X-ray Spectroscopy of Candidate Ultracompact X-ray Binaries

We present high-resolution spectroscopy of the neutron star/low-mass X-ray binaries (LMXBs) 4U 1850-087 and 4U 0513-40 as part of our continuing study of known and candidate ultracompact binaries. The LMXB 4U 1850-087 is one of four systems in which we had previously inferred an unusual Ne/O ratio in the absorption along the line of sight, most likely from material local to the binaries. However, our recent Chandra X-ray Observatory LETGS spectrum of 4U 1850-087 finds a Ne/O ratio by number of 0.22+/-0.05, smaller than previously measured and consistent with the expected interstellar value. We propose that variations in the Ne/O ratio due to source variability, as previously observed in these sources, can explain the difference between the low- and high-resolution spectral results for 4U 1850-087. Our XMM-Newton RGS observation of 4U 0513-40 also shows no unusual abundance ratios in the absorption along the line of sight. We also present spectral results from a third candidate ultracompact binary, 4U 1822-000, whose spectrum is well fit by an absorbed power-law + blackbody model with absorption consistent with the expected interstellar value. Finally, we present the non-detection of a fourth candidate ultracompact binary, 4U 1905+000, with an upper limit on the source luminosity of < 1 x 10^{32} erg s^{-1}. Using archival data, we show that the source has entered an extended quiescent state.Comment: 8 pages, 3 figures, accepted for publication to the Astrophysical Journa

Chandra Observations of SN 2004et and the X-ray Emission of Type IIp Supernovae

We report the X-ray detection of the Type II-plateau supernova SN 2004et in the spiral galaxy NGC 6946, using the Chandra X-Ray Observatory. The position of the X-ray source was found to agree with the optical position within ~0.4 arcsec. Chandra also surveyed the region before the 2004 event, finding no X-ray emission at the location of the progenitor. For the post-explosion observations, a total of 202, 151, and 158 photons were detected in three pointings, each ~29 ks in length, on 2004 October 22, November 6, and December 3, respectively. The spectrum of the first observation is best fit by a thermal model with a temperature of kT=1.3 keV and a line-of-sight absorption of N_H=1.0 x 10^{22} cm^{-2}. The inferred unabsorbed luminosity (0.4-8 keV) is ~4x10^{38} erg/s, adopting a distance of 5.5 Mpc. A comparison between hard and soft counts on the first and third epochs indicates a softening over this time, although there is an insufficient number of photons to constrain the variation of temperature and absorption by spectral fitting. We model the emission as arising from the reverse shock region in the interaction between the supernova ejecta and the progenitor wind. For a Type IIP supernova with an extended progenitor, the cool shell formed at the time of shock wave breakout from the star can affect the initial evolution of the interaction shell and the absorption of radiation from the reverse shock. The observed spectral softening might be due to decreasing shell absorption. We find a pre-supernova mass loss rate of (2-2.5)x 10^{-6} M_{\odot} /yr for a wind velocity of 10 kms, which is in line with expectations for a Type IIP supernova.Comment: total 19 pages including 7 figures. ApJ, in press. See http://spider.ipac.caltech.edu/staff/rho/preprint/SN2004etms.ps for the paper including full resolution image

A Deep Chandra Observation of Kepler's Supernova Remnant: A Type Ia Event with Circumstellar Interaction

We present initial results of a 750 ks Chandra observation of the remnant of Kepler's supernova of AD 1604. The strength and prominence of iron emission, together with the absence of O-rich ejecta, demonstrate that Kepler resulted from a thermonuclear supernova, even though evidence for circumstellar interaction is also strong. We have analyzed spectra of over 100 small regions, and find that they fall into three classes. (1) The vast majority show Fe L emission between 0.7 and 1 keV and Si and S K alpha emission; we associate these with shocked ejecta. A few of these are found at or beyond the mean blast wave radius. (2) A very few regions show solar O/Fe abundance rations; these we associate with shocked circumstellar medium (CSM). Otherwise O is scarce. (3) A few regions are dominated by continuum, probably synchrotron radiation. Finally, we find no central point source, with a limit about 100 times fainter than the central object in Cas A. The evidence that the blast wave is interacting with CSM may indicate a Ia explosion in a more massive progenitor.Comment: Accepted by ApJ Letter

An adjustable law of motion for relativistic spherical shells

A classical and a relativistic law of motion for an advancing shell are deduced applying the thin layer approximation. A new parameter connected with the quantity of absorbed matter in the expansion is introduced; this allows of matching theory and observation.Comment: 15 pages, 10 figures and article in press; Central European Journal of Physics 201

Why did Supernova 1054 shine at late times?

The Crab nebula is the remnant of supernova 1054 (SN 1054). The progenitor of this supernova has, based on nucleosynthesis arguments, been modeled as an 8-10 solar mass star. Here we point out that the observations of the late light curve of SN 1054, from the historical records, are not compatible with the standard scenario, in which the late time emission is powered by the radioactive decay of small amounts of Ni-56. Based on model calculations we quantify this discrepancy. The rather large mass of Ni-56 needed to power the late time emission, 0.06[-0.03,+0.02] solar masses, seems inconsistent with abundances in the Crab nebula. The late light curve may well have been powered by the pulsar, which would make SN 1054 unique in this respect. Alternatively, the late light curve could have been powered by circumstellar interaction, in accordance with scenarios in which 8-10 solar mass stars are progenitors to `dense wind' supernovae.Comment: 5 pages, 2 figures. Accepted for publication in A&

Coronal X-Ray Emission from the Stellar Companions to Transiently Accreting Black Holes

Observations of soft X-ray transients (SXTs) in quiescence have found that the binaries harboring black holes are fainter than those that contain a neutron star. Narayan and collaborators postulated that the faint X-ray emission from black hole binaries was powered by an advection dominated accretion flow (ADAF). We explore an alternative explanation for the quiescent X-ray emission from the black hole systems: coronal emission from the rapidly rotating optical companion. This is commonly observed and well studied in other tidally locked binaries, such as the RS CVns. We show that two of the three X-ray detected black hole binaries (A0620-00 and GRO J1655-40) exhibit X-ray fluxes entirely consistent with coronal emission. The X-ray spectra of these objects should be best fit with thermal Raymond-Smith models rich in lines when coronal emission predominates. One black hole system (V404 Cyg) is too X-ray bright to be explained as coronal emission. The quiescent X-ray emission from the neutron star binaries is far too bright for coronal emission. It might be that all SXT's have variable accretion rates in quiescence and that the basal quiescent X-ray flux is set by either coronal emission from the companion or -- when present -- by thermal emission from the neutron star. We also show that the lithium abundances in the black hole systems are comparable to those in the RS CVns, reducing the need for production mechanisms that involve the compact object.Comment: ApJ, accepted (v541; Oct 1, 2000); Changes to figures and tables, minor modifications to text. Uses emulateapj.sty. 14 pages, 3 figure

Influence of the r-mode instability on hypercritically accreting neutron stars

We have investigated an influence of the r-mode instability on hypercritically accreting ($\dot{M}\sim 1M_\odot {y}^{-1}$) neutron stars in close binary systems during their common envelope phases based on the scenario proposed by Bethe et al. \shortcite{bethe-brown-lee}. On the one hand neutron stars are heated by the accreted matter at the stellar surface, but on the other hand they are also cooled down by the neutrino radiation. At the same time, the accreted matter transports its angular momentum and mass to the star. We have studied the evolution of the stellar mass, temperature and rotational frequency. The gravitational-wave-driven instability of the r-mode oscillation strongly suppresses spinning-up of the star, whose final rotational frequency is well below the mass-shedding limit, typically as small as 10% of that of the mass-shedding state. On a very short time scale the rotational frequency tends to approach a certain constant value and saturates there as far as the amount of the accreted mass does not exceed a certain limit to collapse to a black hole. This implies that the similar mechanism of gravitational radiation as the so-called Wagoner star may work in this process. The star is spun up by accretion until the angular momentum loss by gravitational radiation balances the accretion torque. The time-integrated dimensionless strain of the radiated gravitational wave may be large enough to be detectable by the gravitational wave detectors such as LIGO II.Comment: 6 pages, 3 figure

Circumstellar interaction in supernovae in dense environments - an observational perspective

In a supernova explosion, the ejecta interacting with the surrounding circumstellar medium (CSM) give rise to variety of radiation. Since CSM is created from the mass lost from the progenitor star, it carries footprints of the late time evolution of the star. This is one of the unique ways to get a handle on the nature of the progenitor star system. Here, I will focus mainly on the supernovae (SNe) exploding in dense environments, a.k.a. Type IIn SNe. Radio and X-ray emission from this class of SNe have revealed important modifications in their radiation properties, due to the presence of high density CSM. Forward shock dominance of the X-ray emission, internal free-free absorption of the radio emission, episodic or non-steady mass loss rate, asymmetry in the explosion seem to be common properties of this class of SNe.Comment: Fixed minor typos. 31 pages, 9 figures, accepted for publication in Space Science Reviews. Chapter in International Space Science Institute (ISSI) Book on "Supernovae" to be published in Space Science Reviews by Springe