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

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

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

Modeling the Radio and X-ray Emission of SN 1993J and SN 2002ap

Modeling of radio and X-ray observations of supernovae interacting with their circumstellar media are discussed, with special application to SN 1993J and SN 2002ap. We emphasize the importance of including all relevant physical mechanisms, especially for the modeling of the radio light curves. The different conclusions for the absorption mechanism (free-free or synchrotron self-absorption), as well as departures from an $\rho \propto r^{-2}$ CSM, as inferred by some authors, are discussed in detail. We conclude that the evidence for a variation in the mass loss rate with time is very weak. The results regarding the efficiencies of magnetic field generation and relativistic particle acceleration are summarized.Comment: 10 pages, 2 figures. Uses svmult.cls. To appear in proceedings of IAU Colloquium 192 "Supernovae (10 years of SN 1993J)", April 2003, Valencia, Spain, eds. J. M. Marcaide and K. W. Weile

Simultaneous XMM-Newton and ESO VLT observations of SN 1995N: probing the wind/ejecta interaction

We present the results of the first {\it XMM-Newton} observation of the interacting type IIn supernova 1995N, performed in July 2003. We find that the 0.2--10.0 keV unabsorbed flux dropped at a value of $\simeq 1.8 \times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$, almost one order of magnitude lower than that of a previous {\it ASCA} observation of January 1998. From all the available X-ray measurements, an interesting scenario emerges where the X-ray light emission may be produced by a two-phase (clumpy/smooth) circumstellar medium. The X-ray spectral analysis shows statistically significant evidence for the presence of two distinct components, that can be modeled with emission from optically thin, thermal plasmas at different temperatures. The exponent of the ejecta density distribution inferred from these temperatures is $n\simeq 6.4$. From the fluxes of the two spectral components we derive an estimate of the mass loss rate of the supernova progenitor, ${\dot M} \sim 2 \times 10^{-4} M_\odot {\rm yr}^{-1}$, at the upper end of the interval exhibited by red super-giants. Coordinated optical and infrared observations allow us to reconstruct the simultaneous infrared to X-ray flux distribution of SN 1995N. We find that, at $\sim$ 9 years after explosion, the direct X-ray thermal emission due to the wind/ejecta interaction is $\sim 5$ times larger than the total reprocessed IR/optical flux.Comment: 11 pages, 7 figures, MNRAS, in pres

SN 2007bg: The Complex Circumstellar Environment Around One of the Most Radio-Luminous Broad-Lined Type Ic Supernovae

In this paper we present the results of the radio light curve and X-ray observations of broad-lined Type Ic SN 2007bg. The light curve shows three distinct phases of spectral and temporal evolution, implying that the SNe shock likely encountered at least 3 different circumstellar medium regimes. We interpret this as the progenitor of SN 2007bg having at least two distinct mass-loss episodes (i.e., phases 1 and 3) during its final stages of evolution, yielding a highly-stratified circumstellar medium. Modelling the phase 1 light curve as a freely-expanding, synchrotron-emitting shell, self-absorbed by its own radiating electrons, requires a progenitor mass-loss rate of \dot{M}~1.9x10^{-6}(v_{w}/1000 km s^{-1}) Solar masses per year for the last t~20(v_{w}/1000 km s^{-1}) yr before explosion, and a total energy of the radio emitting ejecta of E\sim1x10^{48} erg after 10 days from explosion. This places SN 2007bg among the most energetic Type Ib/c events. We interpret the second phase as a sparser "gap" region between the two winds stages. Phase 3 shows a second absorption turn-on before rising to a peak luminosity 2.6 times higher than in phase 1. Assuming this luminosity jump is due to a circumstellar medium density enhancement from a faster previous mass-loss episode, we estimate that the phase 3 mass-loss rate could be as high as \dot{M}<~4.3x10^{-4}(v_{w}/1000 km s^{-1}) Solar masses per year. The phase 3 wind would have transitioned directly into the phase 1 wind for a wind speed difference of ~2. In summary, the radio light curve provides robust evidence for dramatic global changes in at least some Ic-BL progenitors just prior (~10-1000 yr) to explosion. The observed luminosity of this SN is the highest observed for a non-gamma-ray-burst broad-lined Type Ic SN, reaching L_{8.46 GHz}~1x10^{29} erg Hz^{-1} s^{-1}, ~567 days after explosion.Comment: 11 pages, 5 figures, accepted for publication in MNRA

Low frequency radio and X-ray properties of core-collapse supernovae

Radio and X-ray studies of young supernovae probe the interaction between the supernova shock waves and the surrounding medium and give clues to the nature and past of the progenitor star. Here we discuss the early emission from type Ic SN 2002ap and argue that repeated Compton boosting of optical photons by hot electrons presents the most natural explanation of the prompt X-ray emission. We describe the radio spectrum of another type Ic SN 2003dh (GRB030329) obtained with combined GMRT and VLA data. We report on the low frequency radio monitoring of SN 1995N and our objectives of distinguishing between competing models of X-ray emission from this SN and the nature of its progenitor by X-ray spectroscopy. Radio studies on SN 2001gd, SN 2001ig and SN 2002hh are mentioned.Comment: 5 pages, 4 figures. Uses svmult.cls. To appear in proceedings of IAU Colloquium 192 "Supernovae (10 years of SN 1993J)", April 2003, Valencia, Spain, eds. J. M. Marcaide and K. W. Weile

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

The high energy gamma-ray emission expected from Tycho's supernova remnant

A nonlinear kinetic model of cosmic ray (CR) acceleration in supernova remnants (SNRs) is used to describe the properties of Tycho's SNR. Observations of the expansion characteristics and of the nonthermal radio and X-ray emission spectra, assumed to be of synchrotron origin, are used to constrain the overall dynamical evolution and the particle acceleration parameters of the system, in addition to what is known from independent estimates of the distance and thermal X-ray observations. It is shown that a very efficient production of nuclear cosmic rays, leading to strong shock modification, and a large downstream magnetic field strength B_d approx 240muG are required to reproduce the observed synchrotron emission from radio to X-ray frequencies. This field strength is still well within the upper bound for the effective magnetic field, consistent with the acceleration process. The pi^0-decay gamma-ray flux turns out to be somewhat greater than the inverse Compton (IC) flux off the Cosmic Microwave Background at energies below 1 TeV, dominating it strongly at 10 TeV. The predicted TeV gamma-ray flux is consistent with but close to the very low upper limit recently obtained by HEGRA. A future detection at E_gamma ~ 10 TeV would clearly indicate hadronic emission.Comment: 8 pages, 6 figures. Accepted in Astronomy and Astrophyic

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