arXiv subjects

Robert I. Hynes

Publications and source records attributed to Robert I. Hynes.

15 recordsLinked to original sources

arXiv2026

Optical Counterparts to X-ray sources in LSST DP1

We present a crossmatch between a combined catalog of X-ray sources and the Vera C. Rubin Observatory Data Preview 1 (DP1) to identify optical counterparts. The six fields targeted as part of DP1 include the Extended Chandra Deep Field South (E-CDF-S), the Euclid Deep Field South (EDF-S), the Fornax Dwarf Spheroidal Galaxy (Fornax dSph), 47 Tucanae (47 Tuc) and science validation fields with low galactic and ecliptic latitude (SV\_95\_-25 and SV\_38\_7, respectively). We find matches to 2314 of 3830 X-ray sources. We also compare our crossmatch to DP1 in the E-CDF-S field to previous efforts to identify optical counterparts. The probability of a chance coincidence match varies across each DP1 field, with overall high reliability in the E-CDF-S field, and lower proportion of high-reliability matches in the other fields. The majority of previously known sources that we detect are, unsurprisingly, active galaxies. We plot the X-ray-to-optical flux ratio against optical magnitude and color in an effort to identify Galactic accreting compact objects using a {\em Gaia} color threshold transformed to LSST $g$--$i$, but do not find any strong candidates in these primarily extragalactic counterparts. The DP1 dataset contains high-cadence photometry collected over a number of nights. We calculate the Stetson $ J $ variability index for each object under the hypothesis that X-ray counterparts tend to exhibit higher optical variability; however, the evidence is inconclusive whether our sample is more variable over DP1 timescales when compared to field objects.

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arXiv2025

Multidisciplinary Science in the Multimessenger Era

Astrophysical observations of the cosmos allow us to probe extreme physics and answer foundational questions on our universe. Modern astronomy is increasingly operating under a holistic approach, probing the same question with multiple diagnostics including how sources vary over time, how they appear across the electromagnetic spectrum, and through their other signatures, including gravitational waves, neutrinos, cosmic rays, and dust on Earth. Astrophysical observations are now reaching the point where approximate physics models are insufficient. Key sources of interest are explosive transients, whose understanding requires multidisciplinary studies at the intersection of astrophysics, gravity, nuclear science, plasma physics, fluid dynamics and turbulence, computation, particle physics, atomic, molecular, and optical science, condensed matter and materials science, radiation transport, and high energy density physics. This white paper provides an overview of the major scientific advances that lay at the intersection of physics and astronomy and are best probed through time-domain and multimessenger astrophysics, an exploration of how multidisciplinary science can be fostered, and introductory descriptions of the relevant scientific disciplines and key astrophysical sources of interest.

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arXiv2025

Ultraviolet spectroscopy of the black hole X-ray binary MAXI J1820+070 across a state transition

We present ultraviolet (UV) spectroscopic observations covering three distinct accretion states of the low-mass X-ray binary (LMXB) MAXI J1820+070: the luminous hard state, a hard-intermediate state and the soft state. Our observations were obtained during the 2018 eruption of MAXI J1820+070 with the Hubble Space Telescope (HST) and AstroSat observatory. The extinction towards the source turns out to be low - $\rm E_{B-V} = 0.2 \pm 0.05$ - making it one of the best UV accretion laboratories among LMXBs. Remarkably, we observe only moderate differences between all three states, with all spectra displaying similar continuum shapes and emission lines. Moreover, the continua are not well-described by physically plausible irradiated disc models. All of this challenges the standard reprocessing picture for UV emission from erupting LMXBs. The UV emission lines are double-peaked, with high-ionization lines displaying higher peak-to-peak velocities. None of the lines display obvious outflow signatures, even though blue-shifted absorption features have been seen in optical and near-infrared lines during the hard state. The emission line ratios are consistent with normal abundances, suggesting that the donor mass at birth was low enough to avoid CNO processing ($\rm M_{2,i} \lesssim 1.0 - 1.5 {\mathrm M_{\odot}}$). Finally, we study the evolution of UV variability in our time-resolved HST observations (hard and hard-intermediate states). All UV power spectra can be modelled with a broken power-law, superposed on which we tentatively detect the $\simeq 18$s quasi-periodic oscillation (QPO) that has been seen in other spectral bands.

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arXiv2023

Links Between Optical and X-ray Light in Scorpius X-1

We observed the low-mass X-ray binary Sco X-1 for 12 nights simultaneously using the Rossi X-Ray Timing Explorer and the Otto Struve Telescope at McDonald Observatory at 1 second time resolution. This is among the most comprehensive simultaneous X-Ray/optical data sets of Sco X-1. Evidence of reprocessing was observed in the form of nine positive, near-zero lag peaks in the cross correlation function, eight of which were relatively small and took the shape of piecewise exponential functions. These peaks were initially identified by eye, after which a computational identification scheme was developed to confirm their significance. Based on their short lags (less than 4 seconds), as well as their occurrence on the flaring branch and soft apex, the small cross correlation features are likely to be caused by reprocessing off the outer disc, although the companion could still make a contribution to their tails. The Z track was parameterized using a rank number scheme so that the system's location on the track could be numerically defined. Plotting the results against the optical reveals an increasing step function when moving from the horizontal to the normal to the flaring branch, with differential optical levels at ~0.47, ~0.57, and ~1.1 respectively. An additional correlation between Z track location and the optical was found on the upper flaring branch. An optical intensity histogram reveals a transition region between the normal and flaring branches with only intermediate fluxes.

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arXiv2021

Dynamical Modeling of CXOGBS J175553.2-281633: A 10 Hour Long Orbital Period Cataclysmic Variable

We present modeling of the long-term optical light curve and radial velocity curve of the binary stellar system CXOGBS J175553.2-281633, first detected in X-rays in the \textit{Chandra} Galactic Bulge Survey. We analyzed 7 years of optical I-band photometry from OGLE and found long-term variations from year to year. These long-term variations can most likely be explained with by either variations in the luminosity of the accretion disk or a spotted secondary star. The phased light curve has a sinusoidal shape, which we interpret as being due to ellipsoidal modulations. We improve the orbital period to be $P = 10.34488 \pm 0.00006$ h with a time of inferior conjunction of the secondary star $T_0 = {\rm HJD\ } 2455260.8204 \pm 0.0008$. Moreover, we collected 37 spectra over 6 non-consecutive nights. The spectra show evidence for an evolved K7 secondary donor star, from which we obtain a semi-amplitude for the radial velocity curve of $K_2 = 161 \pm 6 $ km s$^{-1}$. Using the light curve synthesis code {\tt XRbinary}, we derive the most likely orbital inclination for the binary of $i = 63.0\pm0.7$ deg, a primary mass of $M_1 = 0.83 \pm 0.06$ M$_\odot$, consistent with a white dwarf accretor, and a secondary donor mass of $M_2 = 0.65 \pm 0.07$ M$_\odot$, consistent with the spectral classification. Therefore, we identify the source as a long orbital period cataclysmic variable star.

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arXiv2017

Simultaneous multiwavelength observations of V404 Cygni during its 2015 June outburst decay strengthen the case for an extremely energetic jet-base

We present results of multiband optical photometry of the black hole X-ray binary system V404 Cygni obtained using Wheaton College Observatory's 0.3m telescope, along with strictly simultaneous INTEGRAL and Swift observations during 2015 June 25.15--26.33 UT, and 2015 June 27.10--27.34 UT. These observations were made during the 2015 June outburst of the source when it was going through an epoch of violent activity in all wavelengths ranging from radio to $γ$-rays. The multiwavelength variability timescale favors a compact emission region, most likely originating in a jet outflow, for both observing epochs presented in this work. The simultaneous INTEGRAL/Imager on Board the Integral Satellite (IBIS) 20--40 keV light curve obtained during the June 27 observing run correlates very strongly with the optical light curve, with no detectable delay between the optical bands as well as between the optical and hard X-rays. The average slope of the dereddened spectral energy distribution was roughly flat between the $I_C$- and $V$-bands during the June 27 run, even though the optical and X-ray flux varied by $>$25$\times$ during the run, ruling out an irradiation origin for the optical and suggesting that the optically thick to optically thin jet synchrotron break during the observations was at a frequency larger than that of $V$-band, which is quite extreme for X-ray binaries. These observations suggest that the optical emission originated very close to the base of the jet. A strong H$α$ emission line, probably originating in a quasi-spherical nebula around the source, also contributes significantly in the $R_C$-band. Our data, in conjunction with contemporaneous data at other wavelengths presented by other groups, strongly suggest that the jet-base was extremely compact and energetic during this phase of the outburst.

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arXiv2017

An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

Relativistic plasma jets are observed in many accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot be easily disentangled from other accreting components. Here, we show that rapid optical flux variations from a Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by ~0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these sub-second optical variations has hitherto been controversial. Not only does our work strongly support a jet origin for the optical variations, it also sets a characteristic elevation of <~10$^3$ Schwarzschild radii for the main inner optical emission zone above the black hole, constraining both internal shock and magnetohydrodynamic models. Similarities with blazars suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black hole binaries show very similar optical lags, so this size scale may be a defining feature of such systems.

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arXiv2013

The Galactic Bulge Survey: completion of the X-ray survey observations

We provide the Chandra source list for the last ~quarter of the area covered by the Galactic Bulge Survey (GBS). The GBS targets two strips of 6\degr x 1\degr (12 square degrees in total), one above (1\degr<b<2\degr) and one below (-2\degr<b<-1\degr) the Galactic plane in the direction of the Galactic Center at X-ray, optical and near-infrared wavelengths. For the X-ray part of the survey we use 2 ks per Chandra pointing. We find 424 X-ray sources in the 63 Chandra observations we report on here. These sources are in addition to the 1216 X-ray sources discovered in the first part of the GBS survey described before. We discuss the characteristics and the X-ray variability of the brightest of the sources as well as the radio properties from existing radio surveys. We point out an interesting asymmetry in the number of X-ray sources as a function of their Galactic l and b coordinates which is probably caused by differences in average extinction towards the different parts of the GBS survey area.

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arXiv2012

Radio sources in the Chandra Galactic Bulge survey

We discuss radio sources in the Chandra Galactic Bulge survey region. By cross-matching the X-ray sources in this field with the NVSS archival data, we find 12 candidate matches. We present a classification scheme for radio/X-ray matches in surveys taken in or near the Galactic Plane, taking into account other multi-wavelength data. We show that none of the matches found here is likely to be due to coronal activity from normal stars because the radio to X-ray flux ratios are systematically too high. We show that one of the sources could be a radio pulsar, and that one could be a planetary nebula, but that the bulk of the sources are likely to be background active galactic nuclei (AGN), with many confirmed through a variety of approaches. Several of the AGN are bright enough in the near infrared (and presumably in the optical) to use as probes of the interstellar medium in the inner Galaxy.

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arXiv2012

The Orbital Period of Scorpius X-1

The orbital period of Sco X-1 was first identified by Gottlieb et al. (1975). While this has been confirmed on multiple occasions, this work, based on nearly a century of photographic data, has remained the reference in defining the system ephemeris ever since. It was, however, called into question when Vanderlinde et al. (2003) claimed to find the one-year alias of the historical period in RXTE/ASM data and suggested that this was the true period rather than that of Gottlieb et al. (1975). We examine data from the All Sky Automated Survey (ASAS) spanning 2001-2009. We confirm that the period of Gottlieb et al. (1975) is in fact the correct one, at least in the optical, with the one-year alias strongly rejected by these data. We also provide a modern time of minimum light based on the ASAS data.

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arXiv2012

Optical and Infrared Light Curves of the Eclipsing X-ray Binary V395 Car = 2S 0921-630

We present results of optical and infrared photometric monitoring of the eclipsing low-mass X-ray binary V395 Car (2S 0921-630). Our observations reveal a clear, repeating orbital modulation with an amplitude of about one magnitude in B, and V and a little less in J. Combining our data with archival observations spanning about 20 years, we derive an updated ephemeris with orbital period 9.0026+/-0.0001d. We attribute the modulation to a combination of the changing aspect of the irradiated face of the companion star and eclipses of the accretion disk around the neutron star. Both appear to be necessary as a secondary eclipse of the companion star is clearly seen. We model the B, V, and J lightcurves using a simple model of an accretion disk and companion star and find a good fit is possible for binary inclinations of 82.2+/-1.0 degrees. We estimate the irradiating luminosity to be about 8x10^35 erg/s, in good agreement with X-ray constraints.

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arXiv2012

Gemini/GMOS Spectroscopy of EXO 0748-676 (=UY Vol) in Outburst

We present a phase-resolved, optical, spectroscopic study of the eclipsing low-mass X-ray binary, EXO 0748-676 = UY Vol. The sensitivity of Gemini combined with our complete phase coverage makes for the most detailed blue spectroscopic study of this source obtained during its extended twenty-four year period of activity. We identify 12 optical emission lines and present trailed spectra, tomograms, and the first modulation maps of this source in outburst. The strongest line emission originates downstream of the stream-impact point, and this component is quite variable from night-to-night. Underlying this is weaker, more stable axisymmetric emission from the accretion disk. We identify weak, sharp emission components moving in phase with the donor star, from which we measure Kem = 329+/-26 km/s. Combining all the available dynamical constraints on the motion of the donor star with our observed accretion disk velocities we favor a neutron star mass close to canonical (M1~1.5Msun) and a very low mass donor (M2~0.1$Msun). We note that there is no evidence for CNO processing that is often associated with undermassive donor stars, however. A main sequence donor would require both a neutron star more massive than 2Msun and substantially sub-Keplerian disk emission.

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arXiv2010

Multiwavelength Observations of Accretion in Low-Mass X-ray Binary Systems

This work is intended to provide an introduction to multiwavelength observations of low-mass X-ray binaries and the techniques used to analyze and interpret their data. The focus will primarily be on ultraviolet, optical, and infrared observations and their connections to other wavelengths. The topics covered include: outbursts of soft X-ray transients, accretion disk spectral energy distributions, orbital lightcurves in luminous and quiescent states, super-orbital and sub-orbital variability, line spectra, system parameter determinations, and echo-mapping and other rapid correlated variability.

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arXiv2009

The Structure of the Accretion Disk in the ADC X-Ray Binary 4U 1822-371 at Optical and Ultraviolet Wavelengths

The eclipsing low-mass X-ray binary 4U 1822-371 is the prototypical accretion disk corona (ADC) system. We have obtained new time-resolved UV spectroscopy of 4U 1822-371 with the Advanced Camera for Surveys/Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope (HST) and new V- and J-band photometry with the 1.3-m SMARTS telescope at CTIO. We use the new data to construct its UV/optical spectral energy distribution and its orbital light curve in the UV, V, and J bands. We derive an improved ephemeris for the optical eclipses and confirm that the orbital period is changing rapidly, indicating extremely high rates of mass flow in the system; and we show that the accretion disk in the system has a strong wind with projected velocities up to 4000 km/s. We show that the disk has a vertically-extended, optically-thick component at optical wavelengths.This component extends almost to the edge of the disk and has a height equal to ~0.5 of the disk radius. As it has a low brightness temperature, we identify it as the optically-thick base of a disk wind, not as the optical counterpart of the ADC. Like previous models of 4U 1822-371, ours needs a tall obscuring wall near the edge of the accretion disk, but we interpret the wall as a layer of cooler material at the base of the disk wind, not as a tall, luminous disk rim.

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arXiv2006

VW Hyi: optical spectroscopy and Doppler tomography

We present high quality optical spectroscopy of the SU UMa-subtype dwarf nova, VW Hyi taken while the system was in quiescence. An S-wave is executed by the emission cores of the Hydrogen Balmer lines and by the emission lines of He I, Ca II, Fe II and He II. Using Doppler tomography we show it originates in the accretion stream-disc impact region. The He II emission is strongly phase-dependent, suggesting it originates exclusively within a hot cavity at the initial impact. We map the ionization structure of the stream-disc interaction region. One possible interpretation of this is that the Balmer hotspot lies downstream of the He II hotspot in the outer accretion disc, with the He I and Ca II hotspots at intermediate locations between the two. This suggests that Balmer emission is suppressed until material has cooled somewhat downstream of the impact site and is able to recombine. We favour a phase offset of 0.15+/-0.04 between the photometric ephemeris and inferior conjunction of the mass donor. The white dwarf contributes significantly to the optical continuum, with broad Balmer absorption and narrow Mg II 4481 absorption clearly apparent. This latter feature yields the gravitational redshift: v_grav=38+/-21 km s^-1, so M_1=0.71+0.18-0.26 M_sun. This implies M_2=0.11 +/- 0.03 M_sun and hence the donor is not a brown dwarf. A prominent Balmer jump is also observed. We note that the previously accepted system parameters for both VW Hyi and WX Hyi incoporate an algebraic error, and we provide a recalculated M_1(q) plane for WX Hyi.

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