arXiv subjects

Jeroen Homan

Publications and source records attributed to Jeroen Homan.

At least 91 records · Page 5Linked to original sources

arXiv2009

X-ray Timing of Stellar Mass Black Holes

X-ray timing observations of accreting stellar mass black holes have shown that they can produce signals with such short time scales that we must be probing very close to the innermost stable circular orbit that is predicted by the theory of General Relativity (GR). These signals are quasi-periodic oscillations (QPOs), and both the high-frequency variety (HFQPOs, which have frequencies in the 40-450 Hz range) as well as the 0.1-10 Hz low-frequency type have the potential to provide tests of GR in the strong field limit. An important step on the path to GR tests is to constrain the physical black hole properties, and the straightforward frequency measurements that are possible with X-ray timing may provide one of the cleanest measurements of black hole spins. While current X-ray satellites have uncovered these phenomenona, the HFQPOs are weak signals, and future X-ray timing missions with larger effective area are required for testing the candidate theoretical QPO mechanisms. Another main goal in the study of accreting black holes is to understand the production of relativistic jets. Here, we have also made progress during the past decade by finding clear connections between the radio emission that traces the strength of the jet and the properties of the X-ray emission. With new radio capabilities just coming on-line, continuing detailed X-ray studies of accreting black holes is crucial for continuing to make progress.

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arXiv2008

Spectral States of XTE J1701-462: Link between Z and Atoll Sources

We have analyzed 866 RXTE observations of the 2006-2007 outburst of the accreting neutron star XTE J1701-462, during which the source evolves from super-Eddington luminosities to quiescence. The X-ray color evolution first resembles the Cyg X-2 subgroup of Z sources, with frequent excursions on the horizontal and normal branches (HB/NB). The source then decays and evolves to the Sco X-1 subgroup, with increasing focus on the flaring branch (FB) and the lower vertex of the "Z". Finally, the FB subsides, and the source transforms into an atoll source, with the lower vertex evolving to the atoll soft state. Spectral analyses suggest that the atoll stage is characterized by a constant inner disk radius, while the Z stages exhibit a luminosity-dependent expansion of the inner disk, which we interpret as effects related to the local Eddington limit. Contrary to the view that the mass accretion rate ($\dot{m}$) changes along the Z, we find that changes in $\dot{m}$ are instead responsible for the secular evolution of the Z and the subclasses. Motion along the Z branches appears to be caused by three different mechanisms that may operate at roughly constant $\dot{m}$. For the Sco X-1-like Z stage, we find that the FB is an instability track that proceeds off the lower vertex when the inner disk radius shrinks from the value set by the X-ray luminosity toward the value measured for the atoll soft state. Excursions up the NB occur when the apparent size of the boundary layer increases while the disk exhibits little change. The HB is associated with Comptonization of the disk emission. The Z branches for the Cyg X-2-like stage are more complicated, and their origin is unclear. Finally, our spectral results lead us to hypothesize that the lower and upper Z vertices correspond to a standard thin disk and a slim disk, respectively.

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arXiv2008

A variable near-infrared counterpart to the neutron-star low-mass X-ray binary 4U 1705-440

We report the discovery of a near-infrared (nIR) counterpart to the persistent neutron-star low-mass X-ray binary 4U 1705-440, at a location consistent with its recently determined Chandra X-ray position. The nIR source is highly variable, with K_s-band magnitudes varying between 15.2 and 17.3 and additional J- and H-band observations revealing color variations. A comparison with contemporaneous X-ray monitoring observations shows that the nIR brightness correlates well with X-ray flux and X-ray spectral state. We also find possible indications for a change in the slope of the nIR/X-ray flux relation between different X-ray states. We discuss and test various proposed mechanisms for the nIR emission from neutron-star low-mass X-ray binaries and conclude that the nIR emission in 4U 1705-440 is most likely dominated by X-ray heating of the outer accretion disk and the secondary star.

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arXiv2008

Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole

We present simultaneous Chandra-HETG and RXTE observations of a moderate flux `soft state' of the black hole candidate 4U1957+11. These spectra, having a minimally discernible hard X-ray excess, are an excellent test of modern disk atmosphere models that include the effects of black hole spin. The HETG data show that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X 10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies a highly inclined disk around a low mass black hole rapidly rotating with normalized spin a*~1. We show, however, that pure Schwarzschild black hole models describe the data extremely well, albeit with large disk atmosphere ``color-correction'' factors. Standard color-correction factors can be attained if one additionally incorporates mild Comptonization. We find that the Chandra observations do not uniquely determine spin. Similarly, XMM/RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and unknown distance of 4U1957+11; however, it is also driven by the limited bandpass of Chandra and XMM. We therefore present a series of 48 RXTE observations taken over the span of several years and at different brightness/hardness levels. These data prefer a spin of a*~1, even when including a mild Comptonization component; however, they also show evolution of the disk atmosphere color-correction factors. If the rapid spin models with standard atmosphere color-correction factors of h_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11 can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun black hole at 22 kpc with a* ~ 1, with the latter being statistically preferred.

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arXiv2008

The Long-Term Variability of the X-Ray Sources in NGC 6946 and NGC 4485/4490

We analyze data from five Chandra observations of the spiral galaxy NGC 6946 and from three Chandra observations of the irregular/spiral interacting galaxy pair NGC 4485/4490, with an emphasis on investigating the long-term variability exhibited by the source populations. We detect 90 point sources coincident with NGC 6946 down to luminosities of a few times 10^36 erg/s, and 38 sources coincident with NGC 4485/90 down to a luminosity of ~1x10^37 erg/s. Twenty-five (15) sources in NGC 6946 (NGC 4485/90) exhibit long-term (i.e., weeks to years) variability in luminosity; 11 (4) are transient candidates. The single ultraluminous X-ray source (ULX) in NGC 6946 and all but one of the eight ULXs in NGC 4485/90 exhibit long-term flux variability. Two of the ULXs in NGC 4485/90 have not been identified before as ultraluminous sources. The widespread variability in both systems is indicative of the populations being dominated by X-ray binaries, and this is supported by the X-ray colors of the sources. The distribution of colors among the sources indicates a large fraction of high-mass X-ray binaries in both systems. The shapes of the X-ray luminosity functions of the galaxies do not change significantly between observations and can be described by power laws with cumulative slopes ~0.6-0.7 (NGC 6946) and ~0.4 (NGC 4485/90).

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arXiv2008

Luminosity dependence of the electron temperature in the bright hard state of the black hole candidate GX 339--4

We have analyzed 200 Rossi X-ray Timing Explorer observations of the black hole candidate GX 339--4, all from the bright hard state periods between 1996 and 2005. Purpose of our study is to investigate the radiation mechanisms in the hard state of GX 339--4. The broadband 3--200 keV spectra were successfully modeled by a simple analytic model, power--law with an exponential cut-off modified with a smeared edge. The obtained energy cut-off ($E_{\rm{cut}}$) was distributed over 50--200 keV, and the photon index over 1.4--1.7. We found a clear anti-correlation ($E_{\rm{cut}} \propto L^{-0.70\pm0.06}$) between the X-ray luminosity ($L$) in 2--200 keV and $E_{\rm{cut}}$, when $L$ is larger than $7 \times 10^{37}$ erg s$^{-1}$ (assuming a distance of 8 kpc), while $E_{\rm{cut}}$ is roughly constant at around 200 keV when $L$ is smaller than $7 \times 10^{37}$ erg s$^{-1}$. This anti-correlation remained unchanged by adopting a more physical thermal Comptonization model, which resulted in the anti-correlation that can be expressed as $kT_{\rm{e}} \propto L^{-0.24\pm0.06}$. These anti-correlations can be quantitatively explained by a picture in which the energy-flow rate from protons to electrons balances with the inverse Compton cooling.

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arXiv2008

Chandra detection of XTE J1650-500 in quiescence and the minimum luminosity of black hole X-ray binaries

The Galactic black hole X-ray binary XTE J1650-500 entered a quiescent regime following the decline from the 2001-2002 outburst that led to its discovery. Here we report on the first detection of its quiescent counterpart in a 36 ks observation taken in 2007 July with the Chandra X-ray Observatory. The inferred 0.5-10 keV unabsorbed flux is in the range 2.5-5.0x1e-15 erg/s/cm^2. Notwithstanding large distance uncertainties, the measured luminosity is comparable to that of the faintest detected black hole X-ray binaries, all having orbital periods close to the expected bifurcation period between j- and n-driven low-mass X-ray binaries. This suggests that a few 1e30 erg/s might be a limiting luminosity value for quiescent black holes.

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arXiv2007

Relativistic iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii

Using Suzaku observations of three neutron star low-mass X-ray binaries (Ser X-1, 4U 1820-30 and GX 349+2) we have found broad, asymmetric, relativistic Fe K emission lines in all three objects. These Fe K lines can be well fit by a model for lines from a relativistic accretion disk ('diskline'), allowing a measurement of the inner radius of the accretion disk, and hence an upper limit on the neutron star radius. These upper limits correspond to 14.5 - 16.5 km for a 1.4 M(solar) neutron star. The inner disk radii we measure with Fe K lines are in good agreement with the inner disk radii implied by kHz QPOs observed in both 4U 1820-30 and GX 349+2, supporting the inner disk nature of kHz QPOs. Additionally, the Fe K lines observed in these neutron stars are narrower than those in the black holes that are thought to be close to maximally spinning, as one would expect if inferences for spin are robust.

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arXiv2007

Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients

We propose a hybrid model to fit the X-ray spectra of atoll-type X-ray transients in the soft and hard states. This model uniquely produces luminosity tracks that are proportional to T^4 for both the accretion disk and boundary layer. The model also indicates low Comptonization levels for the soft state, gaining a similarity to black holes in the relationship between Comptonization level and the strength of integrated rms variability in the power density spectrum. The boundary layer appears small, with a surface area that is roughly constant across soft and hard states. This result may suggestion that the NS radius is smaller than its inner-most stable circular orbit.

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arXiv2007

RossiXTE monitoring of 4U 1636-53: I. Long-term evolution and kHz Quasi-Periodic Oscillations

We have monitored the atoll-type neutron star low-mass X-ray binary 4U 1636-53 with the Rossi X-Ray Timing Explorer (RXTE) for more than 1.5 years. Our campaign consisted of short (~2 ks) pointings separated by two days, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ~30-40 days, already seen in the light curves from the RXTE All-Sky Monitor, which corresponded to regular transitions between the hard (island) and soft (banana) states. We detected kHz QPOs in about a third of the observations, most of which were in the soft (banana) state. The distribution of the frequencies of the peak identified as the lower kHz QPO is found to be different from that previously observed in an independent data set. This suggests that the kHz QPOs in the system shows no intrinsically preferred frequency.

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arXiv2007

RXTE spectra of the Galactic microquasar GRO J1655-40 during the 2005 outburst

We report on the results of a detailed spectral analysis of 389 RXTE observations of the Galactic microquasar GRO J1655-40, performed during its 2005 outburst. The maximum luminosity reached during this outburst was 1.4 times higher than in the previous (1996-1997) outburst. However, the spectral behavior during the two outbursts was very similar. In particular, Ldisk was proportional to Tin^4 up to the same critical luminosity and in both outbursts there were periods during which the energy spectra were very soft, but could not be fit with standard disk models.

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arXiv2006

RXTE observations of the first transient Z source XTE J1701-462: shedding new light on mass accretion in luminous neutron star LMXBs

(Abridged) We report on ten weeks of RXTE observations of the X-ray transient XTE J1701-462. Comparisons with other sources suggest it had all the characteristics of the neutron star Z sources (the brightest persistent neutron star LMXBs). These include Z tracks in X-ray color diagrams and typical variability components detected in the power spectra. XTE J1701-462 is the first transient Z source and provides unique insights into mass accretion rate (Mdot) and luminosity dependencies in neutron star LMXBs. As its overall luminosity decreased, we observed a switch between two types of Z-source behavior, with most of the branches of the Z-track changing their shape and/or orientation. We interpret this switch as an extreme case of the longterm changes seen in the persistent Z sources and suggest that these result from changes in Mdot. We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1, and GX 340+0) might be more luminous (> 50%) than the Sco-like Z sources (Sco X-1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior of kHz QPOs, which involves a prompt as well as a filtered response to changes in Mdot, we propose that changes in Mdot can explain both movement along the Z track and changes in the shape of the Z track. We discuss some consequences of this and consider the possibility that the branches of the Z will evolve into the branches observed in the X-ray color diagrams of the less luminous atoll sources, but not in a way that was previously suggested.

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arXiv2006

Cooling of the quasi-persistent neutron star X-ray transients KS 1731-260 and MXB 1659-29

We present Chandra and XMM-Newton X-ray observations that monitor the neutron star cooling of the quasi-persistent neutron star X-ray transients KS 1731-260 and MXB 1659-29 for approximately 4 years after these sources returned to quiescence from prolonged outbursts. In both sources the outbursts were long enough to significantly heat the neutron star crust out of thermal equilibrium with the core. We analyse the X-ray spectra by fitting absorbed neutron star atmosphere models to the observations. The results of our analysis strengthen the preliminary findings of Wijnands et al. that in both sources the neutron star crust cools down very rapidly suggesting it has a high heat conductivity and that the neutron star core requires enhanced core cooling processes. Importantly, we now detect the flattening of the cooling in both sources as the crust returns to thermal equilibrium with the core. We measure the thermal equilbrium flux and temperature in both sources by fitting a curve that decays exponentially to a constant level. The cooling curves cannot be fit with just a simple exponential decay without the constant offset. We find the constant bolometric flux and effective temperature components to be (9.2 +/- 0.9)E-14 ergs/cm^2/s and 70.0 +/- 1.6 eV in KS 1731-260 and (1.7 +/- 0.3)E-14 ergs/cm^2/s and 51.6 +/- 1.4 eV in MXB 1659-29. In addition, we find that the crust of KS 1731-260 cools faster than that of MXB 1659-29 by a factor of ~2, likely due to different crustal properties. This is the first time that the cooling of a neutron star crust into thermal equilibrium with the core has been observed in such detail.

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arXiv2005

A Precessing Ring Model for Low-Frequency Quasi-periodic Oscillations

We develop a simple physical model to describe the most common type of low-frequency quasi-periodic oscillations (QPOs) seen in a number of accreting black hole systems, as well as the shape of the relativistically broadened iron emission lines that often appear simultaneously in such sources. The model is based on an inclined ring of hot gas that orbits the black hole along geodesic trajectories. For spinning black holes, this ring will precess around the spin axis of the black hole at the Lense-Thirring (``frame-dragging'') frequency. Using a relativistic ray-tracing code, we calculate X-ray light curves and observed energy spectra as a function of the radius and tilt angle of the ring, the spin magnitude, and the inclination of the black hole. The model predicts higher-amplitude QPOs for systems with high inclinations, as seen in a growing number of black hole binary systems. We find that the Rossi X-ray Timing Explorer observations of low-frequency QPOs in GRS 1915+105 are consistent with a ring of radius R ~ 10M orbiting a black hole with spin a/M ~0.5 and inclination angle of i ~ 70 deg. Finally, we describe how future X-ray missions may be able to use simultaneous timing and spectroscopic observations to measure the black hole spin and probe the inner-most regions of the accretion disk.

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arXiv2005

XMM-Newton observations of the black hole X-ray transient XTE J1650-500 in quiescence

We report the result of an XMM-Newton observation of the black-hole X-ray transient XTE J1650-500 in quiescence. The source was not detected and we set upper limits on the 0.5-10 keV luminosity of 0.9e31-1.0e31 erg/s (for a newly derived distance of 2.6 kpc). These limits are in line with the quiescent luminosities of black-hole X-ray binaries with similar orbital periods (~7-8 hr)

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arXiv2005

The distribution of kHz QPO frequencies in bright LMXBs

We analyzed all published frequencies, nu_1 and nu_2, of the twin kilohertz quasi-periodic oscillations (kHz QPOs) in bright neutron star low-mass X-ray binaries. The two frequencies are well correlated but, contrary to recent suggestions, the frequency-frequency correlation is significantly different from a nu_2 = (3/2) nu_1 relation. To check whether, although not following the the 3/2 relation, the QPO frequencies cluster around a region where nu_2/nu_1 ~ 3/2, we re-analyzed the Sco X-1 data that were used to report that ratio and show that, because the distribution of ratios of linearly correlated measurements is intrinsically biased, although the significance of the clustering around nu_2/nu_1 ~ 3/2 previously reported in the case of Sco X-1 is formally correct, it does not provide any useful information about a possible underlying resonance mechanism in this source. Using the same data, we then show that the (unbiased) distribution of QPO frequencies is consistent with a uniform distribution at a 2.4 sigma level. To investigate this further, we analyzed a larger data set of Sco X-1 and four other sources, 4U 1608-52, 4U 1636-53, 4U 1728-34 and 4U 1820-30. We find that for all five sources the distribution of the kHz QPO frequencies is not uniform and has multiple peaks, which have no analogy in the distribution of points in the spectral color-color diagrams of these sources. Finally, we demonstrate that a simple random walk of the QPO frequencies can reproduce qualitatively the observed distributions in frequency and frequency ratio. This result weakens the support for resonance models of kHz QPOs in neutron stars.

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arXiv2005

Iron-line and continuum flux variations in the RXTE spectra of the black-hole candidate XTE J1650-500

We present the results of spectral fits made to 57 pointed observations of the Galactic black hole candidate and X-ray transient XTE J1650-500, made with the Rossi X-ray Timing Explorer in 2001 when the source was in a transition from the hard state to the soft state. A strong and variable Fe K-alpha emission line is detected in these spectra. The line flux varies in a non-linear way with the hard X-ray flux, in apparent contradiction to the predictions of simple disc reflection models. We observe a change in the overall trend that coincides with changes in t he continuum X-ray spectrum and the fast X-ray variability. The Fe-line flux versus hard X-ray flux variations are consistent with the predictions of reflection models which consider high disc-ionization states and with a model which considers gravitational light-bending effects. Indications for an anti-correlation between the Fe-line flux and the hard X-ray flux in the spectrally hardest observations and weak variations in the Fe-line energy (as observed with XMM-Newton and BeppoSAX) slightly favour the light-bending interpretation.

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arXiv2005

Multi-wavelength observations of the 2002 outburst of GX 339-4: two patterns of X-ray-optical/near-infrared behavior

(Abridged) We report on RXTE and optical/near-infrared (nIR) observations of the black hole transient GX 339-4, made over a time span of 8 months in 2002 and covering the initial rise and transition from a hard to a soft spectral state. Two 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 vs. X-ray flux relation similar to the relation between X-ray and radio fluxes found in previous studies of GX 339-4 and other black hole binaries. As GX 339-4 went through an intermediate state the optical/nIR fluxes decreased rapidly and once it had entered the spectrally soft state, the optical/nIR spectrum 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 nIR-changes preceded those in the X-rays by more than 2 weeks, indicating a disk origin of the nIR. We present SEDs, 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 X-ray reprocessing is the dominant source of optical/nIR emission. Finally, comparing the light curves from the ASM and 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.

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