor the sky for optical transients. The core of the ystem is composed of two telescope arrays, separated by 38 kilometers, that stereoscopically view the same 1500 square-degree field with a wide-field imaging array and a central 4 square-degree field with a more sensitive narrow-field ``fovea imager. Coupled to each telescope array is a real-time data analysis pipeline that is designed to identify interesting transients on timescales of seconds and, when a celestial transient is identified, to command the rapidly slewing robotic mounts to point the narrow-field ``fovea imagers at the transient. The two narrow-field telescopes then image the transient with higher spatial resolution and at a faster cadence to gather light curve information. Each ``fovea camera also images the transient through a different filter to provide color information. This stereoscopic monitoring array is supplemented by a rapidly slewing telescope with a low resolution spectrograph for follow-up observations of transients and a sky patrol telescope that nightly monitors about 10,000 square-degrees for variations, with timescales of a day or longer, to a depth about 100 times fainter. In addition to searching for fast transients, we will use the data stream from RAPTOR as a real-time sentinel for recognizing important variations in known sources. All of the data will be publically released through a virtual observatory called SkyDOT (Sky Database for Objects in the Time Domain) that we are developing for studying variability of the optical sky. Altogether, the RAPTOR project aims to construct a new type of system for discovery in optical astronomy---one that explores the time domain by mining the sky in real time.

On the Correlated X-Ray and Optical Evolution of SS Cygni

We have analyzed the variability and spectral evolution of the prototype dwarf nova system SS Cygni using RXTE data and AAVSO observations. A series of pointed RXTE/PCA observations allow us to trace the evolution of the X-ray spectrum of SS Cygni in unprecedented detail, while 6 years of optical AAVSO and RXTE/ASM light curves show long-term patterns. Employing a technique in which we stack the X-ray flux over multiple outbursts, phased according to the optical light curve, we investigate the outburst morphology. We find that the 3-12 keV X-ray flux is suppressed during optical outbursts, a behavior seen previously but only in a handful of cycles. The several outbursts of SS Cygni observed with the more sensitive RXTE/PCA also show a depression of the X-rays during optical outburst. We quantify the time lags between the optical and X-ray outbursts and the timescales of the X-ray recovery from outburst. The optical light curve of SS Cygni exhibits brief anomalous outbursts. During these events the hard X-rays and optical flux increase together. The long-term data suggest that the X-rays decline between outbursts. Our results are in general agreement with modified disk instability models, which invoke a two-component accretion flow consisting of a cool optically thick accretion disk truncated at an inner radius, and a quasi-spherical hot corona-like flow extending to the surface of the white dwarf. We discuss our results in the framework of one such model, involving the evaporation of the inner part of the optically thick accretion disk, proposed by Meyer & Meyer-Hofmeister.

Detection of Pulsed X-Ray Emission from XMM-Newton Observations of PSR J0538+2817

We report on the XMM-Newton observations of the 143 ms pulsar PSR J0538+2817. We present evidence for the first detections of pulsed X-rays from the source at a frequency that is consistent with the predicted radio frequency. The pulse profile is broad and asymmetric, with a pulse fraction of 18% ± 3%. We find that the spectrum of the source is well-fitted with a blackbody with T∞ = (2.12) × 106 K and NH = 2.5 × 1021 cm-2. The radius determined from the model fit of 1.68 ± 0.05 km suggests that the emission is from a heated polar cap. A fit to the spectra with an atmospheric model reduces the inferred temperature and, hence, increases the radius of the emitting region; however, the pulsar distance determined from the fit is then smaller than the dispersion distance.

EVIDENCE FOR SURFACE COOLING EMISSION IN THE XMM-NEWTON SPECTRUM OF THE X-RAY PULSAR PSR B2334+61

We report on the first XMM-Newton observation of the Vela-like pulsar PSR B2334+61. Spectral analysis reveals soft X-ray emission, with the bulk of the photons emitted at energies below ~1.5 keV. We find that the spectrum has a thermal origin and is well-fitted with either a blackbody or a magnetized, pure H atmospheric model. In the latter case, for a neutron star with a radius of 13 km and a magnetic field of 1013 G, the best fit gives a hydrogen column density NH = 0.33 × 1022 cm-2 and an effective temperature T = 0.65 × 106 K, as measured at Earth. A comparison of the surface temperature of PSR B2334+61 obtained from this fit with cooling curves favors a medium-mass neutron star with M ~ 1.45 M☉ or M ~ 1.6 M☉, depending on which of two different models of proton superfluidity in the interior is used. We do not detect any pulsed emission from the source and determine an upper limit of 5% for the modulation amplitude of the emission on the pulsars radio frequency.

XMM spectroscopy of the transient supersoft source RX J0513.9 − 6951: probing the dynamic white dwarf photosphere

The highly luminous (>1037 erg s?1) supersoft X-ray sources (SSSs) are believed to be Eddington-limited accreting white dwarfs undergoing surface hydrogen burning. The current paradigm for SSSs involves thermally unstable mass transfer from a 1–2M companion. However, this model has never been directly confirmed and yet is crucial for the evolution of cataclysmic variables (CVs) in general, and for the establishment of SSSs as progenitors of Type Ia supernovae in particular. The key SSS is RX J0513.9?6951 which has recurrent X-ray outbursts every 100–200 d (lasting for ?40 d) during which the optical brightness declines by 1 mag.We present the firstXMM observations of RX J0513.9?6951 through one of its optical low states. Our results show that as the optical low state progresses, the temperature and X-ray luminosity decrease, behaviour that is anti-correlated with the optical and ultraviolet (UV) emission. We find that as the optical (and UV) intensity recovers, the radius implied by thenspectral fits increases. The high-resolution spectra show evidence of deep-absorption features which vary during the optical low state. Our results are consistent with the predictions of the white dwarf photospheric contraction model proposed by Southwell et al.

Searching for Optical Transients in Real‐Time: The RAPTOR Experiment

A rich, but relatively unexplored, region in optical astronomy is the study of transients with durations of less than a day. We describe a wide‐field optical monitoring system, RAPTOR, which is designed to identify and make follow‐up observations of optical transients in real‐time. The system is composed of an array of telescopes that continuously monitor about 1500 square degrees of the sky for transients down to about 12th magnitude in 60 seconds and a central fovea telescope that can reach 16th magnitude in 60 seconds. Coupled to the telescope array is a real‐time data analysis pipeline that is designed to identify transients on timescales of seconds. In a manner analogous to human vision, the entire array is mounted on a rapidly slewing robotic mount so that the fovea of the array can be rapidly directed at transients identified by the wide‐field system. The goal of the project is to develop a ground‐based optical system that can reliably identify transients in real‐time and ultimately generate alerts...

Real-time detection of optical transients with RAPTOR

Fast variability of optical objects is an interesting though poorly explored subject in modern astronomy. Real-time data processing and identification of transient celestial events in the images is very important for such study as it allows rapid follow-up with more sensitive instruments. We discuss an approach which we have developed for the RAPTOR project, a pioneering closed-loop system combining real-time transient detection with rapid follow-up. RAPTORs data processing pipeline is able to identify and localize an optical transient within seconds after the observation. The testing we performed so far have been confirming the effectiveness of our method for the optical transient detection. The software pipeline we have developed for RAPTOR can easily be applied to the data from other experiments.