arXiv subjects

Mariano Méndez

Publications and source records attributed to Mariano Méndez.

At least 55 records · Page 3Linked to original sources

arXiv2022

The evolving properties of the corona of GRS 1915+105: A spectral-timing perspective through variable-Comptonisation modelling

The inverse Compton process by which soft photons are up-scattered by hot electrons in a corona plays a fundamental role in shaping the X-ray spectra of black-hole (BH) low-mass X-ray binaries (LMXBs), particularly in the hard and hard-intermediate states. In these states, the power-density spectra of these sources typically show Type-C low-frequency quasi-periodic oscillations (QPOs). Although several models have been proposed to explain the dynamical origin of their frequency, only a few of those models predict the spectral-timing radiative properties of the QPOs. Here we study the physical and geometrical properties of the corona of the BH-LMXB GRS 1915+105 based on a large sample of observations available in the RXTE archive. We use a recently-developed spectral-timing Comptonisation model to fit simultaneously the energy-dependent fractional rms amplitude and phase-lag spectra of the Type-C QPO in 398 observations. For this, we include spectral information gathered from fitting a Comptonisation model to the corresponding time-averaged spectra. We analyse the dependence of the physical and geometrical properties of the corona upon the QPO frequency and spectral state of the source, the latter characterised by the hardness ratio. We find consistent trends in the evolution of the corona size, temperature, and feedback (the fraction of the corona photons that impinge back onto the disc) that persist for roughly 15~years. By correlating our observations with simultaneous radio-monitoring of the source at 15 GHz, we propose a scenario in which the disc-corona interactions connect with the launching mechanism of the radio jet in this source.

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arXiv2022

The origin of UV/optical emission in the black hole low-mass X-ray binary Swift J1753.5-0127

The emission from the accreting black holes (BHs) in low-mass X-ray binaries (LMXBs) covers a broad energy band from radio to X-rays. Studying the correlations between emission in different energy bands during outbursts can provide valuable information about the accretion process. We analyse the simultaneous optical, ultraviolet (UV) and X-ray data of the BH-LMXB Swift J1753.5-0127 during its $\sim$ 12-year long outburst with the {\it Neil Gehrels Swift Observatory}. We find that the UV/optical and X-ray emission are strongly correlated during the hard states of the outburst. We fit the relation with a power-law function $F_{UV/optical} \propto F_{X}^β$ and find that the power-law index $β$ increases from $\sim$ 0.24 to $\sim$ 0.33 as the UV/optical wavelength decreases from $\sim$ 5400 Å (V) to $\sim$ 2030 Å (UVW2). We explore the possible reasons for this and suggest that in Swift J1753.5-0127 the UV/optical emission is dominated by a viscously heated accretion disc at large radii. We find that the data that deviate from the correlation correspond to the low-intensity peaks appeared in the X-ray band during the outburst, and suggest that these deviations are driven by the emission from the inner part of the accretion disc.

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arXiv2022

Lags of the KiloHertz Quasi-Periodic Oscillations in the transient source XTE J1701$-$462

We analysed 14 observations with kilohertz quasi-periodic oscillations (kHz QPOs) of the neutron star X-ray binary XTE J1701$-$462, the first source to show a clear transition between atoll and Z-like behaviour during a single outburst. We calculated the average cross-spectrum of both atoll and Z-phase observations of XTE J1701$-$462 between a reference/hard band (6.1 - 25.7 keV) and a subject/soft band (2.1 - 5.7 keV) to obtain, using a novel technique, the average time lags of the lower and upper kHz QPOs. During the atoll phase, we found that at the frequency of the lower kHz QPO the soft photons lag behind the hard ones by $18 \pm 8$ $μ$s, whereas during the Z phase the lags are $33\pm35$ $μ$s, consistent with zero. This difference in the lags of both phases suggests that in XTE J1701$-$462, as observed in other sources, the lags decrease with increasing luminosity. We found that for both the atoll and Z phase observations the fractional rms amplitude increases with energy up to $\sim$10 keV and remains more or less constant at higher energies. Since these changes in the variability of XTE J1701$-$462 occur within the same outburst, properties like the mass of the neutron star or the inclination of the system cannot be responsible for the differences in the timing properties of the kHz QPOs in the atoll and Z phase. Here we suggest that these differences are driven by a Comptonizing component or corona, possibly oscillating in a coupled mode with the innermost regions of the accretion disc.

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arXiv2022

The evolution of the high-frequency variability in the black hole candidate GRS 1915+105 as seen by RXTE

GRS 1915+105 can show type-C quasi-periodic oscillations (QPOs) in the power density spectrum. A high-frequency QPO (HFQPO) at 67 Hz has been observed in this source, albeit less often than the type-C QPOs. Besides these features, GRS 1915+105 sometimes shows a broad bump in the power spectrum at around 30-150 Hz. We study the power spectra of GRS 1915+105 with the Rossi X-ray Timing Explorer when the source was in the $χ$ class. We find that the rms amplitude of the bump depends strongly upon both the frequency of the type-C QPO and the hardness ratio, and is correlated with the corona temperature and anti-correlated with the radio flux at 15 GHz. The characteristic frequency of the bump is better correlated with a combination of the frequency of the type-C QPO and the hardness ratio than with the frequency of the type-C QPO alone. The rms amplitude of the bump generally increases with energy from ~1-2% at ~3 keV to ~10-15% at ~30 keV. We suggest that the bump and the high-frequency QPO may be the same variability component but the properties of the corona affect the coherence of this variability, leading either to a HFQPO when the spectrum is in the relatively soft $γ$ class, or to a bump when the spectrum is in the hard $χ$ class. Finally, we discuss the anti-correlation between the rms amplitude of the bump and the radio flux in the context of the relation between the corona and the jet.

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arXiv2022

The evolution of the corona in MAXI J1535-571 through type-C quasi-periodic oscillations with Insight-HXMT

Type-C quasi-periodic oscillations (QPOs) in black hole X-ray transients can appear when the source is in the low-hard and hard-intermediate states. The spectral-timing evolution of the type-C QPO in MAXI J1535-571 has been recently studied with Insight-HXMT. Here we fit simultaneously the time-averaged energy spectrum, using a relativistic reflection model, and the fractional rms and phase-lag spectra of the type-C QPOs, using a recently developed time-dependent Comptonization model when the source was in the intermediate state. We show, for the first time, that the time-dependent Comptonization model can successfully explain the X-ray data up to 100 keV. We find that in the hard-intermediate state the frequency of the type-C QPO decreases from 2.6 Hz to 2.1 Hz, then increases to 3.3 Hz, and finally increases to ~ 9 Hz. Simultaneously with this, the evolution of corona size and the feedback fraction (the fraction of photons up-scattered in the corona that return to the disc) indicates the change of the morphology of the corona. Comparing with contemporaneous radio observations, this evolution suggests a possible connection between the corona and the jet when the system is in the hard-intermediate state and about to transit into the soft-intermediate state.

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arXiv2021

Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity

We study the energy-dependent time lags and rms fractional amplitude of the kilohertz quasi-periodic oscillations (kHz QPOs) of a group of neutron-star low mass X-ray binaries (LMXBs). We find that for the lower kHz QPO the slope of the best-fitting linear model to the time-lag spectrum and the total rms amplitude integrated over the 2 to 25 keV energy band both decrease exponentially with the luminosity of the source. For the upper kHz QPO the slope of the time-lag spectrum is consistent with zero, while the total rms amplitude decreases exponentially with the luminosity of the source. We show that both the slope of the time-lag spectrum and the total rms amplitude of the lower kHz QPO are linearly correlated with a slope of ~1. Finally, we discuss the mechanism that could be responsible for the radiative properties of the kHz QPOs, with the variability originating in a Comptonising cloud or corona that is coupled to the innermost regions of the accretion disc, close to the neutron star.

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arXiv2021

Spectral and timing evolution of MAXI J1631-479 during the 2018-19 outburst with NICER

The X-ray transient MAXI J1631-479 went into outburst on 2018 December 21 and remained active for about seven months. Owing to various constraints it was monitored by NICER only during the decay phase of the outburst for about four months. The NICER observations were primarily in the soft state with a brief excursion to the hard intermediate state. While the soft state spectrum was dominated by thermal disk emission, the hard intermediate state spectrum had maximum contribution from the thermal Comptonization. Almost all intermediate-state power spectra had a Type-C low frequency quasi-periodic oscillation (within 4 - 10 Hz), often accompanied by a harmonic component. The frequency of these oscillations increased and the fractional rms decreased with inner-disk temperature suggesting a geometric origin. One observation in the middle of the outburst during the hard intermediate state had two non-harmonically related peaks. While one of them was definitely a Type-C QPO, the identification of the other one is uncertain. The rms spectra during the intermediate state had a hard shape from above 1 keV. Below 1 keV the shape could not be constrained in most cases, while only a few observations showed a rise in amplitude.

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arXiv2021

A variable corona for GRS 1915+105

Most models of the low frequency quasi periodic oscillations (QPOs) in low-mass X-ray binaries (LMXBs) explain the dynamical properties of those QPOs. On the other hand, in recent years reverberation models that assume a lamp-post geometry have been successfull in explaining the energy-dependent time lags of the broad-band noise component in stellar mass black-holes and active galactic nuclei. We have recently shown that Comptonisation can explain the spectral-timing properties of the kilo-hertz (kHz) QPOs observed in neutron star (NS) LMXBs. It is therefore worth exploring whether the same family of models would be as successful in explaining the low-frequency QPOs. In this work, we use a Comptonisation model to study the frequency dependence of the phase lags of the type-C QPO in the BH LMXB GRS 1915+105. The phase lags of the QPO in GRS 1915+105 make a transition from hard to soft at a QPO frequency of around 1.8 Hz. Our model shows that at high QPO frequencies a large corona of ~ 100-150 R_g covers most of the accretion disc and makes it 100% feedback dominated, thus producing soft lags. As the observed QPO frequency decreases, the corona gradually shrinks down to around 3-17 R_g, and at 1.8 Hz feedback onto the disc becomes inefficient leading to hard lags. We discuss how changes in the accretion geometry affect the timing properties of the type-C QPO.

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arXiv2020

A two-component Comptonisation model for the type-B QPO in MAXI J1348-630

Spectral-timing analysis of the fast variability observed in X-rays is a powerful tool to study the physical and geometrical properties of the accretion/ejection flows in black-hole binaries. The origin of type-B quasi-periodic oscillations (QPO), predominantly observed in black-hole candidates in the soft-intermediate state, has been linked to emission arising from the relativistic jet. In this state, the X-ray spectrum is characterised by a soft-thermal blackbody-like emission due to the accretion disc, an iron emission line (in the 6-7 keV range), and a power-law like hard component due to Inverse-Compton scattering of the soft-photon source by hot electrons in a corona or the relativistic jet itself. The spectral-timing properties of MAXI J1348-630 have been recently studied using observations obtained with the NICER observatory. The data show a strong type-B QPO at ~4.5 Hz with increasing fractional rms amplitude with energy and positive lags with respect to a reference band at 2-2.5 keV. We use a variable-Comptonisation model that assumes a sinusoidal coherent oscillation of the Comptonised X-ray flux and the physical parameters of the corona at the QPO frequency, to fit simultaneously the energy-dependent fractional rms amplitude and phase lags of this QPO. We show that two physically-connected Comptonisation regions can successfully explain the radiative properties of the QPO in the full 0.8-10 keV energy range.

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arXiv2020

The study of multi-peaked type-I X-ray bursts in the neutron-star low mass X-ray binary 4U 1636$-$536 with RXTE

We have found and analysed 16 multi-peaked type-I bursts from the neutron-star low mass X-ray binary 4U 1636$-$53 with the Rossi X-ray Timing Explorer (RXTE). One of the bursts is a rare quadruple-peaked burst which was not previously reported. All 16 bursts show a multi-peaked structure not only in the X-ray light curves but also in the bolometric light curves. Most of the multi-peaked bursts appear in observations during the transition from the hard to the soft state in the colour-colour diagram. We find an anti-correlation between the second peak flux and the separation time between two peaks. We also find that in the double-peaked bursts the peak-flux ratio and the temperature of the thermal component in the pre-burst spectra are correlated. This indicates that the double-peaked structure in the light curve of the bursts may be affected by enhanced accretion rate in the disc, or increased temperature of the neutron star.

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arXiv2020

High-frequency variability in neutron-star low-mass X-ray binaries

Binary systems with a neutron-star primary accreting from a companion star display variability in the X-ray band on time scales ranging from years to milliseconds. With frequencies of up to ~1300 Hz, the kilohertz quasi-periodic oscillations (kHz QPOs) represent the fastest variability observed from any astronomical object. The sub-millisecond time scale of this variability implies that the kHz QPOs are produced in the accretion flow very close to the surface of the neutron star, providing a unique view of the dynamics of matter under the influence of some of the strongest gravitational fields in the Universe. This offers the possibility to probe some of the most extreme predictions of General Relativity, such as dragging of inertial frames and periastron precession at rates that are sixteen orders of magnitude faster than those observed in the solar system and, ultimately, the existence of a minimum distance at which a stable orbit around a compact object is possible. Here we review the last twenty years of research on kHz QPOs, and we discuss the prospects for future developments in this field.

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arXiv2020

The evolution of the broadband temporal features observed in the black-hole transient MAXI J1820+070 with Insight-HXMT

We study the evolution of the temporal properties of MAXI 1820+070 during the 2018 outburst in its hard state from MJD 58190 to 58289 with Insight-HXMT in a broad energy band 1-150 keV. We find different behaviors of the hardness ratio, the fractional rms and time lag before and after MJD 58257, suggesting a transition occurred at around this point. The observed time lags between the soft photons in the 1-5 keV band and the hard photons in higher energy bands, up to 150 keV, are frequency-dependent: the time lags in the low-frequency range, 2-10 mHz, are both soft and hard lags with a timescale of dozens of seconds but without a clear trend along the outburst; the time lags in the high-frequency range, 1-10 Hz, are only hard lags with a timescale of tens of milliseconds; first increase until around MJD 58257 and decrease after this date. The high-frequency time lags are significantly correlated to the photon index derived from the fit to the quasi-simultaneous NICER spectrum in the 1-10 keV band. This result is qualitatively consistent with a model in which the high-frequency time lags are produced by Comptonization in a jet.

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arXiv2020

A Systematic Analysis of the Phase Lags Associated with the Type-C Quasi-periodic Oscillation in GRS 1915+105

We present a systematic analysis of the phase lags associated with the type-C QPOs in GRS 1915+105 using RXTE data. Our sample comprises of 620 RXTE observations with type-C QPOs ranging from ~0.4 Hz to ~6.3 Hz. Based on our analysis, we confirm that the QPO phase lags decrease with QPO frequency, and change sign from positive to negative at a QPO frequency of ~2 Hz. In addition, we find that the slope of this relation is significantly different between QPOs below and above 2 Hz. The relation between the QPO lags and QPO rms can be well fitted with a broken line: as the QPO lags go from negative to positive, the QPO rms first increases, reaching its maximum at around zero lag, and then decreases. The phase-lag behaviour of the subharmonic of the QPO is similar to that of the QPO fundamental, where the subharmonic lags decrease with subharmonic frequency and change sign from positive to negative at a subharmonic frequency of ~1 Hz; on the contrary, the second harmonic of the QPO shows a quite different phase-lag behaviour, where all the second harmonics show hard lags that remain more or less constant. For both the QPO and its (sub)harmonics, the slope of the lag-energy spectra shows a similar evolution with frequency as the average phase lags. This suggests that the lag-energy spectra drives the average phase lags. We discuss the possibility for the change in lag sign, and the physical origin of the QPO lags.

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arXiv2020

A retrograde spin of the black hole in MAXI J1659--152

We present the results of spectral analysis of the galactic black-hole binary MAXI J1659--152 in the rising phase of the outburst that lasted for about 65 days starting on 2010 September 25. The presence of a broad Fe line, verified by Monte-Carlo simulations, and coverage of a wide energy band by utilizing the combined spectral capabilities of XMM-Newton/EPIC-pn and RXTE/PCA allowed us to use a combination of reflection spectroscopy and continuum fitting methods to estimate the spin of the black hole. We explored the entire parameter range allowed by the present uncertainties on black-hole mass, inclination, and distance as well as the accretion rate. We show that for about 95 percentage of parameter space and very reasonable upper limits on mass accretion rate, the spin of the black hole has to be negative. This is the first clear detection of negative spin in a galactic black-hole binary.

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arXiv2019

The Comptonising medium of the neutron star in 4U 1636-53 through its lower kilohertz Quasi-Periodic Oscillations

Inverse Compton scattering dominates the high energy part of the spectra of neutron star (NS) low mass X-ray binaries (LMXBs). It has been proposed that inverse Compton scattering also drives the radiative properties of kilohertz quasi periodic oscillations (kHz QPOs). In this work, we construct a model that predicts the energy dependence of the rms amplitude and time lag of the kHz QPOs. Using this model, we fit the rms amplitude and time lag energy spectra of the lower kHz QPO in the NS LMXB 4U 1636-53 over 11 frequency intervals of the QPO and report three important findings: (i) A medium that extends 1-8 km above the NS surface is required to fit the data; this medium can be sustained by the balance between gravity and radiation pressure, without forcing any equilibrium condition. (ii) We predict a time delay between the oscillating NS temperature, due to feedback, and the oscillating electron temperature of the medium which, with the help of phase resolved spectroscopy, can be used as a probe of the geometry and the feedback mechanism. (iii) We show that the observed variability as a function of QPO frequency is mainly driven by the oscillating electron temperature of the medium. This provides strong evidence that the Comptonising medium in LMXBs significantly affects, if not completely drives, the radiative properties of the lower kHz QPOs regardless of the nature of the dynamical mechanism that produces the QPO frequencies.

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arXiv2019

The amplitude of the kilohertz quasi-periodic oscillations in 4U 1636$-$53 in the frequency-energy space

We present for the neutron-star low-mass X-ray binary 4U 1636$-$53, and for the first time for any source of kilohertz quasi-periodic oscillations (kHz QPOs), the two-dimensional behaviour of the fractional rms amplitude of the kHz QPOs in the parameter space defined by QPO frequency and photon energy. We find that the rms amplitude of the lower kHz QPO increases with energy up to $\sim12$ keV and then decreases at higher energies, while the rms amplitude of the upper kHz QPO either continues increasing or levels off at high energies. The rms amplitude of the lower kHz QPO increases and then decreases with frequency, peaking at $\sim 760$ Hz, while the amplitude of the upper kHz QPO decreases with frequency, with a local maximum at around $\sim 770$ Hz, and is consistent with becoming zero at the same QPO frequency, $\sim1400$ Hz, in all energy bands, thus constraining the neutron-star mass at $M_{NS} \leq 1.6 M_{\odot}$, under the assumption that this QPO reflects the Keplerian frequency at the inner edge of the accretion disc. We show that the slope of the rms energy spectrum is connected to the changing properties of the kHz QPOs in different energy bands as its frequencies change. Finally, we discuss a possible mechanism responsible for the radiative properties of the kHz QPOs and, based on a model in which the QPO arises from oscillations in a Comptonising cloud of hot electrons, we show that the properties of the kHz QPOs can constrain the thermodynamic properties of the inner accretion flow.

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arXiv2019

Study of the X-ray properties of the neutron-star binary 4U 1728$-$34 from the soft to hard state

We studied five XMM-Newton observations of the neutron-star binary 4U 1728$-$34 covering the hard, intermediate and soft spectral states. By jointly fitting the spectra with several reflection models, we obtained an inclination angle of 25$-$53$°$ and an iron abundance up to 10 times the solar. From the fits with reflection models, we found that the fluxes of the reflection and the Comptonised components vary inconsistently; since the latter is assumed to be the illuminating source, this result possibly indicates the contribution of the neutron star surface/boundary layer to the disc reflection. As the source evolved from the relatively soft to the intermediate state, the disc inner radius decreased, opposite to the prediction of the standard accretion disc model. We also explore the possible reasons why the supersolar iron abundance is required by the data and found that this high value is probably caused by the absence of the hard photons in the XMM-Newton data.

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arXiv2018

Dense matter with eXTP

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

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