arXiv subjects

T. Muñoz-Darias

Publications and source records attributed to T. Muñoz-Darias.

At least 91 records · Page 5Linked to original sources

arXiv2013

Black hole spin measurements through the relativistic precession model: XTE J1550-564

We present a systematic analysis of the complete set of observations of the black hole (BH) binary XTE J1550-564 obtained by the Rossi X-ray Timing Explorer. We study the fast time variability properties of the source and determine the spin of the black hole through the relativistic precession model. Similarly to what is observed in the BH binary GRO J1655-40, the frequencies of the QPOs and broad band noise components match the general relativistic frequencies of particle motion close to the compact object predicted by the relativistic precession model. The combination of two simultaneously observed quasi-periodic oscillation (QPO) frequencies together with the dynamical BH mass from optical/infrared observations yields a spin equal to a = 0.34 +/- 0.01, consistent with previous determinations from X-ray spectroscopy. Based on the derived BH parameters, the low frequency QPO emission radii vary from about 30 gravitational radii to the innermost stable orbit for this spin (about 5 gravitational radii), where they sharply disappear as observed for the case of GRO J1655-40.

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arXiv2013

Precise mass and spin measurements for a stellar-mass black hole through X-ray timing: the case of GRO J1655-40

We present a systematic analysis of the fast time variability properties of the transient black hole binary GRO J1655-40, based on the complete set of RossiXTE observations. We demonstrate that the frequencies of the quasi-periodic oscillations and of the broad band noise components and their variations match accurately the strong field general relativistic frequencies of particle motion in the close vicinity of the innermost stable circular orbit, as predicted by the relativistic precession model. We obtain high precision measurements of the black hole mass (M = (5.31 +/- 0.07) M solar masses, consistent with the value from optical/NIR observations) and spin (a/M = 0.290 +/- 0.003), through the sole use of X-ray timing.

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arXiv2013

Time lags in the ultraluminous X-ray source NGC 5408 X-1: implications for the black hole mass

We present the analysis of the X-ray variability and spectral timing properties of the ultraluminous X-ray source (ULX) NGC 5408 X-1, one of the most variable ULXs known so far. The variability properties are used as a diagnostic of the accretion state of the source and to derive estimates of the black hole (BH) mass. The observed high level of fast X-ray variability (fractional root-mean-square variability - rms - amplitude of $\sim$ 30 percent in the hard energy band), the hardening of the fractional rms spectrum, and the properties of the QPO, all resemble those of a source in a hard-intermediate accretion state. We confirm the previous detection of a soft lag in the X-ray light curves of the source during 2006 and 2008 observations and find that the soft lag is still present in the more recent 2010/2011 observations. Using the entire available XMM-Newton data set (public as of February 2012) we observe that the soft lag (of few seconds amplitude) is detected over a relatively large range of frequencies ($ν\sim 5-90$ mHz), which always includes the QPO frequencies. The soft lag displays energy-dependence, with the (absolute) amplitude increasing as a function of energy separation. We find close analogies with soft lags associated to type-C QPOs in BH binary systems (although an association to other types of QPOs cannot be completely excluded), as well as with reverberation lags observed in AGN. In both cases an intermediate mass BH (IMBH) solution appears the most plausible.

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arXiv2013

A 420 day X-ray/optical modulation and extended X-ray dips in the short-period transient Swift J1753.5-0127

We have discovered a \sim420d modulation, with associated X-ray dips, in RXTE-ASM/MAXI/Swift-BAT archival light-curves of the short-period (3.2h) black-hole X-ray transient, Swift J1753.5-0127. This modulation only appeared at the end of a gradual rebrightening, approximately 3 years after the initial X-ray outburst in mid-2005. The same periodicity is present in both the 2-20 keV and 15-50 keV bands, but with a \sim0.1 phase offset (\sim40d). Contemporaneous photometry in the optical and near-IR reveals a weaker modulation, but consistent with the X-ray period. There are two substantial X-ray dips (very strong in the 15-50 keV band, weaker at lower energies) that are separated by an interval equal to the X-ray period. This likely indicates two physically separated emitting regions for the hard X-ray and lower energy emission. We interpret this periodicity as a property of the accretion disc, most likely a long-term precession, where the disc edge structure and X-ray irradiation is responsible for the hard X-ray dips and modulation, although we discuss other possible explanations, including Lense-Thirring precession in the inner disc region and spectral state variations. Such precession indicates a very high mass ratio LMXB, which even for a \sim10M_sun BH requires a brown dwarf donor (\sim0.02M_sun), making Swift J1753.5-0127 a possible analogue of millisecond X-ray pulsars.We compare the properties of Swift J1753.5-0127 with other recently discovered short-period transients, which are now forming a separate population of high latitude BH transients located in the galactic halo.

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arXiv2013

Inclination and relativistic effects in the outburst evolution of black hole transients

We have systematically studied the effect of the orbital inclination in the outburst evolution of black hole transients. We have included all the systems observed by the Rossi X-ray timing explorer in which the thermal, accretion disc component becomes strongly dominant at some point of the outburst. Inclination is found to modify the shape of the tracks that these systems display in the colour/luminosity diagrams traditionally used for their study. Black hole transients seen at low inclination reach softer spectra and their accretion discs look cooler than those observed closer to edge-on. This difference can be naturally explained by considering inclination dependent relativistic effects on accretion discs.

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arXiv2013

The optical counterpart of the bright X-ray transient Swift J1745-26

We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745-26, starting only 19 minutes after the discovery of the source. We observe the system peaking at i' ~17.6 on day 6 (MJD 56192) to then decay at a rate of ~0.04 mag/day. We show that the optical peak occurs at least 3 days later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 magnitudes, which favours an orbital period < 21 h and a companion star with a spectral type later than ~ A0. Spectroscopic observations taken with the GTC-10.4 m telescope reveal a broad (FWHM ~ 1100 km/s), double-peaked H_alpha emission line from which we constrain the radial velocity semi-amplitude of the donor to be K_2 > 250 km/s. The breadth of the line and the observed optical and X-ray fluxes suggest that Swift J1745-26 is a new black hole candidate located closer than ~7 kpc.

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arXiv2013

A Black Hole Nova Obscured by an Inner Disk Torus

Stellar-mass black holes (BHs) are mostly found in X-ray transients, a subclass of X-ray binaries that exhibit violent outbursts. None of the 50 Galactic BHs known show eclipses, which is surprising for a random distribution of inclinations. Swift J1357.2-0933 is a very faint X-ray transient detected in 2011. On the basis of spectroscopic evidence, we show that it contains a BH in a 2.8 h orbital period. Further, high-time resolution optical light curves display profound dips without X-ray counterparts. The observed properties are best explained by the presence of an obscuring toroidal structure moving outwards in the inner disk seen at very high inclination. This observational feature should play a key role in models of inner accretion flows and jet collimation mechanisms in stellar-mass BHs.

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arXiv2012

A complex state transition from the black hole candidate Swift J1753.5-0127

We present our monitoring campaign of the outburst of the black-hole candidate Swift J1753.5-0127, observed with the Rossi X-ray Timing Explorer and the Swift satellites. After ~4.5 years since its discovery, the source had a transition to the hard intermediate state. We performed spectral and timing studies of the transition showing that, unlike the majority of the transient black holes, the system did not go to the soft states but it returned to the hard state after a few months. During this transition Swift J1753.5-0127 features properties which are similar to those displayed by the black hole Cygnus X-1. We compared Swift J1753.5-0127 to one dynamically confirmed black hole and two neutron stars showing that its power spectra are in agreement with the binary hosting a black hole. We also suggest that the prolonged period at low flux that followed the initial flare is reminiscent of that observed in other X-ray binaries, as well as in cataclysmic variables.

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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

The faint 2011 outburst of the black hole X-ray binary candidate MAXI J1543-564

We report on a spectral-timing analysis of the black hole X-ray binary candidate MAXI J1543- 564 during its 2011 outburst. All 99 pointed observations of this outburst obtained with the Rossi X-ray Timing Explorer (RXTE) were included in our study. We computed the fundamental diagrams commonly used to study black hole transients, and fitted power density and energy spectra to study the spectral and timing parameters along the outburst. The determination of timing parameters and hence of exact transitions between different states was hampered by the rather low count rate at which his outburst was observed. We detected two periods of exponential decay, one after the source was brightest, which was interrupted by several flares, and another one during the high/soft state. The detection of these decays allowed us to obtain an estimate for the source distance of at least 8.5 kpc. This leaves two possible explanations for the observed low count rate; either the source has a distance similar to that of other black hole X-ray binary candidates and it is intrinsically faint, or it has a similar luminosity, but is located more than 12 kpc away from us. Furthermore, in the high/soft state the source spectrum appears to be completely disc dominated.

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arXiv2012

The near-infrared counterpart of 4U 1636-53

The optical counterpart of the neutron star X-ray binary and well known X-ray burster, 4U 1636-53 (= 4U 1636-536 = V801 Ara) has been well studied for three decades. However to date, no infrared studies have been reported. Our aims are to identify and investigate the near-infrared (NIR) counterpart of 4U 1636-53. We present deep, K_s-band (2.2 micron) imaging of the region of 4U 1636-53 taken with the Infrared Spectrometer And Array Camera (ISAAC) on the Very Large Telescope. Archival optical and UV data are used to infer the 0.2-2.2 micron spectral energy distribution (SED). One star is located at coordinates alpha =16:40:55.57, delta =-53:45:05.2 (J2000; 1 sigma positional uncertainty of ~ 0.3 arcsec) which is consistent with the known optical position of 4U 1636-53; its magnitude is K_s = 16.14 +- 0.12. This star is also detected in the 2MASS survey in J-band and has a magnitude of J = 16.65 +- 0.22. Under the assumption that the persistent emission is largely unvarying, the 0.4-2.2 micron de-reddened SED can be described by a power law; F_nu propto nu^(1.5 +- 0.3), with some possible curvature (F_nu propto nu^(<= 1.5)) at 0.2-0.4 microns. The SED can be approximated by a blackbody of temperature ~ 27 000 K. This is typical for an active low-mass X-ray binary, and the emission can be explained by the outer regions of a (likely irradiated) accretion disc. We therefore interpret this K_s-band star as the NIR counterpart.

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arXiv2011

The nature of the X-ray transient MAXI J0556-332

Phase-resolved spectroscopy of the newly discovered X-ray transient MAXI J0556-332 has revealed the presence of narrow emission lines in the Bowen region that most likely arise on the surface of the mass donor star in this low mass X-ray binary. A period search of the radial velocities of these lines provides two candidate orbital periods (16.43+/-0.12 and 9.754+/-0.048 hrs), which differ from any potential X-ray periods reported. Assuming that MAXI J0556-332 is a relatively high inclination system that harbors a precessing accretion disk in order to explain its X-ray properties, it is only possible to obtain a consistent set of system parameters for the longer period. These assumptions imply a mass ratio of q~0.45, a radial velocity semi-amplitude of the secondary of K_2~190 km/s and a compact object mass of the order of the canonical neutron star mass, making a black hole nature for MAXI J0556-332 unlikely. We also report the presence of strong N III emission lines in the spectrum, thereby inferring a high N/O abundance. Finally we note that the strength of all emission lines shows a continuing decay over the ~1 month of our observations.

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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

Spectral properties of transitions between soft and hard state in GX 339-4

We present a study of the spectral properties during state transition of GX 339-4. Data are taken from the 2010 outburst of GX 339-4, which is densely covered by Rossi X-ray Timing Explorer, providing an excellent coverage of the state transitions between the low/hard state and the high/soft state. We select all observations within a certain hardness ratio range during the soft intermediate state (SIMS). This sample was chosen in such a way to comprise all observations that show a type-B quasi-periodic oscillation (QPO). In addition, we also investigate the spectra of hard intermediate state observations. The spectra, obtained from Proportional Counter Array data in the 10 to 40 keV range, are fitted with a power law and an additional high energy cut-off if needed. We find that the spectra are significantly harder during the SIMS of the soft-to-hard transition than they are during the hard-to-soft transition. This demonstrates that during the SIMS of the soft-to-hard transition not only the luminosity and peak frequencies of type-B QPOs are lower, but that also the photon index is lower, compared to the hard-to-soft transition. Hence, type-B QPOs can be associated to a different spectral shape even though they appear at the same hardness. However, in each branch only certain combinations of centroid frequency and photon index are realised.

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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

The black hole candidate MAXI J1659-152: spectral and timing analysis during its 2010 outburst

We present a comprehensive spectral-timing study of the black hole candidate MAXI J1659-152 during its 2010 outburst. We analysed 65 RXTE observations taken along this period and computed the fundamental diagrams commonly used to study black hole transients. We fitted power density and energy spectra and studied the evolution of the spectral and timing parameters along the outburst. We discuss the evolution of the variability observed at different energy bands on the basis of the relative contribution of the disc and hard components to the energy spectrum of the source. We conclude that hard emission accounts for the observed fast variability, it being strongly quenched when type-B oscillations are observed. We find that both disc and hard emission are responsible for local count-rate peaks until the system reaches the soft state. From that point, the peaks are only observed in the hard component, whereas the thermal component drops monotonically probably following the accretion rate decrease. We have also computed time-lags between soft and hard X-ray variability confirming that lags are larger during the hard-to-soft transition than during the hard state.

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arXiv2010

New Optical Results on gamma-ray Binaries

We present new optical spectroscopy of the gamma-ray binary LS 5039. Our data show evidence for sub-orbital modulation in the radial velocities with amplitude ~7 km/s and period ~Porb/4. This short-term oscillation is stable over at least 7 years and it is likely triggered by non-radial oscillations of the O6.5V optical star. We also present the results of a spectroscopic campaign on MWC 148, the optical counterpart of the new gamma-ray binary candidate HESS J0632+067. Long-term variations in the Halpha and Hbeta emission line parameters are clearly detected which, if modulated with the binary orbit, would imply a period >200 days.

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arXiv2010

Fast variability as a tracer of accretion regimes in black hole transients

We present the rms-intensity diagram for black hole transients. Using observations taken with the Rossi X-ray timing explorer we study the relation between the root mean square (rms) amplitude of the variability and the net count-rate during the 2002, 2004 and 2007 outbursts of the black hole X-ray binary GX 339-4. We find that the rms-flux relation previously observed during the hard state in X-ray binaries does not hold for the other states, when different relations apply. These relations can be used as a good tracer of the different accretion regimes. We identify the hard, soft and intermediate states in the rms-intensity diagram. Transitions between the different states are seen to produce marked changes in the rms-flux relation. We find that one single component is required to explain the ~ 40 per cent variability observed at low count rates, whereas no or very low variability is associated to the accretion-disc thermal component.

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