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The quasi-persistent neutron star low-mass X-ray binary MXB 1659-29 went into quiescence in 2001, and we have followed its quiescent X-ray evolution since. Observations over the first 4 years showed a rapid drop in flux and temperature of…
In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 started a transition from outburst to quiescence, after it had been actively accreting for more than 24 years. In a previous work, we…
We present the results of detailed analysis of X-ray data in 3-20 keV range from a ~70 day outburst of the neutron star transient Aquila X-1 during September-November 2000. Optical monitoring with the YALO 1m telescope was used to trigger…
We have observed the quasi-persistent neutron-star X-ray transient and eclipsing binary MXB 1659-29 in quiescence on three occasions with Chandra. The purpose of our observations was to monitor the quiescent behavior of the source after its…
We conducted the first long-term (60 days), multiwavelength (optical, ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift observations from June to August 2012, with the goal of understanding variability in the low…
We explore the thermal evolution of a neutron star undergoing episodes of intense accretion, separated by long periods of quiescence. By using an exact cooling code we follow in detail the flow of heat in the star due to the time-dependent…
Transient Low-Mass X-ray Binaries (LMXBs) are usually first detected by all-sky X-ray monitors when they enter new outbursts, typically at X-ray luminosities above $\sim$10$^{36}$ erg/s. Observations of these sources during the early rise…
We present HST/ACS ultraviolet photometry of three quiescent black hole X-ray transients: X-ray Nova Muscae 1991 (GU Mus), GRO J0422+32 (V518 Per), and X-ray Nova Vel 1993 (MM Vel), and one neutron star system, Aql X-1. These are the first…
We have monitored the Seyfert galaxy NGC 3227 with the Rossi X-ray Timing Explorer (RXTE) since January 1999. During late 2000 and early 2001 we observed an unusual hardening of the 2-10 keV X-ray spectrum which lasted several months. The…
We present X-ray observations of novae V2491 Cyg and KT Eri about 9 years post-outburst, of the dwarf nova and post-nova candidate EY Cyg, and of a VY Scl variable. The first three objects were observed with XMM-Newton, KT Eri also with the…
Obscuration from clumpy accretion-disk winds has been invoked to explain the extreme X-ray weakness and X-ray variability observed in a substantial fraction of super-Eddington accreting quasars. We present a comprehensive study of the…
We present results from a joint Xmm-Newton/NuSTAR monitoring of the Seyfert 1 NGC 4593, consisting of 5x20 ks simultaneous observations spaced by two days, performed in January 2015. The source is variable, both in flux and spectral shape,…
The neutron-star Low-Mass X-ray Binary Aquila X-1 was observed seven times in total with the Suzaku X-ray observatory from September 28 to October 30 in 2007, in the decaying phase of an outburst. In order to constrain the flux-dependent…
The Seyfert 1 galaxy Mrk 279 was observed by XMM-Newton in November 2005 in three consecutive orbits, showing significant short-scale variability (average soft band variation in flux ~20%). The source is known to host a two-component warm…
On January 10 and 13, 2001, Venus was observed for the first time with an X-ray astronomy satellite. The observation, performed with the ACIS-I and LETG/ACIS-S instruments on Chandra, yielded data of high spatial, spectral, and temporal…
We present an analysis of all observations of Cygnus X-1 by the BATSE (20-300 keV) and by ASM (1.5-12 keV) until 2002 June, including 1200 days of simultaneous data. We find a number of correlations between fluxes and hardnesses in…
We report Chandra observations of the dwarf nova WX Hyi in quiescence. The X-ray spectrum displays strong and narrow emission lines of N, O, Mg, Ne, Si, S and Fe. The various ionization states implied by the lines suggest that the emission…
We present a study of the variations in the absorbing column density of 25 X-ray defined Seyfert 2 galaxies, as inferred from hard X-ray observations, on timescales from months to several years. We show that a significant variation of N$_H$…
We present a detailed re-analysis of the two {\it ASCA} Performance Verification observations of the nearby Seyfert 1 galaxy MCG-6-30-15. Confirming the results of Fabian et al. (1994), we find definite evidence for the {\sc O\,vii} and…
Periodic dips observed in approx. 20% of low-mass X-ray binaries are thought to arise from obscuration of the neutron star by the outer edge of the accretion disk. We report the detection with the Rossi X-ray Timing Explorer of two dipping…