arXiv subjects

C. Brocksopp

Publications and source records attributed to C. Brocksopp.

At least 19 recordsLinked to original sources

arXiv2013

Multiwavelength observations of the black hole transient XTE J1752-223 during its 2010 outburst decay

Galactic black hole transients show many interesting phenomena during outburst decays. We present simultaneous X-ray (RXTE, Swift, and INTEGRAL), and optical/near-infrared (O/NIR) observations (SMARTS) of the X-ray transient XTE J1752-223 during its outburst decay in 2010. The multiwavelength observations over 150 days in 2010 cover the transition from soft to hard spectral state. We discuss the evolution of radio emission is with respect to the O/NIR light curve which shows several flares. One of those flares is bright and long, starting about 60 days after the transition in X-ray timing properties. During this flare, the radio spectral index becomes harder. Other smaller flares occur along with the X-ray timing transition, and also right after the detection of the radio core. We discuss the significances of these flares. Furthermore, using the simultaneous broadband X-ray spectra including INTEGRAL, we find that a high energy cut-off with a folding energy near 250 keV is necessary around the time that the compact jet is forming. The broad band spectrum can be fitted equally well with a Comptonization model. In addition, using photoelectric absorption edges in the XMM-Newton RGS X-ray spectra and the extinction of red clump giants in the direction of the source, we find a lower limit on the distance of > 5 kpc.

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arXiv2013

Formation of the compact jets in the black hole GX 339-4

Galactic black hole binaries produce powerful outflows with emit over almost the entire electromagnetic spectrum. Here, we report the first detection with the Herschel observatory of a variable far-infrared source associated with the compact jets of the black hole transient GX 339-4 during the decay of its recent 2010-2011 outburst, after the transition to the hard state. We also outline the results of very sensitive radio observations conducted with the Australia Telescope Compact Array, along with a series of near-infrared, optical (OIR) and X-ray observations, allowing for the first time the re-ignition of the compact jets to be observed over a wide range of wavelengths. The compact jets first turn on at radio frequencies with an optically thin spectrum that later evolves to optically thick synchrotron emission. An OIR reflare is observed about ten days after the onset of radio and hard X-ray emission, likely reflecting the necessary time to build up enough density, as well as to have acceleration (e.g. through shocks) along an extended region in the jets. The Herschel measurements are consistent with an extrapolation of the radio inverted power-law spectrum, but they highlight a more complex radio to OIR spectral energy distribution for the jets.

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arXiv2012

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.

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arXiv2012

Spectral evolution of the X-ray nova XTE J1859+226 during its outburst observed by BeppoSAX and RXTE

We report results of an extensive analysis of the X-ray nova XTE J1859+226 observed with BeppoSAX and RXTE during its 1999 outburst. We modelled the source spectrum with a multicolour blackbody-like feature plus the generic Comptonization model BMC which has the advantage of providing spectral description of the emitted-radiation properties without assumptions on the underlying physical process. The multicolour component is attributed to the geometrically thin accretion disk, while the Comptonization spectrum is claimed to originate in the innermost sub-Keplerian region of the system (transition layer). We find that XTE J1859+226 covers all the spectral states typical of black-hole sources during its evolution across the outburst, however during the very high state, when the disk contribution to the total luminosity is more than 70% and the root mean square variability > 5%, the high-energy photon index is closer to a hard state value (Gamma ~ 1.8). The BMC normalization and photon index Gamma well correlate with the radio emission, and we also observed a possible saturation effect of Gamma at the brightest radio emission levels. A strong positive correlation was found between the fraction of Comptonized seed photons and the observed integrated root mean square variability, which strengthens the idea that most of the fast variability in these systems is provided by the innermost Compton cloud, which may be also identified as a jet.

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arXiv2011

Transient Relativistic Ejections and Stationary Core in XTE J1752-223

The Galactic X-ray transient XTE J1752-223 was shown to have properties of black hole binary candidates. As reported in our previous paper, we identified transient and decelerating ejecta in multi-epoch Very Long Baseline Interferometry (VLBI) observations with the European VLBI Network (EVN) and the NRAO Very Long Baseline Array (VLBA). Here we present new EVN and VLBA data in which a new transient ejection event and later a stationary component are identified. The latter is interpreted as a reappearance of the radio core/compact jet during the transition from soft to hard X-ray state. This component appears to be highly variable in brightness although effects of tropospheric instabilities might play a role too. We also re-analyze the earlier VLBI data and find that the transient ejecta closer to the core position has significantly higher proper motion, further strengthening the case for strongly decelerating ejecta on the scale of several hundred milli-arcsecond, never observed in X-ray binaries before. Although the distance of the source is not well constrained, it is clear that these ejectas are at least mildly relativistic at the early stages. Moreover, we show the large scale environment of the transient from the Westerbork synthesis array data recorded in parallel during the EVN run.

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arXiv2011

Discovery and evolution of the new black hole candidate Swift J1539.2-6227 during its 2008 outburst

We report on the discovery by the Swift Gamma-Ray Burst Explorer of the black hole candidate Swift J1539.2-6227 and the subsequent course of an outburst beginning in November 2008 and lasting at least seven months. The source was discovered during normal observations with the Swift Burst Alert Telescope (BAT) on 2008 November 25. An extended observing campaign with the Rossi X-Ray Timing Explorer (RXTE) and Swift provided near-daily coverage over 176 days, giving us a good opportunity to track the evolution of spectral and timing parameters with fine temporal resolution through a series of spectral states. The source was first detected in a hard state during which strong low-frequency quasi-periodic oscillations (QPOs) were detected. The QPOs persisted for about 35 days and a signature of the transition from the hard to soft intermediate states was seen in the timing data. The source entered a short-lived thermal state about 40 days after the start of the outburst. There were variations in spectral hardness as the source flux declined and returned to a hard state at the end of the outburst. The progression of spectral states and the nature of the timing features provide strong evidence that Swift J1539.2-6227 is a candidate black hole in a low-mass X-ray binary system.

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arXiv2011

Swift observations of black hole candidate XTE J1752-223 during outburst

Here we summarise the Swift broadband observations of the recently discovered X-ray transient and black hole candidate, XTE J1752-223,obtained over the period of outburst from October 2009 to June 2010. We offer a phenomenological treatment of the spectra as an indication of the canonical spectral state of the source during different periods of the outburst. We find that the high energy hardness-intensity diagrams over two separate bands follows the canonical behavior, confirming the spectral states. From Swift-UVOT data we confirm the presence of an optical counterpart which displays variability correlated, in the soft state, to the X-ray emission observed by Swift-XRT. The optical counterpart also displays hysteretical behaviour between the states not normally observed in the optical bands, suggesting a possible contribution from a synchrotron emitting jet to the optical emission in the rising hard state. Our XRT timing analysis shows that in the hard state there is significant variability below 10Hz which is more pronounced at low energies, while during the soft state the level of variability is consistent with being minimal.These properties of XTE J1752-223 support its candidacy as a black hole in the Galactic centre region.

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arXiv2011

An accurate position for the black hole candidate XTE J1752-223: re-interpretation of the VLBI data

Using high-precision astrometric optical observations from the Walter Baade Magellan Telescope in conjunction with high-resolution very long baseline interferometric (VLBI) radio imaging with the Very Long Baseline Array (VLBA), we have located the core of the X-ray binary system XTE J1752-223. Compact radio emission from the core was detected following the state transition from the soft to the hard X-ray state. Its position to the south-east of all previously-detected jet components mandated a re-analysis of the existing VLBI data. Our analysis suggests that the outburst comprised at least two ejection events prior to 2010 February 26. No radio-emitting components were detected to the south-east of the core at any epoch, suggesting that the receding jets were Doppler-deboosted below our sensitivity limit. From the ratio of the brightness of the detected components to the measured upper limits for the receding ejecta, we constrain the jet speed to be greater than 0.66c and the inclination angle to the line of sight to be less than 49 degrees. Assuming that the initial ejection event occurred at the transition from the hard intermediate state to the soft intermediate state, an initial period of ballistic motion followed by a Sedov phase (i.e. self-similar adiabatic expansion) appears to fit the motion of the ejecta better than a uniform deceleration model. The accurate core location can provide a long time baseline for a future proper motion determination should the system show a second outburst, providing insights into the formation mechanism of the compact object.

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arXiv2011

Radiatively efficient accreting black holes in the hard state: the case study of H1743-322

In recent years, much effort has been devoted to unraveling the connection between the accretion flow and the jets in accreting compact objects. In the present work, we report new constraints on these issues, through the long term study of the radio and X-ray behaviour of the black hole candidate H1743-322. This source is known to be one of the `outliers' of the universal radio/X-ray correlation, i.e. a group of accreting stellar-mass black holes displaying fainter radio emission for a given X-ray luminosity than expected from the correlation. Our study shows that the radio and X-ray emission of H1743-322 are strongly correlated at high luminosity in the hard spectral state. However, this correlation is unusually steep for a black hole X-ray binary: b ~ 1.4 (with L_{Radio} \propto L_{X}^{b}). Below a critical luminosity, the correlation becomes shallower until it rejoins the standard correlation with b ~ 0.6. Based on these results, we first show that the steep correlation can be explained if the inner accretion flow is radiatively efficient during the hard state, in contrast to what is usually assumed for black hole X-ray binaries in this spectral state. The transition between the steep and the standard correlation would therefore reflect a change from a radiatively efficient to a radiatively inefficient accretion flow. Finally, we investigate the possibility that the discrepancy between `outliers' and `standard' black holes arises from the outflow properties rather than from the accretion flow.

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arXiv2010

A decelerating jet observed by the EVN and VLBA in the X-ray transient XTE J1752-223

The recently discovered Galactic X-ray transient XTE J1752-223 entered its first known outburst in 2010, emitting from the X-ray to the radio regimes. Its general X-ray properties were consistent with those of a black hole candidate in various spectral states, when ejection of jet components is expected. To verify this, we carried out very long baseline interferometry (VLBI) observations. The measurements were carried out with the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA) at four epochs in 2010 February. The images at the first three epochs show a moving jet component that is significantly decelerated by the last epoch, when a new jet component appears that is likely to be associated with the receding jet side. The overall picture is consistent with an initially mildly relativistic jet, interacting with the interstellar medium or with swept-up material along the jet. The brightening of the receding ejecta at the final epoch can be well explained by initial Doppler deboosting of the emission in the decelerating jet.

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arXiv2010

Black hole candidate XTE J1752-223: Swift observations of canonical states during outburst

We present Swift broadband observations of the recently discovered black hole candidate, X-ray transient, XTE J1752-223, obtained over the period of outburst from October 2009 to June 2010. From Swift-UVOT data we confirm the presence of an optical counterpart which displays variability correlated, in the soft state, to the X-ray emission observed by Swift-XRT. The optical counterpart also displays hysteretical behaviour between the states not normally observed in the optical bands, suggesting a possible contribution from a synchrotron emitting jet to the optical emission in the rising hard state. We offer a purely phenomenological treatment of the spectra as an indication of the canonical spectral state of the source during different periods of the outburst. We find that the high energy hardness-intensity diagrams over two separate bands follows the canonical behavior, confirming the spectral states. Our XRT timing analysis shows that in the hard state there is significant variability below 10Hz which is more pronounced at low energies, while during the soft state the level of variability is consistent with being minimal. These properties of XTE J1752-223 support its candidacy as a black hole in the Galactic centre region.

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arXiv2010

Disentangling jet and disc emission from the 2005 outburst of XTE J1118+480

The black hole X-ray transient, XTE J1118+480, has now twice been observed in outburst - 2000 and 2005 - and on both occasions remained in the low/hard X-ray spectral state. Here we present radio, infrared, optical, soft X-ray and hard X-ray observations of the more recent outburst. We find that the lightcurves have very different morphologies compared with the 2000 event and the optical decay is delayed relative to the X-ray/radio. We attribute this lesser degree of correlation to contributions of emission from multiple components, in particular the jet and accretion disc. Whereas the jet seemed to dominate the broadband spectrum in 2000, in 2005 the accretion disc seems to be more prominent and we use an analysis of the lightcurves and spectra to distinguish between the jet and disc emission. There also appears to be an optically thin component to the radio emission in the 2005 data, possibly associated with multiple ejection events and decaying as the outburst proceeds. These results add to the discussion that the term "low/hard state'" covers a wider range of properties than previously thought, if it is to account for XTE J1118+480 during these two outbursts.

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arXiv2009

Constraining Jet/Disk Geometry and Radiative Processes in Stellar Black Holes XTE J1118+480 and GX 339-4

We present results from modeling of quasi-simultaneous broad band (radio through X-ray) observations of the galactic stellar black hole (BH) transient X-ray binary (XRB) systems XTE J1118+480 and GX 339-4 using an irradiated disc + compact jet model. In addition to quantifying the physical properties of the jet, we have developed a new irradiated disc model which also constrains the geometry and temperature of the outer accretion disc by assuming a disc heated by viscous energy release and X-ray irradiation from the inner regions. For the source XTE J1118+480, which has better spectral coverage of the two in optical and near-IR (OIR) wavelengths, we show that the entire broad band continuum can be well described by an outflow-dominated model + an irradiated disc. The best-fit radius of the outer edge of the disc is consistent with the Roche lobe geometry of the system, and the temperature of the outer edge of the accretion disc is similar to those found for other XRBs. Irradiation of the disc by the jet is found to be negligible for this source. For GX 339-4, the entire continuum is well described by the jet-dominated model only, with no disc component required. For the two XRBs, which have very different physical and orbital parameters and were in different accretion states during the observations, the sizes of the jet base are similar and both seem to prefer a high fraction of non-thermal electrons in the acceleration/shock region and a magnetically dominated plasma in the jet. These results, along with recent similar results from modeling other galactic XRBs and AGNs, may suggest an inherent unity in diversity in the geometric and radiative properties of compact jets from accreting black holes.

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arXiv2007

Three episodes of jet activity in the FRII radio galaxy B0925+420

We present Very Large Array images of a "Double-Double Radio Galaxy", a class of objects in which two pairs of lobes are aligned either side of the nucleus. In this object, B0925+420, we discover a third pair of lobes, close to the core and again in alignment with the other lobes. This first-known "Triple-Double" object strongly increases the likelihood that these lobes represent mutiple episodes of jet activity, as opposed to knots in an underlying jet. We model the lobes in terms of their dynamical evolution. We find that the inner pair of lobes is consistent with the outer pair having been displaced buoyantly by the ambient medium. The middle pair of lobes is more problematic - to the extent where an alternative model interpreting the middle and inner "lobes" as additional bow shocks within the outer lobes may be more appropriate - and we discuss the implications of this on our understanding of the density of the ambient medium.

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arXiv2006

Recent observational progress in AM CVn binaries

We present the results of some recent research on AM CVn systems. We present: X-ray/UV observations made using XMM-Newton; the X-ray grating spectrum of RX J1914+24; preliminary results of a search for radio emission from AM CVn binaries, and discuss the strategy and first results of the RATS project, whose main aim is to discover AM CVn systems.

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arXiv2006

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 L_OIR is proportional to Lx^0.6. 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. (abridged)

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arXiv2005

Multiwavelength XMM-Newton observations of the Laor et al. sample of PG quasars

We present XMM-Newton/EPIC spectra for the Laor et al. sample of Palomar Green quasars. We find that a power-law provides a reasonable fit to the 2-5 keV region of the spectra. Excess soft X-ray emission below 2 keV is present for all objects, with the exception of those known to contain a warm absorber. A single power-law is, however, a poor fit to the 0.3-10.0 keV spectrum and instead we find that a simple model, consisting of a broken power-law (plus an iron line), provides a reasonable fit in most cases. The equivalent width of the emission line is constrained in just twelve objects but with low (<2 sigma) significance in most cases. For the sources whose spectra are well-fit by the broken power-law model, we find that various optical and X-ray line and continuum parameters are well-correlated; in particular, the power-law photon index is well-correlated with the FWHM of the Hbeta line and the photon indices of the low and high energy components of the broken power-law are well-correlated with each other. These results suggest that the 0.3-10 keV X-ray emission shares a common (presumably non-thermal) origin, as opposed to suggestions that the soft excess is directly produced by thermal disc emission or via an additional spectral component. We present XMM-Newton/OM data which we combine with the X-ray spectra so as to produce broad-band spectral energy distributions, free from uncertainties due to long-term variability in non-simultaneous data. Fitting these optical-UV spectra with a Comptonized disc model indicates that the soft X-ray excess is independent of the accretion disc, confirming our interpretation of the tight correlation between the hard and soft X-ray spectra.

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arXiv2005

The 2005 outburst of GRO J1655-40: spectral evolution of the rise, as observed by Swift

We present Swift observations of the black hole X-ray transient, GRO J1655-40, during the recent outburst. With its multiwavelength capabilities and flexible scheduling, Swift is extremely well-suited for monitoring the spectral evolution of such an event. GRO J1655-40 was observed on 20 occasions and data were obtained by all instruments for the majority of epochs. X-ray spectroscopy revealed spectral shapes consistent with the ``canonical'' low/hard, high/soft and very high states at various epochs. The soft X-ray source (0.3-10 keV) rose from quiescence and entered the low/hard state, when an iron emission line was detected. The soft X-ray source then softened and decayed, before beginning a slow rebrightening and then spending $\sim 3$ weeks in the very high state. The hard X-rays (14-150 keV) behaved similarly but their peaks preceded those of the soft X-rays by up to a few days; in addition, the average hard X-ray flux remained approximately constant during the slow soft X-ray rebrightening, increasing suddenly as the source entered the very high state. These observations indicate (and confirm previous suggestions) that the low/hard state is key to improving our understanding of the outburst trigger and mechanism. The optical/ultraviolet lightcurve behaved very differently from that of the X-rays; this might suggest that the soft X-ray lightcurve is actually a composite of the two known spectral components, one gradually increasing with the optical/ultraviolet emission (accretion disc) and the other following the behaviour of the hard X-rays (jet and/or corona).

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