arXiv subjects

J. Homan

Publications and source records attributed to J. Homan.

At least 91 records · Page 5Linked to original sources

arXiv2009

Jets from black hole X-ray binaries: testing, refining and extending empirical models for the coupling to X-rays

In this paper we study the relation of radio emission to X-ray spectral and variability properties for a large sample of black hole X-ray binary systems. This is done to test, refine and extend -- notably into the timing properties -- the previously published `unified model' for the coupling of accretion and ejection in such sources. In 14 outbursts from 11 different sources we find that in every case the peak radio flux, on occasion directly resolved into discrete relativistic ejections, is associated with the bright hard to soft state transition near the peak of the outburst. We also note the association of the radio flaring with periods of X-ray flaring during this transition in most, but not all, of the systems. In the soft state, radio emission is in nearly all cases either undetectable or optically thin, consistent with the suppression of the core jet in these states and `relic' radio emission from interactions of previously ejected material and the ambient medium. However, these data cannot rule out an intermittent, optically thin, jet in the soft state. In attempting to associate X-ray timing properties with the ejection events we find a close, but not exact, correspondence between phases of very low integrated X-ray variability and such ejections. In summary we find no strong evidence against the originally proposed model, confirming and extending some aspects of it with a much larger sample, but note that several aspects remain poorly tested. (ABRIDGED)

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arXiv2009

Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence

The quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 recently started the transition to quiescence following an accretion outburst that lasted more than 24 years. We report on two Chandra and twelve Swift observations performed within five months after the end of the outburst. The Chandra spectrum is composed of a soft, thermal component that fits to a neutron star atmosphere model with kT^inf~0.12 keV, joined by a hard powerlaw tail that contributes ~20% of the total 0.5-10 keV unabsorbed flux. The combined Chandra/Swift data set reveals a relatively hot and luminous quiescent system with a temperature of kT^inf~0.11-0.13 keV and a bolometric thermal luminosity of ~8.1E33-1.6E34 (d/7.4 kpc)^2 erg/s. We discuss our results in the context of cooling neutron star models.

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arXiv2009

Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?

We report the discovery of one-hour long tails on the few-minutes long X-ray bursts from the `clocked burster' GS 1826-24. We propose that the tails are due to enduring thermal radiation from the neutron star envelope. The enduring emission can be explained by cooling of deeper NS layers which were heated up through inward conduction of heat produced in the thermonuclear shell flash responsible for the burst. Similar, though somewhat shorter, tails are seen in bursts from EXO 0748-676 and 4U 1728-34. Only a small amount of cooling is detected in all these tails. This is either due to compton up scattering of the tail photons or, more likely, to a NS that is already fairly hot due to other, stable, nuclear processes.

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arXiv2008

Broad-band X-ray spectral evolution of GX 339-4 during a state transition

We report on X-ray and soft gamma-ray observations of the black-hole candidate GX 339-4 during its 2007 outburst, performed with the RXTE and INTEGRAL satellites. The hardness-intensity diagram of all RXTE/PCA data combined shows a q-shaped track similar to that observed in previous outbursts.The evolution in the diagram suggested that a transition from hard-intermediate state to soft-intermediate state occurred, simultaneously with INTEGRAL observations performed in March. The transition is confirmed by the timing analysis presented in this work, which reveals that a weak type-A quasi-periodic oscillation (QPO) replaces a strong type-C QPO. At the same time, spectral analysis shows that the flux of the high-energy component shows a significant decrease in its flux. However, we observe a delay (roughly one day) between variations of the spectral parameters of the high-energy component and changes in the flux and timing properties. The changes in the high-energy component can be explained either in terms the high-energy cut-off or in terms of a variations in the reflection component. We compare our results with those from a similar transition during the 2004 outburst of GX 339-4.

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arXiv2008

A search for iron emission lines in the Chandra X-ray spectra of neutron star low-mass X-ray binaries

While iron emission lines are well studied in black hole systems, both in X-ray binaries and Active Galactic Nuclei, there has been less of a focus on these lines in neutron star low-mass X-ray binaries (LMXBs). However, recent observations with Suzaku and XMM-Newton have revealed broad asymmetric iron line profiles in 4 neutron star LMXBs, confirming an inner disk origin for these lines in neutron star systems. Here, we present a search for iron lines in 6 neutron star LMXBs. For each object we have simultaneous Chandra and RXTE observations at 2 separate epochs, allowing for both a high resolution spectrum, as well as broadband spectral coverage. Out of the six objects in the survey, we only find significant iron lines in two of the objects, GX 17+2 and GX 349+2. However, we cannot rule out that there are weak, broad lines present in the other sources. The equivalent width of the line in GX 17+2 is consistent between the 2 epochs, while in GX 349+2 the line equivalent width increases by a factor of ~3 between epochs as the source flux decreases by a factor of 1.3. This suggests that the disk is highly ionized, and the line is dominated by recombination emission. We find that there appears to be no specific locations in the long-term hardness-intensity diagrams where iron emission lines are formed, though more sources and further observations are required.

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arXiv2008

Science with the XEUS High Time Resolution Spectrometer

XEUS has been recently selected by ESA for an assessment study. XEUS is a large mission candidate for the Cosmic Vision program, aiming for a launch date as early as 2018. XEUS is a follow-on to ESA's Cornerstone X-Ray Spectroscopy Mission (XMM-Newton). It will be placed in a halo orbit at L2, by a single Ariane 5 ECA, and comprises two spacecrafts. The Silicon pore optics assembly of XEUS is contained in the mirror spacecraft while the focal plane instruments are contained in the detector spacecraft, which is maintained at the focus of the mirror by formation flying. The main requirements for XEUS are to provide a focused beam of X-rays with an effective aperture of 5 m^2 at 1 keV, 2 m^2 at 7 keV, a spatial resolution better than 5 arcsec, a spectral resolution ranging from 2 to 6 eV in the 0.1-8 keV energy band, a total energy bandpass of 0.1-40 keV, ultra-fast timing, and finally polarimetric capabilities. The High Time Resolution Spectrometer (HTRS) is one of the five focal plane instruments, which comprises also a wide field imager, a hard X-ray imager, a cryogenic spectrometer, and a polarimeter. The HTRS is unique in its ability to cope with extremely high count rates (up to 2 Mcts/s), while providing sub-millisecond time resolution and good (CCD like) energy resolution. In this paper, we focus on the specific scientific objectives to be pursued with the HTRS: they are all centered around the key theme "Matter under extreme conditions" of the Cosmic Vision science program. We demonstrate the potential of the HTRS observations to probe strong gravity and matter at supra-nuclear densities. We conclude this paper by describing the current implementation of the HTRS in the XEUS focal plane.

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arXiv2008

Bright flares from the X-ray pulsar SWIFT J1626.6-5156

We have performed a timing and spectral analysis of the X-ray pulsar SWIFT J1626.6-5156 during a major X-ray outburst in order to unveil its nature and investigate its flaring activity. Epoch- and pulse-folding techniques were used to derive the spin period. Time-average and pulse-phase spectroscopy were employed to study the spectral variability in the flare and out-of-flare states and energy variations with pulse phase. Power spectra were obtained to investigate the periodic and aperiodic variability. Two large flares, with a duration of ~450 seconds were observed on 24 and 25 December 2005. During the flares, the X-ray intensity increased by a factor of 3.5, while the peak-to-peak pulsed amplitude increased from 45% to 70%. A third, smaller flare of duration ~180 s was observed on 27 December 2005. The flares seen in SWIFT J1626.6-5156 constitute the shortest events of this kind ever reported in a high-mass X-ray binary. In addition to the flaring activity, strong X-ray pulsations with Pspin=15.3714+-0.0003 s characterise the X-ray emission in SWIFT J1626.6-5156. After the major outburst, the light curve exhibits strong long-term variations modulated with a 45-day period. We relate this modulation to the orbital period of the system or to a harmonic. Power density spectra show, in addition to the harmonic components of the pulsation, strong band-limited noise with an integrated 0.01-100 Hz fractional rms of around 40% that increased to 64% during the flares. A weak QPO (fractional rms 4.7%) with characteristic frequency of 1 Hz was detected in the non-flare emission. The timing (short X-ray pulsations, long orbital period) and spectral (power-law with cut off energy and neutral iron line) properties of SWIFT J1626.6-5156 are characteristic of Be/X-ray binaries.

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arXiv2008

Initial Measurements of Black Hole Spin in GX 339-4 from Suzaku Spectroscopy

We report on a deep Suzaku observation of the stellar-mass black hole GX 339-4 in outburst. A clear, strong, relativistically-shaped iron emission line from the inner accretion disk is observed. The broad-band disk reflection spectrum revealed is one of the most sensitive yet obtained from an accreting black hole. We fit the Suzaku spectra with a physically-motivated disk reflection model, blurred by a new relativistic line function in which the black hole spin parameter is a variable. This procedure yielded a black hole spin parameter of a = 0.89 +/- 0.04. Joint modeling of these Suzaku spectra and prior XMM-Newton spectra obtained in two different outburst phases yields a spin parameter of a = 0.93 +/- 0.01. The degree of consistency between these results suggests that disk reflection models allow for spin measurements that are not strongly biased by scattering effects. We suggest that the best value of the black hole spin parameter is a = 0.93 +/- 0.01 (statistical) +/- 0.04 (systematic). Although preliminary, these results represent the first direct measurement of non-zero spin in a stellar-mass black hole using relativistic line modeling.

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arXiv2008

The Accretion Disk Wind in the Black Hole GRO J1655-40

We report on simultaneous Chandra/HETGS and RXTE observations of the transient stellar-mass black hole GRO J1655-40, made during its 2005 outburst. Chandra reveals a line-rich X-ray absorption spectrum consistent with a disk wind. Prior modeling of the spectrum suggested that the wind may be magnetically driven, potentially providing insights into the nature of disk accretion onto black holes. In this paper, we present results obtained with new models for this spectrum, generated using three independent photoionization codes: XSTAR, Cloudy, and our own code. Fits to the spectrum in particular narrow wavelength ranges, in evenly spaced wavelength slices, and across a broad wavelength band all strongly prefer a combination of high density, high ionization, and small inner radius. Indeed, the results obtained from all three codes require a wind that originates more than 10 times closer to the black hole and carrying a mass flux that is on the order of 1000 times higher than predicted by thermal driving models. If seminal work on thermally-driven disk winds is robust, magnetic forces may play a role in driving the disk wind in GRO J1655-40. However, even these modeling efforts must be regarded as crude given the complexity of the spectra. We discuss these results in the context of accretion flows in black holes and other compact objects.

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arXiv2008

Investigating the nature of absorption lines in the Chandra X-ray spectra of the neutron star binary 4U 1820-30

We use four Chandra gratings spectra of the neutron star low-mass X-ray binary 4U 1820-30 to better understand the nature of certain X-ray absorption lines in X-ray binaries, including the NeII, NeIII, NeIX, OVII, and OVIII lines. The equivalent widths of the lines are generally consistent between the observations, as expected if these lines originate in the hot interstellar medium. No evidence was found that the lines were blueshifted, again supporting the interstellar medium origin, though this may be due to poor statistics. There is apparent variability in the OVIII Ly-alpha line equivalent width providing some evidence that at least some of the OVIII absorption arises within the system. However, the significance is marginal (2.4-sigma), and the lack of variation in the other lines casts some doubt on the reality of the variability. From calculating the equivalent hydrogen column densities for a range of Doppler parameters, we find they are consistent with the interstellar origin of the lines. Additionally we fit the spectra with photoionization models for locally absorbing material, and find that they can reproduce the spectrum well, but only when there is an extremely low filling factor. We conclude that both the ISM and local absorption remain possible for the origin of the lines, but that more sensitive observations are needed to search for low-level variability.

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arXiv2007

The high energy emission of GRO J1655-40 as revealed with INTEGRAL spectroscopy of the 2005 outburst

We present broadband (3-500 keV) INTEGRAL X-ray spectra and X-ray/optical light curves of the luminous black hole X-ray transient and relativistic jet source GRO J1655-40. Our analysis covers four Target of Opportunity observations of the outburst that started in February 2005. We find that the high energy emission of GRO J1655-40 can be modelled well with an unbroken power-law (with photon indices of 1.72+-0.03,2.21+-0.04 for the first and the second observations, respectively). These correspond to hard and thermal dominant states, respectively. In contrast to many other black hole spectra, high energy complexity in the form of a break or cut-off is not required for the hard state, contrary to previous expectations for this state. We show for the first time that Comptonization by non-thermal electrons is the dominant process for the high energy emission in the hard state. We discuss our results in terms of models for broad-band emission and accretion flows in stellar-mass black holes.

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arXiv2007

Linking Jet Emission, X-ray States and Hard X-ray Tails in the Neutron Star X-ray Binary GX 17+2

We present the results from simultaneous radio (Very Large Array) and X-ray (Rossi-X-ray Timing Explorer) observations of the Z-type neutron star X-ray binary GX~17+2. The aim is to assess the coupling between X-ray and radio properties throughout its three rapidly variable X-ray states and during the time-resolved transitions. These observations allow us, for the first time, to investigate quantitatively the possible relations between the radio emission and the presence of the hard X-ray tails and the X-ray state of the source. The observations show: 1) a coupling between the radio jet emission and the X-ray state of the source, i.e. the position in the X-ray hardness-intensity diagram (HID); 2) a coupling between the presence of a hard X-ray tail and the position in the HID, qualitatively similar to that found for the radio emission; 3) an indication for a quantitative positive correlation between the radio flux density and the X-ray flux in the hard-tail power law component; 4) evidence for the formation of a radio jet associated with the Flaring Branch-to-Normal Branch X-ray state transition; 5) that the radio flux density of the newly-formed jet stabilizes when also the normal-branch oscillation (NBO) in the X-ray power spectrum stabilizes its characteristic frequency, suggesting a possible relation between X-ray variability associated to the NBO and the jet formation. We discuss our results in the context of jet models.

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arXiv2007

An XMM-Newton observation of the neutron star X-ray transient 2S 1803-245 in quiescence

We observed the neutron star X-ray transient 2S 1803-245 in quiescence with the X-ray satellite XMM-Newton, but did not detect it. An analysis of the X-ray bursts observed during the 1998 outburst of 2S 1803-245 gives an upper-limit to the distance of <7.3 kpc, leading to an upper-limit on the quiescent 0.5-10 keV X-ray luminosity of <2.8x10^32 erg/s (3sigma). Since the expected orbital period of 2S 1803-245 is several hrs, this limit is not much higher than those observed for the quiescent black hole transients with similar orbital periods.

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arXiv2007

Parallel tracks in infrared versus X-ray emission in black hole X-ray transient outbursts: a hysteresis effect?

We report the discovery of a new hysteresis effect in black hole X-ray binary state transitions, that of the near-infrared (NIR) flux (which most likely originates in the jets) versus X-ray flux. We find, looking at existing data sets, that the infrared emission of black hole X-ray transients appears to be weaker in the low/hard state rise of an outburst than the low/hard state decline of an outburst at a given X-ray luminosity. We discuss how this effect may be caused by a shift in the radiative efficiency of the inflowing or outflowing matter, or variations in the disc viscosity or the spectrum/power of the jet. In addition we show that there is a correlation (in slope but not in normalisation) between infrared and X-ray luminosities on the rise and decline, for all three low-mass black hole X-ray binaries with well-sampled infrared and X-ray coverage: L_NIR propto L_x^(0.5-0.7). In the high/soft state this slope is much shallower; L_NIR propto Lx^(0.1-0.2), and we find that the NIR emission in this state is most likely dominated by the viscously heated (as opposed to X-ray heated) accretion disc in all three sources.

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arXiv2007

On the kHz QPO frequency correlations in bright neutron star X-ray binaries

We re-examine the correlation between the frequencies of upper and lower kHz quasi-periodic oscillations (QPO) in bright neutron-star low-mass X-ray binaries. By including the kHz QPO frequencies of the X-ray binary Cir X-1 and two accreting millisecond pulsars in our sample, we show that the full sample does not support the class of theoretical models based on a single resonance, while models based on relativistic precession or Alfven waves describe the data better. Moreover, we show that the fact that all sources follow roughly the same correlation over a finite frequency range creates a correlation between the linear parameters of the fits to any sub-sample.

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arXiv2006

A Long, Hard Look at the Low-Hard State in Accreting Black Holes

We present the first results of coordinated multi-wavelength observations of the Galactic black hole GX 339-4 in a canonical low-hard state, obtained during its 2004 outburst. XMM-Newton observed the source for 2 revolutions, or approximately 280 ksec; RXTE monitored the source throughout this long stare. The resulting data offer the best view yet obtained of the inner accretion flow geometry in the low-hard state, which is thought to be analogous to the geometry in low-luminosity active galactic nuclei. The XMM-Newton spectra clearly reveal the presence of a cool accretion disk component, and a relativistic Fe K emission line. The results of fits made to both components strongly suggest that a standard thin disk remains at or near to the innermost stable circular orbit, at least in bright phases of the low-hard state. These findings indicate that potential links between the inner disk radius and the onset of a steady compact jet, and the paradigm of a radially-recessed disk in the low-hard state, do not hold universally. The results of our observations can best be explained if a standard thin accretion disk fuels a corona which is closely related to, or consistent with, the base of a compact jet. In a brief examination of archival data, we show that Cygnus X-1 supports this picture of the low/hard state. We discuss our results within the context of disk-jet connections and prevailing models for accretion onto black holes.

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arXiv2006

A Prominent Accretion Disk in the Low-Hard State of the Black Hole Candidate SWIFT J1753.5-0127

We report on simultaneous XMM-Newton and RXTE observations of the stellar-mass black hole candidate SWIFT J1753.5-0127. The source was observed in the low-hard state, during the decline of a hard outburst. The inner accretion disk is commonly assumed to be radially truncated in the low-hard state, and it has been suggested that this property may be tied the production of steady, compact jets. Fits to the X-ray spectra of SWIFT J1753.5-0127 with a number of simple models clearly reveal a cool (kT ~ 0.2 keV) accretion disk. The disk component is required at more than the 8 sigma level of confidence. Although estimates of inner disk radii based on continuum spectroscopy are subject to considerable uncertainty, fits with a number of models suggest that the disk is observed at or close to the innermost stable circular orbit. Recently, an observation of GX 339-4 revealed a disk extending to the innermost stable circular orbit at L_X/L_Edd ~ 0.05; our results from SWIFT J1753.5-0127 may extend this finding down to L_X/L_Edd ~ 0.003 (d/8.5 kpc)^2 (M/Msun). We discuss our results within the context of low-luminosity accretion flow models and disk-jet connections.

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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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