arXiv subjects

J. Homan

Publications and source records attributed to J. Homan.

At least 73 records · Page 4Linked to original sources

arXiv2012

LOFT: the Large Observatory For X-ray Timing

The LOFT mission concept is one of four candidates selected by ESA for the M3 launch opportunity as Medium Size missions of the Cosmic Vision programme. The launch window is currently planned for between 2022 and 2024. LOFT is designed to exploit the diagnostics of rapid X-ray flux and spectral variability that directly probe the motion of matter down to distances very close to black holes and neutron stars, as well as the physical state of ultra-dense matter. These primary science goals will be addressed by a payload composed of a Large Area Detector (LAD) and a Wide Field Monitor (WFM). The LAD is a collimated (<1 degree field of view) experiment operating in the energy range 2-50 keV, with a 10 m^2 peak effective area and an energy resolution of 260 eV at 6 keV. The WFM will operate in the same energy range as the LAD, enabling simultaneous monitoring of a few-steradian wide field of view, with an angular resolution of <5 arcmin. The LAD and WFM experiments will allow us to investigate variability from submillisecond QPO's to year-long transient outbursts. In this paper we report the current status of the project.

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arXiv2012

Discovery of two simultaneous non-harmonically related Quasi-Periodic Oscillations in the 2005 outburst of the black-hole binary GRO J1655-40

We studied the low-frequency quasi-periodic oscillations (LFQPOs) in the black hole GRO J1655-40 during the 2005 outburst, using data from the Rossi X-ray Timing Explorer. All LFQPOs could be identified as either type B or type C using previously proposed classification schemes. In the soft state of the outburst the type-C LFQPOs reached frequencies that are among the highest ever seen for LFQPOs in black holes. At the peak of the outburst, in the ultra-luminous state, the power spectrum showed two simultaneous, non-harmonically related peaks which we identified as a type-B and a type-C QPO. The simultaneous presence of a type-C and type-B QPO shows that at least two of the three known LFQPO types are intrinsically different and likely the result of distinct physical mechanisms. We also studied the properties of a broad peaked noise component in the power spectra of the ultra-luminous state. This noise component becomes more coherent with count rate and there are strong suggestions that it evolves into a type-B QPO at the highest observed count rates.

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arXiv2012

A four-year XMM-Newton/Chandra monitoring campaign of the Galactic Centre: analysing the X-ray transients

We report on the results of a four-year long X-ray monitoring campaign of the central 1.2 square degrees of our Galaxy, performed with Chandra and XMM-Newton between 2005 and 2008. Our study focuses on the properties of transient X-ray sources that reach 2-10 keV luminosities of >1E34 erg/s for an assumed distance of 8 kpc. There are 17 known X-ray transients within the field of view of our campaign, eight of which were detected in outburst during our observations: the transient neutron star low-mass X-ray binaries GRS 1741-2853, AX J1745.6-2901, SAX J1747.0-2853, KS 1741-293 and GRO J1744-28, and the unclassified X-ray transients XMM J174457-2850.3, CXOGC J174535.5-290124 and CXOGC J174541.0-290014. We present their X-ray spectra and flux evolution during our campaign, and discuss these results in light of their historic activity. Our main results include the detection of two thermonuclear X-ray bursts from SAX J1747.0-2853 that were separated by an unusually short time interval of 3.8 min. We detected a thermonuclear X-ray burst and a ~1600-s X-ray eclipse from AX J1745.6-2901. Both XMM J174457-2850.3 and GRO J1744-28 displayed weak X-ray activity above their quiescent levels that is indicative of low-level accretion. In addition to the eight known X-ray transients, we discovered a previously unknown X-ray source that we designate XMMU J174654.1-291542. Based on its X-ray properties and the possible association with an infrared source, we tentatively classify this object as a cataclysmic variable. No new transients were found during our campaign, reinforcing the conclusion of previous authors that most X-ray transients recurring on a time scale of less than a decade have now been identified near the Galactic centre.

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arXiv2012

A superburst candidate in EXO 1745--248 as a challenge to thermonuclear ignition models

We report on Chandra, RXTE, Swift/BAT and MAXI observations of a ~1 day X-ray flare and subsequent outburst of a transient X-ray source observed in October-November 2011 in the globular cluster Terzan 5. We show that the source is the same as the transient that was active in 2000, i.e., the neutron star low-mass X-ray binary EXO 1745-248. For the X-ray flare we estimate a 6-11 hr exponential decay time and a radiated energy of 2-9 x 10^42 erg. These properties, together with strong evidence of decreasing blackbody temperature during the flare decay, are fully consistent with what is expected for a thermonuclear superburst. We use the most recent superburst models and estimate an ignition column depth of ~10^12 g cm^-2 and an energy release between 0.1-2 x 10^18 erg g^-1, also consistent with expected superburst values. We conclude therefore that the flare was most probably a superburst. We discuss our results in the context of theoretical models and find that even when assuming a few days of low level accretion before the superburst onset (which is more than what is suggested by the data), the observations of this superburst are very challenging for current superburst ignition models.

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arXiv2012

The black hole candidate MAXIJ1659-152 in and towards quiescence in X-ray and radio

In this paper we report on Expanded Very Large Array radio and Chandra and Swift X-ray observations of the outburst decay of the transient black hole candidate MAXI J1659-152 in 2011. We discuss the distance to the source taking the high inclination into account and we conclude that the source distance is probably 6+-2 kpc. The lowest observed flux corresponds to a luminosity of 2x10^31 (d/6 kpc)^2 erg/s This, together with the orbital period of 2.4 hr reported in the literature, suggests that the quiescent X-ray luminosity is higher than predicted on the basis of the orbital period -- quiescent X-ray luminosity relationship. The relation between the accretion and ejection mechanisms can be studied using the observed correlation between the radio and X-ray luminosities as these evolve over an outburst. We determine the behaviour of MAXI J1659-152 in the radio -- X-ray diagram at low X-ray luminosities using the observations reported in this paper and at high X-ray luminosities using values reported in the literature. At high X-ray luminosities the source lies closer to the sources that follow a correlation index steeper than 0.6-0.7. However, when compared to other sources that follow a steeper correlation index, the X-ray luminosity in MAXI J1659-152 is also lower. The latter can potentially be explained by the high inclination of MAXI J1659-152 if the X-ray emission comes from close to the source and the radio emission is originating in a more extended region. However, it is probable that the source was not in the canonical low-hard state during these radio observations and this may affect the behaviour of the source as well. At intermediate X-ray luminosities the source makes the transition from the radio underluminous sources in the direction of the relation traced by the 'standard' correlation similar to what has been reported for H1743-322. (abridged)

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arXiv2012

The black hole candidate XTE J1752-223 towards and in quiescence: optical and simultaneous X-ray - radio observations

We present optical, X-ray and radio observations of the black hole transient (BHT) XTE J1752-223 towards and in quiescence. Optical photometry shows that the quiescent magnitude of XTE J1752-223 is fainter than 24.4 magnitudes in the i'-band. A comparison with measurements of the source during its 2009-2010 outburst shows that the outburst amplitude is more than 8 magnitudes in the i'-band. Known X-ray properties of the source combined with the faintness of the quiescence optical counterpart and the large outburst optical amplitude point towards a short orbital period system (Porb<~6.8 h) with an M type (or later) mass donor, at a distance of 3.5<~d<~8 kpc. Simultaneous X-ray and radio data were collected with Chandra and the EVLA, allowing constraints to be placed on the quiescent X-ray and radio flux of XTE J1752-223. Furthermore, using data covering the final stage of the outburst decay, we investigated the low luminosity end of the X-ray - radio correlation for this source and compared it with other BHTs. We found that XTE J1752-223 adds to the number of outliers with respect to the `standard' X-ray - radio luminosity relation. Furthermore, XTE J1752-223 is the second source, after the BHT H1743-322, that shows a transition from the region of the outliers towards the `standard' correlation at low luminosity. Finally, we report on a faint, variable X-ray source we discovered with Chandra at an angular distance of ~2.9" to XTE J1752-223 and at a position angle consistent with that of the radio jets previously observed from the BHT. We discuss the possibility that we detected X-ray emission associated with a jet from XTE J1752-223.

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arXiv2011

A late jet rebrightening revealed from multi-wavelength monitoring of the black hole candidate XTE J1752-223

We present optical monitoring of the black hole candidate XTE J1752-223 during its 2009 - 2010 outburst and decay to quiescence. The optical light curve can be described by an exponential decay followed by a plateau, then a more rapid fade towards quiescence. The plateau appears to be due to an extra component of optical emission that brightens and then fades over ~ 40 days. We show evidence for the origin of this optical 'flare' to be the synchrotron jet during the decaying hard state, and we identify and isolate both disc and jet components in the spectral energy distributions. The optical flare has the same morphology and amplitude as a contemporaneous X-ray rebrightening. This suggests a common origin, but no firm conclusions can be made favouring or disfavouring the jet producing the X-ray flare. The quiescent optical magnitudes are B >= 20.6, V >= 21.1, R >= 19.5, i' >= 19.2. From the optical outburst amplitude we estimate a likely orbital period of < 22 h. We also present near-infrared (NIR) photometry and polarimetry and rare mid-infrared imaging (8 - 12 microns) when the source is nearing quiescence. The fading jet component, and possibly the companion star may contribute to the NIR flux. We derive deep mid-IR flux upper limits and NIR linear polarization upper limits. With the inclusion of radio data, we measure an almost flat jet spectral index between radio and optical; F_nu ~ nu^(~ +0.05). The data favour the jet break to optically thin emission to reside in the infrared, but may shift to frequencies as high as the optical or UV during the peak of the flare.

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arXiv2011

Low frequency oscillations in black holes: a spectral-timing approach to the case of GX 339-4

We analyzed RXTE/PCA and HEXTE data of the transient black hole binary GX 339-4, col- lected over a time span of eight years. We studied the properties and the behavior of low frequency quasi periodic oscillations (QPOs) as a function of the integrated broad-band variability and the spectral parameters during four outbursts (2002, 2004, 2007, 2010). Most of the QPOs could be classified following the ABC classification that has been proposed before. Our results show that the ABC classification can be extended to include spectral dependencies and that the three QPO types have indeed intrinsically different properties. In terms of the relation between QPO frequency and power-law flux, type-A and -C QPOs may follow the same relation, whereas the type-B QPOs trace out a very different relation. Type-B QPO frequencies clearly correlate with the powerlaw-flux and are connected to local increases of the count rate. The frequency of all QPOs observed in the rising phase of the 2002, 2007 and 2010 outburst correlate with the disk flux. Our results can be interpreted within the framework of recently proposed QPO models involving Lense-Thirring precession. We suggest that type- C and -A QPOs might be connected and could be interpreted as being the result of the same phenomenon observed at different stages of the outburst evolution, while a different physical process produces type-B QPOs.

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arXiv2011

X-ray softening in the new X-ray transient XTE J1719-291 during its 2008 outburst decay

The X-ray transient XTE J1719-291 was discovered with RXTE/PCA during its outburst in 2008 March, which lasted at least 46 days. Its 2-10 keV peak luminosity is 7E35 erg/s assuming a distance of 8 kpc, which classifies the system as a very faint X-ray transient. The outburst was monitored with Swift, RXTE, Chandra and XMM-Newton. We analysed the X-ray spectral evolution during the outburst. We fitted the overall data with a simple power-law model corrected for absorption and found that the spectrum softened with decreasing luminosity. However, the XMM-Newton spectrum can not be fitted with a simple one-component model, but it can be fitted with a thermal component (black body or disc black body) plus power-law model affected by absorption. Therefore, the softening of the X-ray spectrum with decreasing X-ray luminosity might be due to a change in photon index or alternatively it might be due to a change in the properties of the soft component. Assuming that the system is an X-ray binary, we estimated a long-term time-averaged mass accretion rate of ~ 7.7E-13 M_sun/yr for a neutron star as compact object and ~ 3.7E10-13 M_sun/yr in the case of a black hole. Although no conclusive evidence is available about the nature of the accretor, based on the X-ray/optical luminosity ratio we tentatively suggest that a neutron star is present in this system.

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arXiv2011

Luminosity Functions and Point Source Properties from Multiple Chandra Observations of M81

We present an analysis of 15 Chandra observations of the nearby spiral galaxy M81 taken over the course of six weeks in May--July 2005. Each observation reaches a sensitivity of ~10^37 erg/s. With these observations and one previous deeper Chandra observation, we compile a master source list of 265 point sources, extract and fit their spectra, and differentiate basic populations of sources through their colors. We also carry out variability analyses of individual point sources and of X-ray luminosity functions in multiple regions of M81 on timescales of days, months, and years. We find that, despite measuring significant variability in a considerable fraction of sources, snapshot observations provide a consistent determination of the X-ray luminosity function of M81. We also fit the X-ray luminosity functions for multiple regions of M81 and, using common parametrizations, compare these luminosity functions to those of two other spiral galaxies, M31 and the Milky Way.

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arXiv2011

The identification of MAXI J1659-152 as a black hole candidate

We report on the analysis of all 65 pointed Rossi X-ray Timing Explorer observations of the recently discovered soft X-ray transient MAXI J1659-152 (initially referred to as GRB 100925A). The source was studied in terms of its evolution through the hardness-intensity diagram (HID) as well as its X-ray variability properties. MAXI J1659-152 traced out an anti-clockwise loop in the HID, which is commonly seen in transient low-mass X-ray binaries. The variability properties of the source, in particular the detection of type-B and type-C low-frequency quasi-periodic oscillations, and the way they evolve along the HID track, indicate that MAXI J1659-152 is a black hole candidate. The spectral and variability properties of MAXI J1659-152 imply that the source was observed in the hard and soft intermediate states during the RXTE observations, with several transitions between these two states.

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arXiv2011

Quiescent X-ray variability from the neutron star transient Aql X-1

A number of studies have revealed variability from neutron star low-mass X-ray binaries during quiescence. Such variability is not well characterised, or understood, but may be a common property that has been missed due to lack of multiple observations. One such source where variability has been observed is Aql X-1. Here, we analyse 14 Chandra and XMM-Newton observations of Aql X-1 in quiescence, covering a period of approximately 2 years. There is clear variability between the epochs, with the most striking feature being a flare-like increase in the flux by a factor of 5. Spectral fitting is inconclusive as to whether the power-law and/or thermal component is variable. We suggest that the variability and flare-like behaviour during quiescence is due to accretion at low rates which might reach the neutron star surface.

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arXiv2010

Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust

In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 started a transition from outburst to quiescence, after it had been actively accreting for more than 24 years. In a previous work, we discussed Chandra and Swift observations obtained during the first five months after this transition. Here, we report on further X-ray observations of EXO 0748-676, extending the quiescent monitoring to 1.6 years. Chandra and XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well-fitted by a neutron star atmosphere model. An additional hard powerlaw tail is detected that changes non-monotonically over time, contributing between 4 and 20 percent to the total unabsorbed 0.5-10 keV flux. The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal bolometric luminosity fades from ~1E34 to 6E33 (D/7.4 kpc)^2 erg/s, whereas the inferred neutron star effective temperature decreases from ~124 to 109 eV. We interpret the observed decay as cooling of the neutron star crust and show that the fractional quiescent temperature change of EXO 0748-676 is markedly smaller than observed for three other neutron star X-ray binaries that underwent prolonged accretion outbursts.

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arXiv2010

The return of the bursts: Thermonuclear flashes from Circinus X-1

We report the detection of 15 X-ray bursts with RXTE and Swift observations of the peculiar X-ray binary Circinus X-1 during its May 2010 X-ray re-brightening. These are the first X-ray bursts observed from the source after the initial discovery by Tennant and collaborators, twenty-five years ago. By studying their spectral evolution, we firmly identify nine of the bursts as type I (thermonuclear) X-ray bursts. We obtain an arcsecond location of the bursts that confirms once and for all the identification of Cir X-1 as a type I X-ray burst source, and therefore as a low magnetic field accreting neutron star. The first five bursts observed by RXTE are weak and show approximately symmetric light curves, without detectable signs of cooling along the burst decay. We discuss their possible nature. Finally, we explore a scenario to explain why Cir X-1 shows thermonuclear bursts now but not in the past, when it was extensively observed and accreting at a similar rate.

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arXiv2010

The kilohertz quasi-periodic oscillations during the Z and atoll phases of the unique transient XTE J1701--462

We analysed 866 observations of the neutron-star low-mass X-ray binary XTE J1701-462 during its 2006-2007 outburst. XTE J1701-462 is the only example so far of a source that during an outburst showed, beyond any doubt, spectral and timing characteristics both of the Z and atoll type. We found that the lower kHz QPO in the atoll phase has a significantly higher coherence and fractional rms amplitude than any of the kHz QPOs seen during the Z phase, and that in the same frequency range, atoll lower kHz QPOs show coherence and fractional rms amplitude, respectively, 2 and 3 times larger than the Z kHz QPOs. Out of the 707 observations in the Z phase, there is no single observation in which the kHz QPOs have a coherence or rms amplitude similar to those seen when XTE J1701-462 was in the atoll phase, even though the total exposure time was about 5 times longer in the Z than in the atoll phase. Since it is observed in the same source, the difference in QPO coherence and rms amplitude between the Z and atoll phase cannot be due to neutron-star mass, magnetic field, spin, inclination of the accretion disk, etc. If the QPO frequency is a function of the radius in the accretion disk in which it is produced, our results suggest that in XTE J1701-462 the coherence and rms amplitude are not uniquely related to this radius. Here we argue that this difference is instead due to a change in the properties of the accretion flow around the neutron star. Regardless of the precise mechanism, our result shows that effects other than the geometry of space time around the neutron star have a strong influence on the coherence and rms amplitude of the kHz QPOs, and therefore the coherence and rms amplitude of the kHz QPOs cannot be simply used to deduce the existence of the innermost stable circular orbit around a neutron star.

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arXiv2009

Measuring the Spin of GRS 1915+105 with Relativistic Disk Reflection

GRS 1915+105 harbors one of the most massive known stellar black holes in the Galaxy. In May 2007, we observed GRS 1915+105 for 117 ksec in the low/hard state using Suzaku. We collected and analyzed the data with the HXD/PIN and XIS cameras spanning the energy range from 2.3-55 keV. Fits to the spectra with simple models reveal strong disk reflection through an Fe K emission line and a Compton back-scattering hump. We report constraints on the spin parameter of the black hole in GRS 1915+105 using relativistic disk reflection models. The model for the soft X-ray spectrum (i.e. < 10 keV) suggests a/M = 0.56(2) and excludes zero spin at the 4 sigma level of confidence. The model for the full broadband spectrum suggests that the spin may be higher, a/M = 0.98(1) (1 sigma confidence), and again excludes zero spin at the 2 sigma level of confidence. We discuss these results in the context of other spin constraints and inner disk studies in GRS 1915+105.

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arXiv2009

Following the 2008 outburst decay of the black hole candidate H1743-322 in X-ray and radio

In this Paper we report on radio (VLA and ATCA) and X-ray (RXTE, Chandra and Swift) observations of the outburst decay of the transient black hole candidate H1743-322 in early 2008. We find that the X-ray light curve followed an exponential decay, leveling off towards its quiescent level. The exponential decay timescale is ~4 days and the quiescent flux corresponds to a luminosity of 3x10^32 (d/7.5 kpc)^2 erg/s. This together with the relation between quiescent X-ray luminosity and orbital period reported in the literature suggests that H1743-322 has an orbital period longer than ~10 hours. Both the radio and X-ray light curve show evidence for flares. The radio - X-ray correlation can be well described by a power-law with index ~0.18. This is much lower than the index of 0.6-0.7 found for the decay of several black hole transients before. The radio spectral index measured during one of the radio flares while the source is in the low-hard state, is -0.5+-0.15, which indicates that the radio emission is optically thin. This is unlike what has been found before in black hole sources in the low-hard state. We attribute the radio flares and the low index for the radio - X-ray correlation to the presence of shocks downstream the jet flow, triggered by ejection events earlier in the outburst. We find no evidence for a change in X-ray power law spectral index during the decay, although the relatively high extinction of N_H =2.3x10^22 cm^-2 limits the detected number of soft photons and thus the accuracy of the spectral fits.

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arXiv2009

The evolution of the high-energy cut-off in the X-Ray spectrum of the GX 339-4 across a hard-to-soft transition

We report on X-ray observations of the black-hole candidate GX 339-4 during its 2006/2007 outburst. The hardness-intensity diagram of all RXTE/PCA data combined shows a q-shaped track similar to that observed in previous outbursts. The evolution through the HID suggests that in the early phase of the outburst the source underwent a sequence of state transitions, from the hard to the soft state, which is supported by our timing analysis. Broadband (4-200 keV) spectra, fitted with an exponentially cutoff powerlaw, show that the hard spectral component steepens during the transition from the hard to the soft state. The high-energy cutoff decreased monotonically from 120 to 60 keV during the brightening of the hard state, but increased again to 100 keV during the softening in the hard intermediate state. In the short-lived soft intermediate state the cutoff energy was ~ 130 keV, but was no longer detected in the soft state. This is one of the first times that the high-energy cut-off has been followed in such detail across several state transitions. We find that in comparison to several other spectral parameters, the cut-off energy changes more rapidly, just like the timing properties. The observed behaviour of the high energy cutoff of GX 339-4 is also similar to that observed with RXTE-INTEGRAL-Swift during the 2005 outburst of GRO J1655-40. These results constitute a valuable reference to be considered when testing theoretical models for the production of the hard component in these systems.

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