Michelle M. Buxton

Global optical/infrared–X-ray correlations in X-ray binaries: quantifying disc and jet contributions

The optical/near-infrared (OIR) region of the spectra of low-mass X-ray binaries appears to lie at the intersection of a variety of different emission processes. In this paper we present quasi-simultaneous OIR–X-ray observations of 33 XBs in an attempt to estimate the contributions of various emission processes in these sources, as a function of X-ray state and luminosity. A global correlation is found between OIR and X-ray luminosity for low-mass black hole candidate XBs (BHXBs) in the hard X-ray state, of the form LOIR / L 0.6 X . This correlation holds over 8 orders of magnitude in LX and includes data from BHXBs in quiescence and at large distances (LMC and M31). A similar correlation is found in low-mass neutron star XBs (NSXBs) in the hard state. For BHXBs in the soft state, all the near-infrared (NIR) and some of the optical emission is suppressed below the correlation, a behaviour indicative of the jet switching off/on in transition to/from the soft state. We compare these relations to theoretical models of a number of emission processes. We find that X-ray reprocessing in the disc and emission from the jets both predict a slope close to 0.6 for BHXBs, and both contribute to the OIR in BHXBs in the hard state, the jets producing � 90 percent of the NIR emission at high luminosities. X-ray reprocessing dominates the OIR in NSXBs in the hard state, with possible contributions from the jets (only at high luminosity) and the viscously heated disc. We also show that the optically thick jet spectrum of BHXBs extends to near the K-band. OIR spectral energy distributions of 15 BHXBs help us to confirm these interpretations. We present a prediction of the LOIR–LX behaviour of a BHXB outburst that enters the soft state, where the peak LOIR in the hard state rise is greater than in the hard state decline (the well known hysteretical behaviour). In addition, it is possible to estimate the X-ray, OIR and radio luminosity and the mass accretion rate in the hard state quasi-simultaneously, from observations of just one of these wavebands, since they are all linked through correlations. Finally, we have discovered that the nature of the compact object, the mass of the companion and the distance/reddening can be constrained by quasi-simultaneous OIR and X-ray luminosities.

Multiwavelength Observations of the 2002 Outburst of GX 339–4: Two Patterns of X-Ray-Optical/Near-Infrared Behavior

We report on quasi-simultaneous Rossi X-Ray Timing Explorer and optical/near-infrared (NIR) observations of the black hole candidate X-ray transient GX 339-4. Our observations were made over a time span of more than 8 months in 2002 and cover the initial rise and transition from a hard to a soft spectral state in X-rays. Two distinct patterns of correlated X-ray-optical/NIR behavior were found. During the hard state, the optical/NIR and X-ray fluxes correlated well, with a NIR versus X-ray flux power-law slope similar to that of the correlation found between X-ray and radio fluxes in previous studies of GX 339-4 and other black hole binaries. As the source went through an intermediate state, the optical/NIR fluxes decreased rapidly, and once it had entered the spectrally soft state, the optical/NIR spectrum of GX 339-4 was much bluer, and the ratio of X-ray to NIR flux was higher by a factor of more than 10 compared to the hard state. In the spectrally soft state, changes in the NIR preceded those in the soft X-rays by more than 2 weeks, indicating a disk origin of the NIR emission and providing a measure of the viscous timescale. A sudden onset of NIR flaring of ~0.5 mag on a timescale of 1 day was also observed during this period. We present spectral energy distributions, including radio data, and discuss possible sources for the optical/NIR emission. We conclude that, in the hard state, this emission probably originates in the optically thin part of a jet and that in none of the X-ray states is X-ray reprocessing the dominant source of optical/NIR emission. Finally, comparing the light curves from the all-sky monitor (ASM) and Proportional Counter Array (PCA) instruments, we find that the X-ray/NIR delay depends critically on the sensitivity of the X-ray detector, with the delay inferred from the PCA (if present at all) being a factor of 3-6 times shorter than the delay inferred from the ASM; this may be important in interpreting previously reported X-ray-optical/NIR lags.

The 'universal' radio/X-ray flux correlation : the case study of the black hole GX 339-4

The existing radio and X-ray flux correlation for Galactic black holes in the hard and quiescent states relies on a sample which is mostly dominated by two sources (GX 339-4 and V404 Cyg) observed in a single outburst. In this paper, we report on a series of radio and X-ray observations of the recurrent black hole GX 339-4 with the Australia Telescope Compact Array, the Rossi X-ray Timing Explorer and the Swift satellites. With our new long term campaign, we now have a total of 88 quasi-simultaneous radio and X-ray observations of GX 339-4 during its hard state, covering a total of seven outbursts over a 15--year period. Our new measurements represent the largest sample for a stellar mass black hole, without any bias from distance uncertainties, over the largest flux variations and down to a level that could be close to quiescence, making GX 339-4 the reference source for comparison with other accreting sources (black holes, neutrons stars, white dwarfs and active galactic nuclei). Our results demonstrate a very strong and stable coupling between radio and X-ray emission, despite several outbursts of different nature and separated by a period of quiescence. The radio and X-ray luminosity correlation of the form L_X ~L_Rad^0.62 +/-0.01 confirms the non-linear coupling between the jet and the inner accretion flow powers and better defines the standard correlation track in the radio-X-ray diagram for stellar mass black holes. We further note epochs of deviations from the fit that significantly exceed the measurement uncertainties, especially during the formation and destruction of the compact jets ...[abridged]. We incorporated our new data in a more global study of black hole candidates strongly supporting a scale invariance in the jet-accretion coupling of accreting black holes, and confirms the existence of two populations of sources in the radio/X-ray diagram.

The detailed optical light curve of GRB 030329

We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst (GRB) 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical data set contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. This data set allows us to follow the photometric evolution of the transient with unprecedented detail. We use the data to constrain the light curve of the underlying supernova (SN) 2003dh and show that it evolved faster than and was probably somewhat fainter than the Type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early- and late-time decay slopes are determined to be alpha(1) approximate to 1.1 and alpha(2) approximate to 2. Assuming this single-break power-law model, we constrain the break to lie between similar to3 and similar to8 days after the burst. This simple, singly broken power-law model, derived only from the analysis of our optical observations, may also account for available multiband data, provided that the break happened similar to8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, millimeter, and radio observations of this event. The unique early coverage available for this event allows us to trace the color evolution of the afterglow during the first hours after the burst. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling-break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations (less than or similar to8 days after the burst) are asymmetric, with a steep rise followed by a relatively slower ( by a factor of about 2) decline. The variations maintain a similar timescale during the first 4 days and then get significantly longer. The structure of these variations is similar to those previously detected in the afterglows of several GRBs.

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 that 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 β of the companion star, where β = 1 indicates a completely filled Roche lobe. In previous work a range of β between 0.9 and 1.0 was usually adopted. We use optical ellipsoidal light-curve data to constrain β. We find neutron star masses of 1.77 ± 0.08 M ☉ for Vela X-1, 0.87 ± 0.07 M ☉ for 4U 1538-52 (eccentric orbit), 1.00 ± 0.10 M ☉ for 4U 1538-52 (circular orbit), 1.04 ± 0.09 M ☉ for SMC X-1, 1.29 ± 0.05 M ☉ for LMC X-4, 1.49 ± 0.08 M ☉ for Cen X-3, and 1.07 ± 0.36 M ☉ 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.

Multiwavelength observations of the Galactic black hole transient 4U 1543-47 during outburst decay: state transitions and jet contribution

Multiwavelength observations of Galactic black hole (GBH) transients during state transitions and in the low/hard state may provide detailed information on the accretion structure of these systems. The object 4U 1543-47 is a GBH transient that was covered exceptionally well in X-rays and the infrared (daily observations) and reasonably well in the optical and radio during its outburst decay in 2002. When all the available information is gathered from the intermediate and the low/hard states, 4U 1543-47 makes an important contribution to our understanding of state transitions and the role of outflows in the high-energy emission properties of black hole binaries. The evolution of the X-ray spectral and temporal properties and the infrared light curve place strong constraints on different models to explain the overall emission from accreting black holes. The overall spectral energy distribution is consistent with a synchrotron origin for the optical and infrared emission; however, the X-ray flux is above the power-law continuation of the optical and infrared flux. The infrared light curve, the HEXTE light curve, and the evolution of the X-ray photon index indicate that the major source of hard X-rays cannot be direct synchrotron radiation from an acceleration region in a jet for most of the outburst decay.

The infrared/X-ray correlation of GX 339-4: Probing hard X-ray emission in accreting black holes.

GX 339-4 has been one of the key sources for unravelling the accretion ejection coupling in accreting stellar mass black holes. After a long period of quiescence between 1999 and 2002, GX 339-4 underwent a series of four outbursts that have been intensively observed by many ground-based observatories [radio/infrared (IR)/optical] and satellites (X-rays). Here, we present results of these broad-band observational campaigns, focusing on the optical-IR (OIR)/X-ray flux correlations over the four outbursts. We found tight OIR/X-ray correlations over four decades with the presence of a break in the IR/X-ray correlation in the hard state. This correlation is the same for all four outbursts. This can be interpreted in a consistent way by considering a synchrotron self-Compton origin of the X-rays in which the break frequency varies between the optically thick and thin regime of the jet spectrum. We also highlight the similarities and differences between optical/X-ray and IR/X-ray correlations which suggest a jet origin of the near-IR emission in the hard state while the optical is more likely dominated by the blackbody emission of the accretion disc in both hard and soft state. However, we find a non-negligible contribution of 40 per cent of the jet emission in the V band during the hard state. We finally concentrate on a soft-to-hard state transition during the decay of the 2004 outburst by comparing the radio, IR, optical and hard X-rays light curves. It appears that unusual delays between the peak of emission in the different energy domains may provide some important constraints on jet formation scenario.

A NEW DYNAMICAL MODEL FOR THE BLACK HOLE BINARY LMC X-1*

We present a dynamical model of the high mass X-ray binary LMC X-1 based on high-resolution optical spectroscopy and extensive optical and near-infrared photometry. From our new optical data we find an orbital period of P = 3.90917 +/- 0.00005 days. We present a refined analysis of the All Sky Monitor data from RXTE and find an X-ray period of P = 3.9094 +/- 0.0008 days, which is consistent with the optical period. A simple model of Thomson scattering in the stellar wind can account for the modulation seen in the X-ray light curves. The V-K color of the star (1.17 +/- 0.05) implies A(V) = 2.28 +/- 0.06, which is much larger than previously assumed. For the secondary star, we measure a radius of R-2 = 17.0 +/- 0.8 R-circle dot and a projected rotational velocity of V-rot sin i = 129.9 +/- 2.2 km s(-1). Using these measured properties to constrain the dynamical model, we find an inclination of i = 36 degrees.38 +/- 1 degrees.92, a secondary star mass of M-2 = 31.79 +/- 3.48 M-circle dot, and a black hole mass of 10.91 +/- 1.41 M-circle dot. The present location of the secondary star in a temperature-luminosity diagram is consistent with that of a star with an initial mass of 35 M-circle dot that is 5 Myr past the zero-age main sequence. The star nearly fills its Roche lobe (approximate to 90% or more), and owing to the rapid change in radius with time in its present evolutionary state, it will encounter its Roche lobe and begin rapid and possibly unstable mass transfer on a timescale of a few hundred thousand years.

Optical and X-ray observations of the neutron star soft X-ray transient XTE J1709-267

In this paper we report on the discovery of the optical counterpart to the neutron star soft X-ray transient (SXT) XTE J1709-267 at an R-band magnitude of R = 20.5 ± 0.1 and 22.24 ± 0.03, in outburst and quiescence, respectively. We further report the detection of type I X-ray bursts in RXTE data obtained during an outburst of the source in 2002. These bursts show a precursor before the onset of the main burst event, reminiscent of photospheric radius expansion bursts. Sifting through the archival RXTE data for the burster 4U 1636-53, we found a nearly identical burst with precursor in 4U 1636-53. A comparison of this burst to true photospheric radius expansion bursts in 4U 1636-53 leads us to conclude that these bursts-with-precursor do not reach the Eddington limit. Nevertheless, from the burst properties we can derive that the distance to XTE J1709-267 is consistent with the distance of the Globular Cluster NGC 6293. We further report on the analysis of a 22.7 ks observation of XTE J1709-267 obtained with the Chandra satellite when the source was in quiescence. We found that the source has a soft quiescent spectrum which can be fit well by an absorbed black body or neutron star atmosphere model. A power law contributes less than ∼20 per cent to the 0.5-10 keV unabsorbed flux of (1.0 ± 0.3) × 10 -13 erg cm -2 s -1 . This flux is only slightly lower than the flux measured right after the outburst in 2002. This is in contrast to the recent findings for MXB 1659-29, where the quiescent source flux decreased gradually by a factor of ∼7-9 over a period of 18 months. Finally, we compared the fractional power-law contribution to the unabsorbed 0.5-10 keV luminosity for neutron star SXTs in quiescence for which the distance is well-known. We find that the power-law contribution is low only when the source quiescent luminosity is close to ∼1-2 x 10 33 erg s -1 . Both at higher and lower values the power-law contribution to the 0.5-10 keV luminosity increases. We discuss how models for the quiescent X-ray emission can explain these trends.

The 2002 Outburst of the Black Hole X-Ray Binary 4U 1543–47: Optical and Infrared Light Curves

We obtained simultaneous optical and near-infrared observations of 4U 1543-47 during its 2002 outburst. The most striking feature of the outburst light curve is the secondary maximum, which appears after the object transitions into the low-hard state. This secondary maximum is much stronger in the infrared bands than in the optical. We suggest that the origin of the secondary maximum flux may be synchrotron radiation associated with a jet. Close infrared monitoring may lead to reliable triggers for simultaneous multiwavelength campaigns to study jet formation processes.