Kajal K. Ghosh

THE ULTRALUMINOUS X-RAY SOURCE POPULATION FROM THE CHANDRA ARCHIVE OF GALAXIES

One hundred fifty-four discrete non-nuclear ultraluminous X-ray (ULX) sources, with spectroscopically determined intrinsic X-ray luminosities greater than 10 39 ergs s � 1 , are identified in 82 galaxies observed with Chandra’s Advanced CCD Imaging Spectrometer. Source positions, X-ray luminosities, and spectral and timing characteristics are tabulated. Statistical comparisons between these X-ray properties and those of the weaker discrete sources in the same fields (mainly neutron star and stellar-mass black hole binaries) are made. Sources above � 10 38 ergs s � 1 display similar spatial, spectral, color, and variability distributions. In particular, there is no compelling evidence in the sample for a new and distinct class of X-ray object such as the intermediate-mass black holes. Eighty-three percent of ULX candidates have spectra that can be described as absorbed power laws ) �

Optical Variability of Blazars

In this paper we show the results of our monitoring the flux variability of 15 blazars, within the redshift range of 0.07-2.06, all with a visual magnitude range between 14 and 20 mag. Because blazars have displayed variability on diverse timescales, we have chosen to study them over both short and long timescales, ranging from minutes to years. These blazars were observed on 58 nights between 1995 March and 1998 June. Individual sources are discussed in detail. For example, one of our sources, Mrk 501, showed microvariability of as much as 0.13 mag change within 12 minutes. The usual parameters for size, mass, and luminosity are calculated. Relativistic beaming is implied by the data. We also searched for correlations between amplitude of variability, redshift, and luminosity. We found few, if any, such correlations. Discussion and conclusions are given for these and other results as implied by the data.

Chandra X-Ray Observations of the Spiral Galaxy M81

A Chandra X-Ray Observatory ACIS-S imaging observation is used to study the population of X-ray sources in the nearby Sab galaxy M81 (NGC 3031). A total of 177 sources are detected with 124 located within the D25 isophote to a limiting X-ray luminosity of � 3 � 10 36 ergs s � 1 . Source positions, count rates, luminosities in the 0.3–8.0 keV band, limiting optical magnitudes, and potential counterpart identifications are tabulated. Spectral and timing analysis of the 36 brightest sources are reported including the low-luminosity active galactic nucleus, SN 1993J, and the Einstein-discovered ultraluminous X-ray source X6. The nucleus accounts for � 86%, or 5 � 10 40 ergs s � 1 , of the total X-ray emission from M81. Its spectrum is well fitted by an absorbed power law with photon index 1:98 � 0:08, consistent with previous observations (average index 1.9). SN 1993J has softened and faded since its discovery. At an age of 2594 days, SN 1993J displayed a complex thermal spectrum from a reverse shock rich in Fe L and highly ionized Mg, Si, and S but lacking O. A hard X-ray component, emitted by a forward shock, is also present. X6 is spatially coincident with a stellar object with optical brightness and colors consistent with an O9–B1 main-sequence star. It is also coincident with a weak radio source with a flux density of � 95 lJy at � ¼ 3:6 cm. The continuum-dominated X-ray spectrum of X6 is most closely reproduced by a blackbody disk model suggesting the X-ray source is

CHANDRA DISCOVERY OF LUMINOUS SUPERSOFT X-RAY SOURCES IN M81

A Chandra ACIS-S imaging observation of the nearby galaxy M81 (NGC 3031) reveals nine luminous soft X-ray sources. The local environments, X-ray spectral properties, and X-ray light curves of the sources are presented and discussed in the context of prevailing physical models for supersoft sources. It is shown that the sample falls within expectations based on population synthesis models taken from the literature, although the high observed luminosities (Lobs � 2 � 10 36 –3 � 10 38 ergs s � 1 in the 0.2–2.0 keV band) and equivalent blackbody temperatures (Teff � 40 80 eV) place the brightest detected M81 objects at the high-luminosity end of the class of supersoft sources defined by previous ROSAT and Einstein studies of nearby galaxies. This is interpreted as a natural consequence of the higher sensitivity of Chandra to hotter and more luminous systems. Most of the sources can be explained as canonical supersoft sources: accreting white dwarfs powered by steady surface nuclear burning with X-ray spectra well fitted by hot white dwarf local thermodynamic equilibrium atmosphere models. An exceptionally bright source is scrutinized in greater detail since its estimated bolometric luminosity, Lbol � 1:5 � 10 39 ergs s � 1 , greatly exceeds theoretical estimates for supersoft sources. This source may be beyond the stability limit and undergoing a phase of mass outflow under extreme conditions. Alternatively, a model in which the observed X-ray spectrum arises from an accretion disk around a black hole of mass � 1200=ðcos iÞ 1=2 M� (viewed at an inclination angle i) cannot be excluded. Subject headings: binaries: symbiotic — stars: atmospheres — stars: evolution — white dwarfs — X-rays: stars

First Images from HERO: A Hard-X-Ray Focusing Telescope

We are developing a balloon-borne hard X-ray telescope that utilizes grazing-incidence optics. Termed HERO, for High-Energy Replicated Optics, the instrument will provide unprecedented sensitivity in the hard X-ray region and will achieve millicrab-level sensitivity in a typical 3 hr balloon-flight observation and 50 μcrab sensitivity on ultralong-duration flights. A recent proof-of-concept flight, featuring a small number of mirror shells, captured the first focused hard X-ray images of galactic X-ray sources. Full details of the payload, its expected future performance, and its recent measurements are provided.

The Herbig Ae Star HD 163296 in X-Rays

Chandra X-ray imaging spectroscopy of the nearby Herbig Ae star HD 163296 at 100 AU angular resolution is reported. A pointlike, soft (kT � 0:5 keV), emission-line source is detected at the location of the star with an X-ray luminosityof4;10 29 ergss � 1 (logLX/Lbol ¼� 5:48).Inaddition,faintemissionalongthedirectionofapreviously detected Ly� -emitting jet and Herbig-Haro outflow may be present. The relatively low luminosity, lack of a hard spectral component, and absence of strong X-ray variability in HD 163296 can be explained as originating from optically thin shock-heated gas accreting onto the stellar surface along magnetic field lines. This would require a (dipole) magnetic field strength at the surface of HD 163296 of at least � 100 G and perhaps as high as several kG. HD 163296 joins the T Tauri star TW Hya in being the only examples known to date of pre‐main-sequence stars whose quiescent X-ray emission appears to be completely dominated by accretion. Subject headings: stars: emission-line, Be — stars: individual (HD 163296) — stars: pre‐main-sequence — X-rays: stars

The Physical Nature of Polar Broad Absorption Line Quasars

In a recent article of Zhou et al., it has been shown, based on radio-variability arguments, that some BALQSOs (broad absorption line quasars) are viewed along the polar axis (orthogonal to accretion disk). These arguments are based on the brightness temperature Tb exceeding 1012 K, which leads to the well-known inverse Compton catastrophe unless the radio jet is relativistic and is viewed along its axis. In this Letter, we expand the Zhou et al. sample of polar BALQSOs to the entire SDSS DR5 (Sloan Digital Sky Survey Data Release 5). In the process, we clarify a mistake in their calculation of the brightness temperature. The expanded sample of high Tb BALQSOs has an inordinately large fraction of LoBALQSOs (low-ionization BALQSOs). We consider this an important clue to understanding the nature of the polar BALQSOs. This is expected in the polar BALQSO analytical/numerical models of Punsly, in which LoBALQSOs occur when the line of sight is very close to the polar axis, where the outflow density is the highest.

Properties of the Chandra Sources in M81

The Chandra X-Ray Observatory obtained a 50 ks observation of the central region of M81 using the ACIS-S in imaging mode. The global properties of the 97 X-ray sources detected in the inner 83 × 83 field of M81 are examined. Roughly half the sources are concentrated within the central bulge. The remainder are distributed throughout the disk, with the brightest disk sources lying preferentially along spiral arms. The average hardness ratios of both bulge and disk sources are consistent with power-law spectra of index Γ ~ 1.6, indicative of a population of X-ray binaries. A group of much softer sources is also present. The background-source-subtracted log N-log S distribution of the disk follows a power law of index ~-0.5 with no change in slope over three decades in flux. The log N-log S distribution of the bulge follows a similar shape but with a steeper slope above ~4 × 1037 ergs s-1. There is unresolved X-ray flux from the bulge with a radial profile similar to that of the bulge sources. This unresolved flux is softer than the average of the bulge sources, and extrapolating the bulge log N-log S distribution toward weaker sources can account for only 20% of the unresolved flux. No strong time variability was observed for any source with the exception of one bright, soft source.

Daytime Aspect Camera for Balloon Altitudes

We have designed, built, and flight-tested a new star camera for daytime guiding of pointed balloon-borne experiments at altitudes around 40 km. The camera and lens are commercially available, off-the-shelf components, but require a custom-built baffle to reduce stray light, especially near the sunlit limb of the balloon. This new camera, which operates in the 600- to 1000-nm region of the spectrum, successfully provides daytime aspect information of approx. 10 arcsec resolution for two distinct star fields near the galactic plane. The detected scattered-light backgrounds show good agreement with the Air Force MODTRAN models used to design the camera, but the daytime stellar magnitude limit was lower than expected due to longitudinal chromatic aberration in the lens. Replacing the commercial lens with a custom-built lens should allow the system to track stars in any arbitrary area of the sky during the daytime.

[ITAL]Chandra[/ITAL] Discovery of a 100 kiloparsec X-Ray Jet in PKS 0637−752

The quasar PKS 0637-753, the first celestial X-ray target of the Chandra X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to 12 arcsec west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous (~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new ATCA radio images at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space VLBI mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. HST images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet, and a radio component to the east, show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extra-nuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton (SSC) model, but this would imply extreme departures from the conventional minimum-energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.