The Orbital Light Curve of Aquila X-1

We obtained R- and I-band CCD photometry of the soft X-ray transient/neutron- star binary Aql X-1 in 1998 June while it was at quiescence. We find that its light curve is dominated by ellipsoidal variations, although the ellipsoidal variations are severely distorted and have unequal maxima. After we correct for the contaminating flux from a field star located only 0.46" away, the peak-to-peak amplitude of the modulation is ~0.25 mag in the R band, which requires the orbital inclination to be greater than 36 degrees. The orbital period we measure is consistent with the 18.95 h period measured by Chevalier & Ilovaisky (1998). During its outbursts the light curve of Aql X-1 becomes single humped. The outburst light curve observed by Garcia et al. (1999) agrees in phase with our quiescent light curve. We show that the single humped variation is caused by a ``reflection effect,'' that is, by heating of the side of the secondary star facing towards the neutron star.Comment: 18 manuscript pages, 7 figures; accepted by A

Frozen Fronts Selection in flow against self-sustained chemical waves

Autocatalytic reaction fronts between two reacting species in the absence of fluid flow, propagate as solitary waves. The coupling between autocatalytic reaction front and forced hydrodynamic flow may lead to stationary front whose velocity and shape depend on the underlying flow field. We focus on the issue of the chemo-hydrodynamic coupling between forced advection opposed to self-sustained chemical waves which can lead to static stationary fronts, i.e Frozen Fronts, $FF$. Towards that purpose, we perform experiments, analytical computations and numerical simulations with the autocatalytic Iodate Arsenious Acid reaction ($IAA$) over a wide range of flow velocities around a solid disk. For the same set of control parameters, we observe two types of frozen fronts: an upstream $FF$ which avoid the solid disk and a downstream $FF$ with two symmetric branches emerging from the solid disk surface. We delineate the range over which we do observe these Frozen Fronts. We also address the relevance of the so-called eikonal, thin front limit to describe the observed fronts and select the frozen front shapes.Comment: draf

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

On the mass of the neutron star in V395 Car/2S 0921-630

We report high-resolution optical spectroscopy of the low-mass X-ray binary V395 Car/2S 0921-630 obtained with the MIKE echelle spectrograph on the Magellan-Clay telescope. Our spectra are obtained near superior conjunction of the mass donor star and we exploit the absorption lines originating from the back-side of the K-type object to accurately derive its rotational velocity. Using K0-K1 III templates, we find vsini=32.9 +/- 0.8 km/s. We show that the choice of template star and the assumed limb darkening coefficient has little impact on the derived rotational velocity. This value is a significant revision downwards compared to previously published values. We derive new system parameter constraints in the light of our much lower rotational velocity. We find M_1=1.44 +/- 0.10 Msun, M_2=0.35 +/- 0.03 Msun, and q=0.24 +/- 0.02 where the errors have been estimated through a Monte-Carlo simulation. A possible remaining systematic effect is the fact that we may be over-estimating the orbital velocity of the mass donor due to irradiation effects. However, any correction for this effect will only reduce the compact object mass further, down to a minimum mass of M_1=1.05 +/- 0.08 Msun. There is thus strong evidence that the compact object in this binary is a neutron star of rather typical mass and that the previously reported mass values of 2-4Msun were too high due to an over-estimate of the rotational broadening.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

HST/STIS UV Spectroscopy of Two Quiescent X-ray Novae: A0620-00 and Centaurus X-4

In 1998 we made UV spectroscopic observations with HST/STIS of A0620-00 and Cen X-4, which are two X-ray novae (aka soft X-ray transients). These binary systems are similar in all respects except that the former contains a black hole and the latter contains a neutron star. A UV spectrum (1700-3100A) is presented for the quiescent state of each system in the context of previously published UV/optical and X-ray data. The non-stellar, continuum spectrum of black hole A0620-00 has a prominent UV/optical peak centered at about 3500A. In contrast the spectrum of neutron-star Cen X-4 lacks a peak and rises steadily with frequency over the entire UV/optical band. In the optical, the two systems are comparably luminous. However, black hole A0620-00 is about 6 times less luminous at 1700A, and about 40 times less luminous in the X-ray band. The broadband spectrum of A0620-00 is discussed in terms of the advection-dominated accretion flow model.Comment: 18 pages including 4 figures; tentatively scheduled for the March 10, 2000 issue of ApJ; minor revision

Eleven years of radio monitoring of the Type IIn supernova SN 1995N

We present radio observations of the optically bright Type IIn supernova SN 1995N. We observed the SN at radio wavelengths with the Very Large Array (VLA) for 11 years. We also observed it at low radio frequencies with the Giant Metrewave Radio Telescope (GMRT) at various epochs within $6.5-10$ years since explosion. Although there are indications of an early optically thick phase, most of the data are in the optically thin regime so it is difficult to distinguish between synchrotron self absorption (SSA) and free-free absorption (FFA) mechanisms. However, the information from other wavelengths indicates that the FFA is the dominant absorption process. Model fits of radio emission with the FFA give reasonable physical parameters. Making use of X-ray and optical observations, we derive the physical conditions of the shocked ejecta and the shocked CSM.Comment: 22 pages, 2 tables, 13 figures, Accepted for publication in Astrophysical Journa

Correlated Timing and Spectral Variations of the Soft X-ray Transient Aquila X-1: Evidence for an Atoll classification

Based on Rossi X-ray Timing Explorer data, we discuss the classification of the soft X-ray transient Aquila X-1 in the Z/atoll scheme, and the relation of its kilohertz quasi-periodic oscillations (kHz QPO) properties to the X-ray colors. The color-color diagram shows one elongated ("banana") structure and several "islands" of data points. The power spectra of the island are best represented by a broken power-law, whereas those of the banana by a power-law below ~ 1 Hz plus an exponentially cut-off component at intermediate frequencies (30-60 Hz). The parameters of these two components change in correlation with the position of the source in the color-color diagram. Based on the pattern that the source shows in the color-color diagram and its aperiodic variability we conclude that Aquila X-1 is an atoll source. We have also investigated the possible correlation between the frequency of the kHz QPO and the position of the source in the color-color diagram. The complexity seen in the frequency versus count rate diagram is reduced to a single track when the frequency is plotted against hard or soft color.Comment: 9 pages, 5 figures. Accepted for publication in Ap

Aql X-1 in Outburst and Quiescence

We present photometry and spectroscopy of the soft x-ray transient Aql X-1. Optical photometry during an active state shows a strong (0.6 mag peak-to-peak) modulation at a period of 19 hours. Infrared (K'-band) photometry during a quiescent state limits any ellipsoidal variations to <0.07 mag (peak-to-peak), which implies an inclination i<31 (90% limit). Spectroscopy in a quiescent state shows at most very small radial velocity variations, which implies a very low inclination of i<12 (90% limit). The low inclination is rather unexpected given the large photometric modulation seen in the active state. The upper limit to the equivalent width of the anomalous Li 6707A line is <0.3A, which is similar to the measured strength of this line in several other x-ray transients.Comment: Accepted for publication in ApJ, 12 pages, 5 figure

Stellar Variability in the Globular Cluster Terzan 5

We present the results of a search for variability in and near the core of the metal-rich, obscured globular cluster Terzan 5, using NICMOS on HST. This extreme cluster has approximately solar metallicity and a central density that places it in the upper few percent of all clusters. It is estimated to have the highest interaction rate of any galactic globular cluster. The large extinction towards Terzan 5 and the severe stellar crowding near the cluster center present a substantial observational challenge. Using time series analysis we discovered two variable stars in this cluster. The first is a RRab Lyrae variable with a period of ~0.61 days, a longer period than that of field stars with similar high metallicities. This period is, however, shorter than the average periods of RR Lyraes found in the metal-rich globular clusters NGC 6441, NGC 6388 and 47 Tuc. The second variable is a blue star with a 7-hour period sinusoidal variation and a likely orbital period of 14 hours. This star is probably an eclipsing blue straggler, or (less likely) the infrared counterpart to the low mass X-ray binary known in Terzan 5. Due to the extreme crowding and overlapping Airy profile of the IR PSF, we fall short of our original goal of detecting CVs via Palpha emission and detecting variable infrared emission from the location of the binary MSP in Terzan 5.Comment: 17 pages including 12 figures. Accepted for publication in Ap

A Possible Explanation of the Radio Afterglow of GRB980519: The Dense Medium Effect

GRB{980519} is characterized by its rapidly declining optical and X-ray afterglows. Explanations of this behavior include models invoking a dense medium environment which makes the shock wave evolve quickly into the sub-relativistic phase, a jet-like outflow, and a wind-shaped circumburst medium environment. Recently, Frail {et al}. (1999a) found that the latter two cases are consistent with the radio afterglow of this burst. Here, by considering the trans-relativistic shock hydrodynamics, we show that the dense medium model can also account for the radio light curve quite well. The potential virtue of the dense medium model for GRB{980519} is that it implies a smaller angular size of the afterglow, which is essential for interpreting the strong modulation of the radio light curve. Optical extinction due to the dense medium is not important if the prompt optical-UV flash accompanying the $\gamma$-ray emission can destroy dust by sublimation out to an appreciable distance. Comparisons with some other radio afterglows are also discussed.Comment: 5 pages, 1 figure, a few minor changes made and references up dated, MNRAS, in pres