arXiv subjects

E. L. Robinson

Publications and source records attributed to E. L. Robinson.

At least 19 recordsLinked to original sources

arXiv2024

Links Between Optical and X-ray Light in Cygnus X-2

We observed the low mass X-ray binary Cyg X-2 for a total of 18 nights over two observing runs in July and September of 2006, using the Otto Struve Telescope at McDonald Observatory and the Rossi X-ray Timing Explorer. Using discrete cross correlations, we found peaks occurring at near-zero lags in the flaring branch of the colour-colour diagram, which could signify reprocessing, in addition to an anti-correlation within the normal branch. When comparing optical flux to the system's placement on the Z track, two distinct behaviors were seen: (1) a state with no correlation, and (2) a multi-valued (horizontal and normal branches)/correlated (flaring branch) state. The correlation was the result of direct steps and more gradual falls to and from the flaring branch respectively. Finally, we modeled timed spectra with 64 second bins with an extended accretion disc corona model. We found that correlations occurred between the optical and the various fitted parameters, particularly the blackbody normalization (and blackbody radius by extension) in higher intensity regions. Despite this, the Z track location was found to be a far better predictor of physical parameters than the optical flux, with clean correlations seen in every branch of the Z track. Where optical correlations are found, the location on the Z track was a better predictor of optical flux than any individual physical parameter.

astro-ph.HE↗

arXiv2019

Optical and X-ray Correlations During the 2015 Outburst of the Black Hole V404 Cyg

We present a serendipitous multiwavelength campaign of optical photometry simultaneous with Integral X-ray monitoring of the 2015 outburst of the black hole V404 Cyg. Large amplitude optical variability is generally correlated with X-rays, with lags of order a minute or less compatible with binary light travel timescales or jet ejections. Rapid optical flaring on time-scales of seconds or less is incompatible with binary light-travel timescales and has instead been associated with synchrotron emission from a jet. Both this rapid jet response and the lagged and smeared one can be present simultaneously. The optical brightness is not uniquely determined by the X-ray brightness, but the X-ray/optical relationship is bounded by a lower-envelope such that at any given optical brightness there is a maximum X-ray brightness seen.} This lower-envelope traces out a Fopt proportional to Fx^0.54 relation which can be approximately extrapolated back to quiescence. Rapid optical variability is only seen near this envelope, and these periods correspond to the hardest hard X-ray colours. This correlation between hard X-ray colour and optical variability (and anti-correlation with optical brightness) is a novel finding of this campaign, and apparently a facet of the outburst behaviour in V404 Cyg. It is likely that these correlations are driven by changes in the central accretion rate and geometry.

astro-ph.HE↗

arXiv2018

Outliers: multicolour photometry guiding the search for evolved binary systems in the globular cluster 47 Tucanae

We use Hubble Space Telescope multicolour photometry of the globular cluster 47 Tucanae to uncover a population of 24 objects with no previous classification that are outliers from the single-star model tracks in the colour-magnitude diagram and yet are likely cluster members. By comparing those sources with evolutionary models and X-ray source catalogues, we were able to show that the majority of those sources are likely binary systems that do not have any X-ray source detected nearby, most possibly formed by a white dwarf and a main-sequence star and a small number of possible double-degenerate systems.

astro-ph.SR↗

arXiv2017

The Spectrum of SS 433 in the H and K Bands

SS~433 is an X-ray binary and the source of sub-relativistic, precessing, baryonic jets. We present high-resolution spectrograms of SS 433 in the infrared H and K bands. The spectrum is dominated by hydrogen and helium emission lines. The precession phase of the emission lines from the jet continues to be described by a constant period, P_jet= 162.375 d. The limit on any secularly changing period is $|\dot P| \lesssim 10^{-5}$. The He I 2.0587 micron line has complex and variable P Cygni absorption features produced by an inhomogeneous wind with a maximum outflow velocity near 900 km/s. The He II emission lines in the spectrum also arise in this wind. The higher members of the hydrogen Brackett lines show a double-peaked profile with symmetric wings extending more than +/-1500 km/s from the line center. The lines display radial velocity variations in phase with the radial velocity variation expected of the compact star, and they show a distortion during disk eclipse that we interpret as a rotational distortion. We fit the line profiles with a model in which the emission comes from the surface of a symmetric, Keplerian accretion disk around the compact object. The outer edge of the disk has velocities that vary from 110 to 190 km/s. These comparatively low velocities place an important constraint on the mass of the compact star: Its mass must be less than 2.2 M_solar and is probably less than 1.6 M_solar.

astro-ph.SR↗

arXiv2013

Multiwavelength Observations of Swift J1753.5-0127

We present contemporaneous X-ray, ultraviolet, optical and near-infrared observations of the black hole binary system, Swift J1753.5-0127, acquired in 2012 October. The UV observations, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope, are the first UV spectra of this system. The dereddened UV spectrum is characterized by a smooth, blue continuum and broad emission lines of CIV and HeII. The system was stable in the UV to <10% during our observations. We estimated the interstellar reddening by fitting the 2175 A absorption feature and fit the interstellar absorption profile of Ly$α$ to directly measure the neutral hydrogen column density along the line of sight. By comparing the UV continuum flux to steady-state thin accretion disk models, we determined upper limits on the distance to the system as a function of black hole mass. The continuum is well fit with disk models dominated by viscous heating rather than irradiation. The broadband spectral energy distribution shows the system has declined at all wavelengths since previous broadband observations in 2005 and 2007. If we assume that the UV emission is dominated by the accretion disk the inner radius of the disk must be truncated at radii above the ISCO to be consistent with the X-ray flux, requiring significant mass loss from outflows and/or energy loss via advection into the black hole to maintain energy balance.

astro-ph.HE↗

arXiv2012

The Mass of the Black Hole in XTE J1118+480

We present contemporaneous, broadband, near-infrared spectroscopy (0.9-2.45 micron) and H-band photometry of the black hole X-ray binary, XTE J1118+480. We determined the fractional dilution of the NIR ellipsoidal light curves of the donor star from other emission sources in the system by comparing the absorption features in the spectrum with field stars of known spectral type. We constrained the donor star spectral type to K7 V - M1 V and determined that the donor star contributed 54+/-27% of the H-band flux at the epoch of our observations. This result underscores the conclusion that the donor star cannot be assumed to be the only NIR emission source in quiescent X-ray binaries. The H-band light curve shows a double-humped asymmetric modulation with extra flux at orbital phase 0.75. The light curve was fit with a donor star model light curve, taking into account a constant second flux component based on the dilution analysis. We also fit models that included emission from the donor star, a constant component from the accretion disk, and a phase-variable component from the bright spot where the mass accretion stream impacts the disk. These simple models with reasonable estimates for the component physical parameters can fully account for the observed light curve, including the extra emission at phase 0.75. From our fits, we constrained the binary inclination to 68 <= i <= 79 deg. This leads to a black hole mass of 6.9 <= M_BH <= 8.2 solar masses. Long-term variations in the NIR light curve shape in XTE J1118+480 are similar to those seen in other X-ray binaries and demonstrate the presence of continued activity and variability in these systems even when in full quiescence.

astro-ph.SR↗

arXiv2012

The Ultraviolet Spectral Energy Distributions of Quiescent Black Holes and Neutron Stars

We present HST/ACS ultraviolet photometry of three quiescent black hole X-ray transients: X-ray Nova Muscae 1991 (GU Mus), GRO J0422+32 (V518 Per), and X-ray Nova Vel 1993 (MM Vel), and one neutron star system, Aql X-1. These are the first quiescent UV detections of these objects. All are detected at a much higher level than expected from their companion stars alone and are significant detections of the accretion flow. Three of the four UV excesses can be characterized by a black body of temperature 5000-13,000K, hotter than expected for the quiescent outer disk. A good fit could not be found for MM Vel. The source of the black-body-like emission is most likely a heated region of the inner disk. Contrary to initial indications from spectroscopy there does not appear to be a systematic difference in the UV luminosity or spectral shape between black holes and neutron star systems. However combining our new data with earlier spectroscopy and published X-ray luminosities there is a significant difference in the X-ray to UV flux ratios with the neutron stars exhibiting Lx/Luv about 10x higher than the black hole systems. Since both bandpasses are expected to be dominated by accretion light this suggests the difference in X-ray luminosities cannot simply reflect differences in quiescent accretion rates and so is a more robust discriminator between the black hole and neutron star populations than the comparison of X-ray luminosities alone.

astro-ph.HE↗

arXiv2011

Millihertz Quasi-periodic Optical Oscillations in 4U 0614+091

We report the discovery of a 1mHz optical quasi-periodic oscillation (QPO) in the candidate ultracompact low-mass X-ray binary 4U 0614+091. The ultra-low frequency QPO has no X-ray counterpart in contemporaneous RXTE/PCA data and is likely a signature of structure in the accretion disk. The QPO can be reasonably fitted with a single sine wave but with a phase jump part way through the observation, indicating that it is not coherent.We also identify a 48 min modulation, approximately consistent with the suggested orbital period of O'Brien (2005) and Shahbaz et al. (2008). If this is indeed orbital, it supports an identification of 4U 0614+091 as an ultra-compact source.

astro-ph.HE↗

arXiv2011

Multiwavelength Observations of A0620-00 in Quiescence

[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 °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.

astro-ph.SR↗

arXiv2011

SN2008am: A Super-Luminous Type IIn Supernova

We present observations and interpretation of the Type IIn supernova SN 2008am discovered by the ROTSE Supernova Verification Project (RSVP). SN 2008am peaked at approximately -22.3 mag at a redshift of z=0.2338, giving it a peak luminosity of 3 x 10^{44}erg/s and making it one of the most luminous supernovae ever observed. The total radiated energy is ~ 2 x 10^{51} erg. Photometric observations in the ultraviolet, optical and infrared bands (J,H,Ks) constrain the SED evolution. We obtained six optical spectra of the supernova, five on the early decline from maximum light and a sixth nearly a year later plus a very late-time spectrum (~2 yr) of the host galaxy. The spectra of SN 2008am show strong Balmer-line and He I lambda 5876A emission with intermediate widths (~25A) in the first ~40 days after optical maximum. We examine a variety of models for the line wings and conclude that multiple scattering is most likely, implying that our spectra contain no specific information on the bulk flow velocity. We examine a variety of models for the ROTSE light curve subject to the rise time and the nature of the spectra, including radioactive decay, shocks in optically-thick and optically-thin circumstellar media (CSM) and a magnetar. The most successful model is one for which the CSM is optically-thick and in which diffusion of forward shock-deposited luminosity gives rise to the observed light curve. Diffusion of the shock-deposited energy from the forward shock is found to be important to account for the rising part of the light curve. Although there are differences in detail, SN 2008am appears to be closely related to other super-luminous Type IIn supernovae, SN 2006gy, SN 2006tf and perhaps SN 2008iy, that may represent the deaths of very massive LBV-type progenitors and for which the luminosity is powered by the interaction of the ejecta with a dense circumstellar medium.

astro-ph.HE↗

arXiv2010

Matching post-Newtonian and numerical relativity waveforms: systematic errors and a new phenomenological model for non-precessing black hole binaries

We present a new phenomenological gravitational waveform model for the inspiral and coalescence of non-precessing spinning black hole binaries. Our approach is based on a frequency domain matching of post-Newtonian inspiral waveforms with numerical relativity based binary black hole coalescence waveforms. We quantify the various possible sources of systematic errors that arise in matching post-Newtonian and numerical relativity waveforms, and we use a matching criteria based on minimizing these errors; we find that the dominant source of errors are those in the post-Newtonian waveforms near the merger. An analytical formula for the dominant mode of the gravitational radiation of non-precessing black hole binaries is presented that captures the phenomenology of the hybrid waveforms. Its implementation in the current searches for gravitational waves should allow cross-checks of other inspiral-merger-ringdown waveform families and improve the reach of gravitational wave searches.

gr-qc↗

arXiv2010

Multi-site Observations of Pulsation in the Accreting White Dwarf SDSS J161033.64-010223.3 (V386 Ser)

Nonradial pulsations in the primary white dwarfs of cataclysmic variables can now potentially allow us to explore the stellar interior of these accretors using stellar seismology. In this context, we conducted a multi-site campaign on the accreting pulsator SDSS J161033.64-010223.3 (V386 Ser) using seven observatories located around the world in May 2007 over a duration of 11 days. We report the best fit periodicities here, which were also previously observed in 2004, suggesting their underlying stability. Although we did not uncover a sufficient number of independent pulsation modes for a unique seismological fit, our campaign revealed that the dominant pulsation mode at 609s is an evenly spaced triplet. The even nature of the triplet is suggestive of rotational splitting, implying an enigmatic rotation period of about 4.8 days. There are two viable alternatives assuming the triplet is real: either the period of 4.8 days is representative of the rotation period of the entire star with implications for the angular momentum evolution of these systems, or it is perhaps an indication of differential rotation with a fast rotating exterior and slow rotation deeper in the star. Investigating the possibility that a changing period could mimic a triplet suggests that this scenario is improbable, but not impossible. Using time-series spectra acquired in May 2009, we determine the orbital period of SDSS J161033.64-010223.3 to be 83.8 +/- 2.9 min. Three of the observed photometric frequencies from our May 2007 campaign appear to be linear combinations of the 609s pulsation mode with the first harmonic of the orbital period at 41.5min. This is the first discovery of a linear combination between nonradial pulsation and orbital motion for a variable white dwarf.

astro-ph.SR↗

arXiv2009

A Catalog of Near Infrared Spectra from Type Ia Supernovae

We present forty-one near infrared (NIR, 0.7-2.5 microns) spectra from normal Type Ia supernovae (SNe Ia) obtained at epochs ranging from fourteen days before to seventy-five days with respect to the maximum light date in the V-band. All data were obtained at the IRTF using the SpeX instrument. We identify many spectral features, measure the Doppler velocities, and discuss the chemical distribution of explosion products in SNe Ia. We describe procedures for smoothing data, fitting continua, and measuring absorption features to insure consistency for measurement and analysis. This sample provides the first opportunity to examine and compare a large number of SNe Ia in this wavelength region. NIR data are a rich source of information about explosion products whose signatures are blended or obscured in other spectral regions and NIR observations probe a greater radial depth than optical wavelengths. We analyze similarities and differences in the spectra and we show that the progressive development of spectral features for normal SNe Ia in the NIR is consistent with time. Measured Doppler velocities indicate that burning products in SNe Ia are distributed in distinct layers with no large scale mixing. Carbon is not detected in these data, in agreement with previous results with NIR data establishing very low limits on carbon abundance in SNe Ia. Carbon burning products, O and Mg, are plentiful in the outer layers suggesting that the entire progenitor is burned in the explosion. The data provide a resource for investigations of cross-correlations with other data libraries that may further constrain SN Ia physics and improve the effectiveness of SNe Ia as cosmological distance indicators.

astro-ph.CO↗

arXiv2008

Search for a stochastic gravitational-wave signal in the second round of the Mock LISA Data Challenges

The analysis method currently proposed to search for isotropic stochastic radiation of primordial or astrophysical origin with the Laser Interferometer Space Antenna (LISA) relies on the combined use of two LISA channels, one of which is insensitive to gravitational waves, such as the symmetrised Sagnac. For this method to work, it is essential to know how the instrumental noise power in the two channels are related to one another; however, no quantitative estimates of this key information are available to date. The purpose of our study is to assess the performance of the symmetrised Sagnac method for different levels of prior information regarding the instrumental noise. We develop a general approach in the framework of Bayesian inference and an end-to-end analysis algorithm based on Markov Chain Monte Carlo methods to compute the posterior probability density functions of the relevant model parameters. We apply this method to data released as part of the second round of the Mock LISA Data Challenges. For the selected (and somewhat idealised) cases considered here, we find that a prior uncertainty of a factor ~2 in the ratio between the power of the instrumental noise contributions in the two channels allows for the detection of isotropic stochastic radiation. More importantly, we provide a framework for more realistic studies of LISA's performance and development of analysis techniques in the context of searches for stochastic signals.

gr-qc↗

arXiv2008

Optical and UV Light Curves of the Accretion Disk Corona Source 4U 1822-371

The eclipsing low-mass X-ray binary 4U is the prototypical accretion disk corona (ADC) system. We have obtained new time-resolved UV spectrograms of 4U with the Hubble Space Telescope and new V- and J-band light curves with the 1.3-m SMARTS telescope at CTIO. We present an updated ephemeris for the times of the optical/UV eclipses. Model light curves do not give acceptable fits to the UV eclipses unless the models include an optically-thick ADC.

astro-ph↗

arXiv2007

Near-Infrared Spectra of the Black Hole X-Ray Binary, A0620-00

We present broadband NIR spectra of A0620-00 obtained with SpeX on the IRTF. The spectrum is characterized by a blue continuum on which are superimposed broad emission lines of HI and HeII and a host of narrower absorption lines of neutral metals and molecules. Spectral type standard star spectra scaled to the dereddened spectrum of A0620-00 in K exceed the A0620-00 spectrum in J and H for all stars of spectral type K7V or earlier, demonstrating that the donor star, unless later than K7V, cannot be the sole NIR flux source in A0620-00. In addition, the atomic absorption lines in the K3V spectrum are too weak with respect to those of A0620-00 even at 100% donor star contribution, restricting the spectral type of the donor star in A0620-00 to later than K3V. Comparison of the A0620-00 spectrum to scaled K star spectra indicates that the CO absorption features are significantly weaker in A0620-00 than in field dwarf stars. Fits of scaled model spectra of a Roche lobe-filling donor star to the spectrum of A0620-00 show that the best match to the CO absorption lines is obtained when the C abundance is reduced to [C/H] = -1.5. The donor star contribution in the H waveband is determined to be 82+-2%. Combined with previous published results from Froning & Robinson (2001) and Marsh et al. (1994), this gives a precise mass for the black hole in A0620-00 of M_BH = 9.7+-0.6 M_solar.

astro-ph↗

arXiv2006

Towards a pure ZZ Ceti instability strip

We have observed again two stars inside the ZZ Ceti instability strip that were previously classified as not-observed-to-vary (NOV) by Mukadam et al. (2004) and found them to be low-amplitude variables. Some evidence points to a pure ZZ Ceti instability strip; other evidence contests it. The two stars previously classified as NOV have Sloan Digital Sky Survey (SDSS) spectroscopic effective temperatures that place them inside the ZZ Ceti instability strip, and they were "contaminating" the strip as constant stars, which could indicate that the instability strip was no longer a simple evolutionary stage. A pure instability strip indicates that pulsation is a normal phase which all DAs must go through. We used effective temperatures derived from SDSS optical spectra by comparing them with model atmospheres to look for pulsators through time-resolved photometry and stars previously classified as NOV. Our new results indicate, but do not prove, a pure instability strip, because there are still other NOV stars that need to be observed again. Additionally, we have discovered five other ZZ Ceti stars based on their effective temperatures.

astro-ph↗

arXiv2006

Further Evidence for Variable Synchrotron Emission in XTE J1118+480 in Outburst

We present simultaneous multicolor infrared and optical photometry of the black hole X-ray transient XTE J1118+480 during its short 2005 January outburst, supported by simultaneous X-ray observations. The variability is dominated by short timescales, ~10s, although a weak superhump also appears to be present in the optical. The optical rapid variations, at least, are well correlated with those in X-rays. Infrared JHKs photometry, as in the previous outburst, exhibits especially large amplitude variability. The spectral energy distribution (SED) of the variable infrared component can be fitted with a power-law of slope alpha=-0.78 where Fnu is proportional to nu^alpha. There is no compelling evidence for evolution in the slope over five nights, during which time the source brightness decayed along almost the same track as seen in variations within the nights. We conclude that both short-term variability, and longer timescale fading, are dominated by a single component of constant spectral shape. We cannot fit the SED of the IR variability with a credible thermal component, either optically thick or thin. This IR SED is, however, approximately consistent with optically thin synchrotron emission from a jet. These observations therefore provide indirect evidence to support jet-dominated models for XTE J1118+480 and also provide a direct measurement of the slope of the optically thin emission which is impossible based on the average spectral energy distribution alone.

astro-ph↗