XMM-Newton observes flaring in the polar UZ For during a low state

During an XMM-Newton observation, the eclipsing polar UZ For was found in a peculiar state with an extremely low X-ray luminosity and occasional X-ray and UV flaring. For most of the observation, UZ For was only barely detected in X-rays and about 800 times fainter than during a high state previously observed with ROSAT. A transient event, which lasted about 900 s, was detected simultaneously by the X-ray instruments and, in the UV, by the Optical Monitor. The transient was likely caused by the impact of 10^17-10^18 g of gas on the main accretion region of the white dwarf. The X-ray spectrum of the transient is consistent with 7 keV thermal bremsstrahlung from the shock-heated gas in the accretion column. A soft blackbody component due to reprocessing of X-rays in the white dwarf atmosphere is not seen. The likely origin of the UV emission during the transient is cyclotron radiation from the accretion column. We conclude from our analysis that the unusual flaring during the low state of UZ For was caused by intermittent increases of the mass transfer rate due to stellar activity on the secondary.Comment: To be published in MNRAS, 8 pages, 4 figure

First XMM-Newton observations of strongly magnetic cataclysmic variables - II. Timing studies of DP Leo and WW Hor

XMM-Newton was used to observe two eclipsing, magnetic cataclysmic variables, DP Leo and WW Hor, continuously for three orbital cycles each. Both systems were in an intermediate state of accretion. For WW Hor we also obtained optical light curves with the XMM-Newton Optical Monitor and from ground-based observations. Our analysis of the X-ray and optical light curves allows us to constrain physical and geometrical parameters of the accretion regions and derive orbital parameters and eclipse ephemerides of the systems. For WW Hor we directly measure horizontal and vertical temperature variations in the accretion column. From comparisons with previous observations we find that changes in the accretion spot longitude are correlated with the accretion rate. For DP Leo the shape of the hard X-ray light curve is not as expected for optically thin emission, showing the importance of optical depth effects in the post-shock region. We find that the spin period of the white dwarf is slightly shorter than the orbital period and that the orbital period is decreasing faster than expected for energy loss by gravitational radiation alone.Comment: Accepted for publication in MNRAS, 12 pages, 6 figure

Policy opportunities

Recommendations are given regarding National Science Foundation (NSF) astronomy programs and the NASA Space Astrophysics program. The role of ground based astronomy is reviewed. The role of National Optical Astronomy Observatories (NOAO) in ground-based night-time astronomical research is discussed. An enhanced Explored Program, costs and management of small and moderate space programs, the role of astrophysics within NASA's space exploration initiative, suborbital and airborne astronomical research, the problems of the Hubble Space Telescope, and astronomy education are discussed. Also covered are policy issues related to the role of science advisory committees, international cooperation and competition, archiving and distribution of astronomical data, and multi-wavelength observations of variable sources

The Nature of the Secondary Star in the Black Hole X-Ray Transient V616 Mon (=A0620-00)

We have used NIRSPEC on Keck II to obtain $K$-band spectroscopy of the low mass X-ray binary V616 Mon (= A0620$-$00). V616 Mon is the proto-typical soft x-ray transient containing a black hole primary. As such it is important to constrain the masses of the binary components. The modeling of the infrared observations of ellipsoidal variations in this system lead to a derived mass of 11.0 M_{\sun} for the black hole. The validity of this derivation has been called into question due to the possiblity that the secondary star's spectral energy distribution is contaminated by accretion disk emission (acting to dilute the variations). Our new $K$-band spectrum of V616 Mon reveals a late-type K dwarf secondary star, but one that has very weak $^{\rm 12}$CO absorption features. Comparison of V616 Mon with SS Cyg leads us to estimate that the accretion disk supplies only a small amount of $K$-band flux, and the ellipsoidal variations are not seriously contaminated. If true, the derived orbital inclination of V616 Mon is not greatly altered, and the mass of the black hole remains large. A preliminary stellar atmosphere model for the $K$-band spectrum of V616 Mon reveals that the carbon abundance is approximately 50% of the solar value. We conclude that the secondary star in V616 Mon has either suffered serious contamination from the accretion of supernova ejecta that created the black hole primary, or it is the stripped remains of a formerly more massive secondary star, one in which the CNO cycle had been active.Comment: 20 pages, 5 figure

The discovery of 2.78 hour periodic modulation of the X-ray flux from globular cluster source Bo 158 in M31

We report the discovery of periodic intensity dips in the X-ray source XMMU J004314.1+410724, in the globular cluster Bo158 in M31. The X-ray flux was modulated by ~83% at a period of 2.78 hr (10017 s) in an XMM-Newton observation taken 2002 Jan 6-7. The X-ray intensity dips show no energy dependence. We detected weaker dips with the same period in observations taken 2000 June 25 (XMM-Newton) and 1991 June 26 (ROSAT/PSPC). The amplitude of the modulation has been found to be anticorrelated with source X-ray flux: it becomes lower when the source intensity rises. The energy spectrum of Bo158 was stable from observation to observation, with a characteristic cutoff at ~4-6 keV. The photo-electric absorption was consistent with the Galactic foreground value. No significant spectral changes were seen in the course of the dips. If the 2.78 hr cycle is the binary period of Bo158 the system is highly compact, with a binary separation of ~10e11 cm. The association of the source with a globular cluster, together with spectral parameters consistent with Galactic neutron star sources, suggests that X-rays are emitted by an accreting neutron star. The properties of Bo 158 are somewhat reminiscent of the Galactic X-ray sources exhibiting a dip-like modulations. We discuss two possible mechanisms explaining the energy-independent modulation observed in Bo 158: i) the obscuration of the central source by highly ionized material that scatters X-rays out of the line of sight; ii) partial covering of an extended source by an opaque absorber which occults varying fractions of the source.Comment: 10 pages, 4 figures, ApJ, submitted, uses emulateapj styl

On the X-ray source luminosity distributions in the bulge and disk of M31: First results from XMM-Newton survey

We present luminosity distributions for the X-ray sources detected with XMM-Newton in the bulge and disk of the Andromeda Galaxy (M31). The disk is clearly lacking the brighter sources which dominate X-ray emission from the bulge. This is the first convincing evidence for a difference between bulge and disk X-ray populations in M31. Our results are in good qualitative agreement with the luminosity distributions for low- and high-mass X-ray binaries recently obtained by Grimm et al.(2001) for our Galaxy. This confirms that X-ray population of the disk of M31 is dominated by fainter HMXB sources, while the bulge is populated with brighter LMXBs.Comment: 5 pages, 2 figures, uses emulateapj.sty, Submitted to ApJ