Andrew G. Cantrell

Yale University, New Haven, Connecticut, United States

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Publications (13)45.53 Total impact

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    ABSTRACT: We examine ~10 years of photometric data and find that the black hole X-ray binary V4641 Sgr has two optical states, passive and active, during X-ray quiescence. The passive state is dominated by ellipsoidal variations and is stable in the shape and variability of the light curve. The active state is brighter and more variable. Emission during the active state varies over the course of the orbital period and is redder than the companion star. These optical/infrared states last for weeks or months. V4641 Sgr spends approximately 85% of X-ray quiescence in the passive state and 15% in the active. We analyze passive colors and spectroscopy of V4641 Sgr and show that they are consistent with a reddened B9III star (with E(B-V) = 0.37 +/- 0.19) with little or no contribution from the accretion disk. We use X-ray observations with an updated ephemeris to place an upper limit on the duration of an X-ray eclipse of <8.3 deg in phase (~1.6 hours). High resolution spectroscopy yields a greatly improved measurement of the rotational velocity of the companion star of V_rot_sin(i) = 100.9 +/- 0.8 km s^-1. We fit ellipsoidal models to the passive state data and find an inclination angle of i = 72.3 +/- 4.1 deg, a mass ratio of Q = 2.2 +/- 0.2, and component masses for the system of M_BH = 6.4 +/- 0.6 M_sun and M_2 = 2.9 +/- 0.4 M_sun. Using these values we calculate an updated distance to V4641 Sgr of 6.2 +/- 0.7 kpc.
    01/2014; 784(1).
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    ABSTRACT: In X-ray binaries, compact jets are known to commonly radiate at radio to infrared frequencies, whereas at optical to gamma-ray energies, the contribution of the jet is debated. The total luminosity, and hence power of the jet is critically dependent on the position of the break in its spectrum, between optically thick (self-absorbed) and optically thin synchrotron emission. This break, or turnover, has been reported in just one black hole X-ray binary (BHXB) thus far, GX 339-4, and inferred via spectral fitting in two others, A0620-00 and Cyg X-1. Here, we collect a wealth of multiwavelength data from the outbursts of BHXBs during hard X-ray states, in order to search for jet breaks as yet unidentified in their spectral energy distributions. In particular, we report the direct detection of the jet break in the spectrum of V404 Cyg during its 1989 outburst, at nu_b = (1.8 +- 0.3) x 10^14 Hz (1.7 +- 0.2 microns). We increase the number of BHXBs with measured jet breaks from three to eight. Jet breaks are found at frequencies spanning more than two orders of magnitude, from nu_b = (4.5 +- 0.8) x 10^12 Hz for XTE J1118+480 during its 2005 outburst, to nu_b > 4.7 x 10^14 Hz for V4641 Sgr in outburst. A positive correlation between jet break frequency and luminosity is expected theoretically; nu_b \propto L_nu,jet^~0.5 if other parameters are constant. With constraints on the jet break in a total of 12 BHXBs including two quiescent systems, we find a large range of jet break frequencies at similar luminosities and no obvious global relation (but such a relation cannot be ruled out for individual sources). We speculate that different magnetic field strengths and/or different radii of the acceleration zone in the inner regions of the jet are likely to be responsible for the observed scatter between sources. (abridged)
    Monthly Notices of the Royal Astronomical Society 11/2012; 429(1). · 5.52 Impact Factor
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    ABSTRACT: Sgr A* is the weakest accreting black hole we have ever observed, yet it is not a particularly unique object. We know that the majority of galaxies are more like Sgr A* than active galactic nuclei (AGN), so it is important to understand what these low-level engines can contribute to their environs. The Fundamental Plane of Black Hole Accretion is an important tool for making comparisons of black hole physics across the mass scale. In this talk, I will present the first ever joint fits between Sgr A* in the flaring state and the stellar-mass quiescent black hole X-ray binary, A0620-00. I will show how the same physical model with exactly the same (mass scaled) parameters can apply to both sources, and discuss what this could mean for our understanding of both objects individually. This new method can help break degeneracies between models, and for these sources in particular shed light on questions of particle acceleration at the lowest accretion rates.
    01/2012;
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    ABSTRACT: 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 5 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. This difference indicates that the low-mass subsample is not consistent with being drawn from the distribution of the combined population. 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. Our analysis generates posterior distributions for M 1%; the best model (the power law) fitted to the low-mass systems has a distribution of lower bounds with M 1%>4.3 M ☉ with 90% confidence, while the best model (the exponential) fitted to all 20 systems has M 1%>4.5 M ☉ 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., although our broad model selection analysis more reliably reveals the best-fit quantitative description of the underlying mass distribution. The results on the combined sample of low- and high-mass systems are in qualitative agreement with Fryer & Kalogera, although the presence of a mass gap remains theoretically unexplained.
    The Astrophysical Journal 10/2011; 741(2):103. · 6.73 Impact Factor
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    ABSTRACT: [Abridged.] We present multiwavelength observations of the black hole binary system, A0620-00. Using the Cosmic Origins Spectrograph on the Hubble Space Telescope, we have obtained the first FUV spectrum of A0620-00. The observed spectrum is flat in the FUV and very faint (with continuum fluxes \simeq 1e - 17 ergs/cm^2/s/A). We compiled the dereddened, broadband spectral energy distribution of A0620-00 and compared it to previous SEDs as well as theoretical models. The SEDs show that the source varies at all wavelengths for which we have multiple samples. Contrary to previous observations, the optical-UV spectrum does not continue to drop to shorter wavelengths, but instead shows a recovery and an increasingly blue spectrum in the FUV. We created an optical-UV spectrum of A0620-00 with the donor star contribution removed. The non-stellar spectrum peaks at \simeq3000 {\deg}A. The peak can be fit with a T=10,000 K blackbody with a small emitting area, probably originating in the hot spot where the accretion stream impacts the outer disk. However, one or more components in addition to the blackbody are needed to fit the FUV upturn and the red optical fluxes in the optical-UV spectrum. By comparing the mass accretion rate determined from the hot spot luminosity to the mean accretion rate inferred from the outburst history, we find that the latter is an order of magnitude smaller than the former, indicating that \sim90% of the accreted mass must be lost from the system if the predictions of the disk instability model and the estimated interoutburst interval are correct. The mass accretion rate at the hot spot is 10^5 the accretion rate at the black hole inferred from the X-ray luminosity. To reconcile these requires that outflows carry away virtually all of the accreted mass, a very low rate of mass transfer from the outer cold disk into the inner hot region, and/or radiatively inefficient accretion.
    The Astrophysical Journal 09/2011; 743. · 6.73 Impact Factor
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    ABSTRACT: V4641 Sgr is a galactic microquasar, or x-ray binary, with a B9III star as secondary and an orbital period of 2.82 days. Although the secondary star is very bright (13th mag.), it is clear that the disk around the black hole also contributes to the optical emission. This makes the determination of the orbital inclination, and thus the mass of the compact object, uncertain. We present simultaneous spectroscopy and photometry from 2009 and 2010, taken at the SMARTS telescopes in Cerro Tololo, Chile, which enables us to determine the disk fraction of the optical emission. Once this disk fraction has been determined, a more definitive mass measurement for the black hole in the system will be possible.
    01/2011;
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    ABSTRACT: We present results of recent observations and theoretical modeling of data from black holes accreting at very low luminosities (L/L_Edd ~ 10^{-8}). We discuss our newly developed time-dependent model for episodic ejection of relativistic plasma within a jet framework, and a successful application of this model to describe the origin of radio flares seen in Sgr A*, the Galactic center black hole. Both the observed time lags and size-frequency relationships are reproduced well by the model. We also discuss results from new Spitzer data of the stellar black hole X-ray binary system A0620-00. Complemented by long term SMARTS monitoring, these observations indicate that once the contribution from the accretion disk and the donor star are properly included, the residual mid-IR spectral energy distribution of A0620-00 is quite flat and consistent with a non-thermal origin. The results above suggest that a significant fraction of the observed spectral energy distribution originating near black holes accreting at low luminosities could result from a mildly relativistic outflow. The fact that these outflows are seen in both stellar-mass black holes as well as in supermassive black holes at the heart of AGNs strengthens our expectation that accretion and jet physics scales with mass. Comment: 4 pages, 2 figures. To appear in Proceedings of IAU Symposium 275 "Jets at all Scales", 13-17 September 2010, Buenos Aires, Argentina
    Proceedings of the International Astronomical Union 10/2010;
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    ABSTRACT: During its year-long outburst in 1975-76, the transient source A0620-00 reached an intensity of 50 Crab, an all-time record for any X-ray binary. The source has been quiescent since then. We have recently determined accurate values for the black hole (BH) mass, orbital inclination angle, and distance. Building on these results, we have measured the radius of the inner edge of the accretion disk around the BH primary by fitting its thermal continuum spectrum to our version of the relativistic Novikov-Thorne thin-disk model. We have thereby estimated the spin of the BH. Although our spin estimate depends on a single high-quality spectrum, which was obtained in 1975 by OSO-8, we are confident of our result because of the consistent values of the inner-disk radius that we have obtained for hundreds of observations of other sources: H1743-322, XTE J1550-564, and notably LMC X-3. We have determined the dimensionless spin parameter of the BH to be a * = 0.12 ± 0.19, with a * < 0.49 and a *> – 0.59 at the 3σ level of confidence. This result takes into account all sources of observational and model-parameter uncertainties. Despite the low spin, the intensity and properties of the radio counterpart, both in outburst and quiescence, attest to the presence of a strong jet. If jets are driven by BH spin, then current models indicate that jet power should be a steeply increasing function of a *. Consequently, the low spin of A0620-00 suggests that its jet may be disk driven.
    The Astrophysical Journal Letters 07/2010; 718(2):L122. · 6.35 Impact Factor
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    ABSTRACT: We analyze photometry of the soft X-ray transient A0620 – 00 spanning nearly 30 years, including previously published and previously unpublished data. Previous attempts to determine the inclination of A0620 using subsets of these data have yielded a wide range of measured values of i. Differences in the measured value of i have been due to changes in the shape of the light curve and uncertainty regarding the contamination from the disk. We give a new technique for estimating the disk fraction and find that disk light is significant in all light curves, even in the infrared. We also find that all changes in the shape and normalization of the light curve originate in a variable disk component. After accounting for this disk component, we find that all the data, including light curves of significantly different shapes, point to a consistent value of i. Combining results from many separate data sets, we find i = 510 ± 09, implying M = 6.6 ± 0.25 M ☉. Using our dynamical model and zero-disk stellar VIH magnitudes, we find d = 1.06 ± 0.12 kpc. Understanding the disk origin of nonellipsoidal variability may assist with making reliable determinations of i in other systems, and the fluctuations in disk light may provide a new observational tool for understanding the three-dimensional structure of the accretion disk.
    The Astrophysical Journal 01/2010; 710(2):1127. · 6.73 Impact Factor
  • Andrew Cantrell, C. Bailyn, J. McClintock, J. Orosz
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    ABSTRACT: We show that X-ray Binaries undergo optical state changes within X-ray quiescence. All states show emission from the accretion flow, but in what we call the "passive" state, we show that it is possible to separate stellar flux from flux due to the accretion flow. We thereby gain a better understanding of both components: We are able to robustly determine binary parameters and distance from the stellar component, while finding new observational constraints on the nature of the accretion flow. We focus on the black hole Binary A0620--00. By accounting for the accretion flow, we improve the uncertainty in its inclination by a factor of 20, the uncertainty in its mass by a factor of 40, and the uncertainty in its distance by a factor of 10. We are optimistic that our methods will produce similar improvements in other systems. This work was supported by NSF Graduate Research Fellowship DGE-0202738 to AGC and NSF/AST grants 0407063 and 0707627 to CDB.
    01/2010;
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    ABSTRACT: We present optical and infrared photometry of the soft X-ray transient A0620–00, obtained by the SMARTS and YALO consortia, spanning the period 1999-2007. Although A0620–00 was X-ray quiescent throughout this period, our data show three distinct optical states, characterized by magnitude, color, and aperiodic variability. In particular, we find that in what we call the "passive" state, A0620–00 exhibits no observable aperiodic variability on any timescale longer than our exposure length. The existence of these states may explain the differences between phased light curves observed by previous authors. We suggest that in order to obtain the purest ellipsoidal light curves, passive-state data should be used in future studies of ellipsoidal variability.
    The Astrophysical Journal 12/2008; 673(2):L159. · 6.73 Impact Factor
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    ABSTRACT: We present optical and infrared photometry of the soft X-ray transient A0620-00, obtained by the SMARTS and YALO consortia, spanning from 1999-2007. Although A0620-00 was X-ray quiescent throughout this period, our data show three distinct optical states, characterized by magnitude, color, and aperiodic variability. In particular, we find that in what we call the ``passive'' state, A0620-00 exhibits no observable aperiodic variability on any timescale longer than our exposure length. The shape of the passive state light curve is consistent throughout our dataset. The other states are brighter than the passive state, and show enhanced aperiodic variability. These characteristics appear in NIR as well as optical data, suggesting that even NIR light curves may be contaminated if they were not obtained during the passive state. We suggest a reanalysis of historical data, using passive state data to determine the inclination and isolate additional sources of variability.
    05/2008;
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    Andrew G. Cantrell, Charles D. Bailyn
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    ABSTRACT: Radial velocity studies of accreting binary stars commonly use accretion disk emission lines to determine the radial velocity of the primary star and therefore the mass ratio. These emission line radial velocity curves are often shifted in phase from the expected motion of the primary. These phase shifts cast doubt on the use of disk emission lines in the determination of mass ratios. We present a systematic study of phase shifts, using data from the literature to distinguish between possible explanations of the phase shift. We find that one widely adopted class of models is contradicted by observations (section 2). We present a generalized form of another class of models, which we call "measurement offset models." We show that these models are quantitatively consistent with existing data (figures 2 and 3, and the discussion in section 4.4). We consider the implications of adopting measurement offset models, for both disk structure and determination of binary parameters. Specifically, we describe in section 6 how measurement offset models may be used improve determinations of the mass ratio based on disk emission lines. This could be a valuable new tool in determining masses of important astrophysical objects such as accreting neutron stars and black holes. Comment: 38 pages, 3 figures, to be published in ApJ
    The Astrophysical Journal 08/2007; · 6.73 Impact Factor