Phillip R. Blanco

In-Flight Performance of the High Energy X-Ray Timing Experiment on the Rossi X-Ray Timing Explorer

The High Energy X-Ray Timing Experiment (HEXTE) is one of three scientific instruments aboard the Rossi X-Ray Timing Explorer (RXTE), which was launched on 1995 December 30. RXTE performs timing and spectral studies of bright X-ray sources to determine the physical parameters of these systems. The HEXTE consists of two independent clusters of detectors, each cluster containing four NaI(Tl)/CsI(Na) phoswich scintillation counters sharing a common 1° FWHM field of view. The field of view of each cluster is switched on and off source to provide near real-time background measurements. The net open area of the eight detectors is 1600 cm2, and each detector covers the energy range 15-250 keV with an average energy resolution of 15.4% at 60 keV. The in-flight performance of the HEXTE is described, the light curve and spectrum of the Crab Nebula/pulsar is given, and the 15-240 keV spectrum of the weak source, active galaxy MCG +8-11-11 is presented to demonstrate the weak source spectral capabilities of HEXTE.

1E 0657–56: A Contender for the Hottest Known Cluster of Galaxies

We identify the extended Einstein IPC X-ray source 1E 0657-56 with a previously unknown cluster of galaxies at a redshift of z=0.296. Optical CCD images show the presence of a gravitational arc in this cluster, and galaxy spectra yield a cluster velocity dispersion of 1213+ 352−191 km s−1. X-ray data obtained with the ROSAT HRI and ASCA indicate that 1E 0657-56 is a highly luminous cluster in which a merger of subclusters may be occurring. The temperature of the hot gas in 1E 0657-56 is kT=17.4±2.5 keV, which makes it an unusually hot cluster, with important cosmological implications.

Structure of the Circumnuclear Region of Seyfert 2 Galaxies Revealed by Rossi X-Ray Timing Explorer Hard X-Ray Observations of NGC 4945

NGC 4945 is one of the brightest Seyfert galaxies on the sky at 100 keV, but is completely absorbed below 10 keV; its absorption column is probably the largest that still allows a direct view of the nucleus at hard X-ray energies. Our observations of it with the Rossi X-Ray Timing Explorer (RXTE) satellite confirm the large absorption, which for a simple phenomenological fit using an absorber with solar abundances implies a column of 4.5+0.4-0.4x1024 cm(-2). Using a more realistic scenario (requiring Monte Carlo modeling of the scattering), we infer the optical depth to Thomson scattering of approximately 2.4. If such a scattering medium were to subtend a large solid angle from the nucleus, it should smear out any intrinsic hard X-ray variability on timescales shorter than the light-travel time through it. The rapid (with a timescale of approximately 1 day) hard X-ray variability of NGC 4945 discovered by us with RXTE implies that the bulk of the extreme absorption in this object does not originate in a parsec-size, geometrically thick molecular torus. Instead, the optically thick material on parsec scales must be rather geometrically thin, subtending a half-angle less than 10 degrees, and it is likely to be the same disk of material that is responsible for the water maser emission observed in NGC 4945. Local number counts of Seyfert 1 and Seyfert 2 galaxies show a large population of heavily obscured active galactic nuclei (AGNs) which are proposed to make up the cosmic X-ray background (CXRB). However, for this to be the case, the absorption geometry in the context of axially symmetric unification schemes must have the obscuring material subtending a large scale height-contrary to our inferences about NGC 4945-implying that NGC 4945 is not a prototype of obscured AGNs postulated to make up the CXRB. The small solid angle of the absorber, together with the black hole mass (of approximately 1.4x106 M( middle dot in circle)) from megamaser measurements, allows a robust determination of the nuclear luminosity, which in turn implies that the source radiates at approximately 10% of the Eddington limit.

H I Absorption toward the Nucleus of the Powerful Radio Galaxy Cygnus A: Evidence for an Atomic Obscuring Torus?

We report the detection of broad (FWHM 270 km s-1) H I 21 cm absorption toward the compact (<15 h-1 pc) radio nucleus of the nearby powerful radio galaxy Cygnus A. The absorption corresponds to a column density of hydrogen atoms of at least 2.54 ± 0.44 × 1019 Tspin cm-2. Observations of OH and H2CO yielded upper limits. While other possibilities exist, we argue that the observed H I absorption plausibly occurs within a circumnuclear obscuring torus which is thought to block our direct view of a quasar nucleus in this object. We have attempted to constrain the properties of the obscuring gas by combining our H I result with upper limits on molecular absorption and estimates of the total obscuring column density from X-ray observations. One possibility is that the majority of the gas is in a hot (≈ 8000 K) mainly atomic phase; we derive limits on the size of such an atomic torus. Alternatively, the H I absorption might be caused by atomic gas within a warm (≈ 1000 K) mainly molecular torus. In this case, the nondetection of molecular absorption can possibly be explained by radiative excitation due to the central radio source. Follow-up VLBI observations are planned which will further constrain the properties of the absorbing gas and distinguish between the competing models.

The x-ray spectrum of the plerionic system psr b1509-58/msh 15-52

We present the results of observations of the PSR B1509

Observation of Centaurus A by the Rossi X-Ray Timing Explorer

-

ROSAT position of the transient bursting pulsar GRO J1744-28 and identification of its near-infrared counterpart

58/MSH 15

A qualitative test of a unified model of Seyfert galaxies with BeppoSAX

-

The hard X-ray spectrum of GRS1915+105

52 system in X-rays (

RXTE observations of Cas A

2-250