OGLE observations of four X-ray binary pulsars in the SMC

This paper presents analysis and interpretation of OGLE photometric data of four X-ray binary pulsar systems in the Small Magellanic Cloud: 1WGA J0054.9-7226, RX J0050.7-7316, RX J0049.1-7250, and 1SAX J0103.2-7209. In each case, the probable optical counterpart is identified on the basis of its optical colours. In the case of RX J0050.7-7316 the regular modulation of its optical light curve appears to reveal an ellipsoidal modulation with a period of 1.416 days. Using reasonable masses for the neutron star and the B star, we show that the amplitude and relative depths of the minima of the I-band light curve of RX J0050.7-7316 can be matched with an ellipsoidal model where the B star nearly fills its Roche lobe. For mass ratios in the range of 0.12 to 0.20, the corresponding best-fitting inclinations are about 55 degrees or larger. The neutron star would be eclipsed by the B star at inclinations larger than 60 degrees for this particular mass ratio range. Thus RX J0050.7-7316 is a good candidate system for further study. In particular, we would need additional photometry in several colours, and most importantly, radial velocity data for the B star before we could draw more quantitative conclusions about the component masses

The Mass Distribution of Stellar-Mass Black Holes

We perform a Bayesian analysis of the mass distribution of stellar-mass black holes using the observed masses of 15 low-mass X-ray binary systems undergoing Roche lobe overflow and five high-mass, wind-fed X-ray binary systems. Using Markov Chain Monte Carlo calculations, we model the mass distribution both parametrically---as a power law, exponential, gaussian, combination of two gaussians, or log-normal distribution---and non-parametrically---as histograms with varying numbers of bins. We provide confidence bounds on the shape of the mass distribution in the context of each model and compare the models with each other by calculating their relative Bayesian evidence as supported by the measurements, taking into account the number of degrees of freedom of each model. The mass distribution of the low-mass systems is best fit by a power-law, while the distribution of the combined sample is best fit by the exponential model. We examine the existence of a "gap" between the most massive neutron stars and the least massive black holes by considering the value, M_1%, of the 1% quantile from each black hole mass distribution as the lower bound of black hole masses. The best model (the power law) fitted to the low-mass systems has a distribution of lower-bounds with M_1% > 4.3 Msun with 90% confidence, while the best model (the exponential) fitted to all 20 systems has M_1% > 4.5 Msun with 90% confidence. We conclude that our sample of black hole masses provides strong evidence of a gap between the maximum neutron star mass and the lower bound on black hole masses. Our results on the low-mass sample are in qualitative agreement with those of Ozel, et al (2010).Comment: 56 pages, 22 figures, 9 tables, as accepted by Ap

End-to-End Safe Reinforcement Learning through Barrier Functions for Safety-Critical Continuous Control Tasks

Reinforcement Learning (RL) algorithms have found limited success beyond simulated applications, and one main reason is the absence of safety guarantees during the learning process. Real world systems would realistically fail or break before an optimal controller can be learned. To address this issue, we propose a controller architecture that combines (1) a model-free RL-based controller with (2) model-based controllers utilizing control barrier functions (CBFs) and (3) on-line learning of the unknown system dynamics, in order to ensure safety during learning. Our general framework leverages the success of RL algorithms to learn high-performance controllers, while the CBF-based controllers both guarantee safety and guide the learning process by constraining the set of explorable polices. We utilize Gaussian Processes (GPs) to model the system dynamics and its uncertainties. Our novel controller synthesis algorithm, RL-CBF, guarantees safety with high probability during the learning process, regardless of the RL algorithm used, and demonstrates greater policy exploration efficiency. We test our algorithm on (1) control of an inverted pendulum and (2) autonomous car-following with wireless vehicle-to-vehicle communication, and show that our algorithm attains much greater sample efficiency in learning than other state-of-the-art algorithms and maintains safety during the entire learning process.Comment: Published in AAAI 201

A Multi-Wavelength, Multi-Epoch Study of the Soft X-Ray Transient Prototype, V616 Mon (A0620-00)

We have obtained optical and infrared photometry of the soft x-ray transient prototype V616 Mon (A0620-00). From this photometry, we find a spectral type of K4 for the secondary star in the system, which is consistent with spectroscopic observations. We present J-, H-, and K-band light curves modeled with WD98 and ELC. Combining detailed, independently run models for ellipsoidal variations due to a spotted, non-spherical secondary star, and the observed ultraviolet to infrared spectral energy distribution of the system, we show that the most likely value for the orbital inclination is 40.75 +/- 3 deg. This inclination angle implies a primary black hole mass of 11.0 +/- 1.9 solar masses.Comment: 29 pages (preprint format), including 7 figures and 4 tables, accepted for publication in the Nov 2001 issue of A

V4641Sgr - Super-Eddington source enshrouded by an extended envelope

An optical spectroscopy of an unusual fast transient V4641 Sgr constrains its mass to 8.7-11.7M_sun (9.6M_sun is the best fit value) and the distance to 7.4--12.3 kpc (Orosz et al. 2001). At this distance the peak flux of 12 Crab in the 2--12 keV energy band, measured by ASM/RXTE, implies the X-ray luminosity exceeding 2-3e39 erg/s, i.e. near or above the Eddington limit for a 9.6M_sun black hole. An optical photometry shows that at the peak of the optical outburst the visual magnitude increased by Delta m_V > 4.7^m relative to the quiescent level and reached m_V < 8.8^m. An assumption that this optical emission is due to irradiated surface of an accretion disk or a companion star with the the black body shape of the spectrum would mean that the bolometric luminosity of the system exceeds L>3e41 erg/s > 300 L_Edd. We argue that the optical data strongly suggest presence of an extended envelope surrounding the source which absorbs primary X-rays flux and reemits it in optical and UV. The data also suggests that this envelope should be optically thin in UV, EUV and soft X-rays. The observed properties of V4641 Sgr at the peak of an optical flare are very similar to those of SS433. This envelope is likely the result of near or super Eddington rate of mass accretion onto the black hole and it vanishes during subsequent evolution of the source when apparent luminosity drops well below the Eddington value. Thus this transient source provides us direct proof of the dramatic change in the character of an accretion flow at the mass accretion rate near or above the critical Eddington value as predicted long time ago by the theoretical models.Comment: 4 pages, 2 figures. Submitted to A&A Letter

Refined Neutron-Star Mass Determinations for Six Eclipsing X-Ray Pulsar Binaries

We present an improved method for determining the mass of neutron stars in eclipsing X-ray pulsar binaries and apply the method to six systems, namely Vela X-1, 4U 1538-52, SMC X-1, LMC X-4, Cen X-3, and Her X-1. In previous studies to determine neutron star mass, the X-ray eclipse duration has been approximated analytically by assuming the companion star is spherical with an effective Roche lobe radius. We use a numerical code based on Roche geometry with various optimizers to analyze the published data for these systems, which we supplement with new spectroscopic and photometric data for 4U 1538-52. This allows us to model the eclipse duration more accurately and thus calculate an improved value for the neutron star mass. The derived neutron star mass also depends on the assumed Roche lobe filling factor beta of the companion star, where beta = 1 indicates a completely filled Roche lobe. In previous work a range of beta between 0.9 and 1.0 was usually adopted. We use optical ellipsoidal lightcurve data to constrain beta. We find neutron star masses of 1.77 +/- 0.08 M_{sun} for Vela X-1, 0.87 +/- 0.07 M_{sun} for 4U 1538-52 (eccentric orbit), 1.00 +/- 0.10 M_{sun} for 4U 1538-52 (circular orbit), 1.04 +/- 0.09 M_{sun} for SMC X-1, 1.29 +/- 0.05 M_{sun} for LMC X-4, 1.49 +/- 0.08 M_{sun} for Cen X-3, and 1.07 +/- 0.36 M_{sun} for Her X-1. We discuss the limits of the approximations that were used to derive the earlier mass determinations, and we comment on the implications our new masses have for observationally refining the upper and lower bounds of the neutron star mass distribution.Comment: 10 figures, accepted for publication in The Astrophysical Journa

Orbital Parameters for the Black Hole Binary XTE J1650-500

(Shortened) We present R-band photometry of the X-ray transient and candidate black hole binary XTE J1650-500 obtained between 2003 May and August with the 6.5m Clay Telescope. A timing analysis of these data reveals a photometric period of 0.3205 +/- 0.0007 days (i.e. 7.63 hr) with a possible alias at 0.3785 days (9.12 hr). Our photometry completely rules out the previously published spectroscopic period of 0.212 days (5.09 hr). Consequently, we reanalyzed the 15 archival ESO/VLT spectra (obtained 2002 June by Sanchez-Fernandez et al.) that were the basis of the previously published spectroscopic period. We used a ``restframe search'' technique that is well suited for cases when the signal-to-noise ratio of individual spectra is low. The results confirmed the photometric period of 0.3205 days, and rule out the alias period near 0.38 days. The best value for the velocity semiamplitude of the companion star is K_2 = 435 +/- 30 km/sec, and the corresponding optical mass function is f(M) = 2.73 +/- 0.56 solar masses. The amplitude of the phased R-band light curve is 0.2 magnitudes, which gives a lower limit to the inclination of 50 +/- 3 degrees in the limiting case of no contribution to the R-band light curve from the accretion disk. If the mass ratio of XTE J1650-500 is similar to the mass ratios of other black hole binaries like A0620-00 or GRS 1124-683 (e.g. Q >~ 10), then our lower limit to the inclination gives an upper limit to the mass of the black hole in XTE J1650-500 of M_1 <~ 7.3 solar masses. However, the mass can be considerably lower if the R-band flux is dominated by the accretion disk. For example, if the accretion disk does contribute 80% of the flux, as our preliminary results suggest, then the black hole mass would be only about 4 solar masses.Comment: Accepted to ApJ. 15 pages, 5 figures (two of degraded quality). Revised after referee's Comments, conclusions are unchange

Stability of Linear Continuous-Time Systems with Stochastically Switching Delays

Necessary and sufficient conditions for the stability of linear continuous-time systems with stochastically switching delays are presented in this paper. It is assumed that the delay random paths are piece-wise constant functions of time where a finite number of values may be taken by the delay. The stability is assessed in terms of the second moment of the state vector of the system. The solution operators of individual linear systems with constant de- lays, chosen from the set of all possible delay values, are extended to form new augmented operators. Then for proper formulation of the second moment in continuous time, tensor products of the augmented solution operators are used. Finally the finite-dimensional versions of the stability conditions, that can be obtained using various time discretization techniques, are presented. Some examples are provided that demonstrate how the stability conditions can be used to assess the stability of linear systems with stochastic delay

Ultraviolet Spectra of CV Accretion Disks with Non-Steady T(r) Laws

An extensive grid of synthetic mid- and far-ultraviolet spectra for accretion disks in cataclysmic variables has been presented by Wade and Hubeny (1998). In those models, the disk was assumed to be in steady-state, that is T_eff(r) is specified completely by the mass M_WD and radius R_WD of the accreting white dwarf star and the mass transfer rate M_dot which is constant throughout the disk. In these models, T_eff(r) is proportional to r^{-3/4} except as modified by a cutoff term near the white dwarf. Actual disks may vary from the steady-state prescription for T_eff(r), however, e.g. owing to outburst cycles in dwarf novae M_dot not constant with radius) or irradiation (in which case T_eff in the outer disk is raised above T_steady). To show how the spectra of such disks might differ from the steady case, we present a study of the ultraviolet (UV) spectra of models in which power-law temperature profiles T_eff(r) is proportional to r^{-gamma} with gamma < 3/4 are specified. Otherwise, the construction of the models is the same as in the Wade & Hubeny grid, to allow comparison. We discuss both the UV spectral energy distributions and the appearance of the UV line spectra. We also briefly discuss the eclipse light curves of the non-standard models. Comparison of these models with UV observations of novalike variables suggests that better agreement may be possible with such modified T_eff(r) profiles.Comment: 13 pages, 6 figures (one reduced quality), ApJ in pres