arXiv subjects

Mariano Mendez

Publications and source records attributed to Mariano Mendez.

At least 55 records · Page 3Linked to original sources

arXiv2012

The time derivative of the kilohertz quasi-periodic oscillations in 4U 1636-53

We analysed all archival RXTE observations of the neutron-star low-mass X-ray binary 4U 1636-53 up to May 2010. In 528 out of 1280 observations we detected kilohertz quasi-periodic oscillations (kHz QPOs), with ~ 65% of these detections corresponding to the so-called lower kHz QPO. Using this QPO we measured, for the first time, the rate at which the QPO frequency changes as a function of QPO frequency. For this we used the spread of the QPO frequency over groups of 10 consecutive measurements, sampling timescales between 320 and 1600 s, and the time derivative of the QPO frequency over timescales of 32 to 160 s. We found that: (i) Both the QPO-frequency spread and the QPO time derivative decrease by a factor ~ 3 as the QPO frequency increases. (ii) The average value of the QPO time derivative decreases by a factor of ~ 2 as the timescale over which the derivative is measured increases from less than 64 s to 160 s. (iii) The relation between the absolute value of the QPO time derivative and the QPO frequency is consistent with being the same both for the positive and negative QPO-frequency derivative. We show that, if either the lower or the upper kHz QPO reflects the Keplerian frequency at the inner edge of the accretion disc, these results support a scenario in which the inner part of the accretion disc is truncated at a radius that is set by the combined effect of viscosity and radiation drag.

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arXiv2011

The Variable Quiescent X-ray Emission of the Neutron Star Transient XTE J1701-462

We present the results of continued monitoring of the quiescent neutron star low-mass X-ray binary XTE J1701-462 with Chandra and Swift. A new Chandra observation from 2010 October extends our tracking of the neutron star surface temperature from ~800 days to ~1160 days since the end of an exceptionally luminous 19 month outburst. This observation indicates that the neutron star crust may still be slowly cooling toward thermal equilibrium with the core; another observation further into quiescence is needed to verify this. The shape of the overall cooling curve is consistent with that of a broken power law, although an exponential decay to a constant level cannot be excluded with the present data. To investigate possible low-level activity, we conducted a monitoring campaign of XTE J1701-462 with Swift during 2010 April-October. Short-term flares - presumably arising from episodic low-level accretion - were observed up to a luminosity of ~1e35 erg/s, ~20 times higher than the normal quiescent level. We conclude that flares of this magnitude are not likely to have significantly affected the equilibrium temperature of the neutron star and are probably not able to have a measurable impact on the cooling curve. However, it is possible that brighter and longer periods of low-level activity have had an appreciable effect on the equilibrium temperature.

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arXiv2010

The cooling phase of Type-I X-ray bursts in 4U 1636-53

Time-resolved spectra during the cooling phase of thermonuclear X-ray bursts in low-mass X-ray binaries (LMXBs) can be used to measure the radii and masses of neutron stars. We analyzed ~ 300 bursts of the LMXB 4U 1636-53 using data from the Rossi X-ray Timing Explorer. We divided the bursts in three groups, photospheric radius expansion (PRE), hard non-PRE and soft non-PRE bursts, based on the properties of the bursts and the state of the source at the time of the burst. For the three types of bursts, we found that the average relation between the bolometric flux and the temperature during the cooling phase of the bursts is significantly different from the canonical $F \propto T^4$ relation that is expected if the apparent emitting area on the surface of the neutron star remains constant as the flux decreases during the decay of the bursts. We also found that a single power law cannot fit the average flux-temperature relation for any of the three types of bursts, and that the flux-temperature relation for the three types of bursts is significantly different. Finally, for the three types of bursts, the temperature distribution at different flux levels during the decay of the bursts is significantly different. From the above we conclude that hard non-PRE bursts ignite in a hydrogen-rich atmosphere, whereas for soft non-PRE and PRE bursts the fuel is helium-rich. We further conclude that the metal abundance in the neutron star atmosphere decreases as the bursts decay, probably because the heavy elements sink faster in the atmosphere than H and He.

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arXiv2010

A strong and broad iron line in the XMM-Newton spectrum of the new X-ray transient and black-hole candidate XTE J1652-453

We observed the new X-ray transient and black-hole candidate XTE J1652-453 simultaneously with XMM-Newton and the Rossi X-ray Timing Explorer (RXTE). The observation was done during the decay of the 2009 outburst, when XTE J1652-453 was in the hard-intermediate state. The spectrum shows a strong and broad iron emission line with an equivalent width of ~ 450 eV. The profile is consistent with that of a line being produced by reflection off the accretion disk, broadened by relativistic effects close to the black hole. The best-fitting inner radius of the accretion disk is ~ 4 gravitational radii. Assuming that the accretion disk is truncated at the radius of the innermost stable circular orbit, the black hole in XTE J1652-453 has a spin parameter of ~ 0.5. The power spectrum of the RXTE observation has an additional variability component above 50 Hz, which is typical for the hard-intermediate state. No coherent quasi-periodic oscillations at low frequency are apparent in the power spectrum, which may imply that we view the system at a rather low inclination angle.

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arXiv2010

The distance and internal composition of the neutron star in EXO 0748-676 with XMM-Newton

Recently, the neutron star X-ray binary EXO 0748-676 underwent a transition to quiescence. We analyzed an XMM-Newton observation of this source in quiescence, where we fitted the spectrum with two different neutron-star atmosphere models. From the fits we constrained the allowed parameter space in the mass-radius diagram for this source for an assumed range of distances to the system. Comparing the results with different neutron-star equations of state, we constrained the distance to EXO 0748-676. We found that the EOS model 'SQM1' is rejected by the atmosphere model fits for the known distance, and the 'AP3' and 'MS1' is fully consistent with the known distance.

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arXiv2010

XTE J1701-462 and its Implications for the Nature of Subclasses in Low-Magnetic-Field Neutron Star Low-Mass X-Ray Binaries

We report on an analysis of RXTE data of the transient neutron star low-mass X-ray binary (NS-LMXB) XTE J1701-462, obtained during its 2006-2007 outburst. The X-ray properties of the source changed between those of various types of NS-LMXB subclasses. At high luminosities the source switched between two types of Z source behavior and at low luminosities we observed a transition from Z source to atoll source behavior. These transitions between subclasses primarily manifest themselves as changes in the shapes of the tracks in X-ray color-color and hardness-intensity diagrams, but they are accompanied by changes in the kHz quasi-periodic oscillations, broad-band variability, burst behavior, and/or X-ray spectra. We find that the low-energy X-ray flux is a good parameter to track the gradual evolution of the tracks in color-color and hardness-intensity diagrams, allowing us to resolve the evolution of the source in greater detail than before and relate the observed properties to other NS-LMXBs. We further find that during the transition from Z to atoll, characteristic behavior known as the atoll upper banana can equivalently be described as the final stage of a weakening Z source flaring branch, thereby blurring the line between the two subclasses. Our findings strongly suggest that the wide variety in behavior observed in NS-LXMBs with different luminosities can be linked through changes in a single variable parameter, namely the mass accretion rate, without the need for additional differences in the neutron star parameters or viewing angle. We briefly discuss the implications of our findings for the spectral changes observed in NS LMXBs and suggest that, contrary to what is often assumed, the position along the color-color tracks of Z sources is not determined by the instantaneous mass accretion rate.

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arXiv2010

Rapid Cooling of the Neutron Star in the Quiescent Super-Eddington Transient XTE J1701-462

We present Rossi X-Ray Timing Explorer and Swift observations made during the final three weeks of the 2006-2007 outburst of the super-Eddington neutron star (NS) transient XTE J1701-462, as well as Chandra and XMM-Newton observations covering the first ~800 days of the subsequent quiescent phase. The source transitioned quickly from active accretion to quiescence, with the luminosity dropping by over 3 orders of magnitude in ~13 days. The spectra obtained during quiescence exhibit both a thermal component, presumed to originate in emission from the NS surface, and a non-thermal component of uncertain origin, which has shown large and irregular variability. We interpret the observed decay of the inferred effective surface temperature of the NS in quiescence as the cooling of the NS crust after having been heated and brought out of thermal equilibrium with the core during the outburst. The interpretation of the data is complicated by an apparent temporary increase in temperature ~220 days into quiescence, possibly due to an additional spurt of accretion. We derive an exponential decay timescale of ~120 (+30/-20) days for the inferred temperature (excluding observations affected by the temporary increase). This short timescale indicates a highly conductive NS crust. Further observations are needed to confirm whether the crust is still slowly cooling or has already reached thermal equilibrium with the core at a surface temperature of ~125 eV. The latter would imply a high equilibrium bolometric thermal luminosity of ~5x10^{33} erg/s for an assumed distance of 8.8 kpc.

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arXiv2009

A very rare triple-peaked type-I X-ray burst in the low-mass X-ray binary 4U 1636-53

We have discovered a triple-peaked X-ray burst from the low-mass X-ray binary (LMXB) 4U 1636-53 with the Rossi X-ray Timing Explorer (RXTE). This is the first triple-peaked burst reported from any LMXB using RXTE, and it is only the second burst of this kind observed from any source. (The previous one was also from 4U 1636-53, and was observed with EXOSAT.) From fits to time-resolved spectra, we find that this is not a radius-expansion burst, and that the same triple-peaked pattern seen in the X-ray light curve is also present in the bolometric light curve of the burst. Similar to what was previously observed in double-peaked bursts from this source, the radius of the emitting area increases steadily during the burst, with short periods in between during which the radius remains more or less constant. The temperature first increases steeply, and then decreases across the burst also showing three peaks. The first and last peak in the temperature profile occur, respectively, significantly before and significantly after the first and last peaks in the X-ray and bolometric light curves. We found no significant oscillations during this burst. This triple-peaked burst, as well as the one observed with EXOSAT and the double-peak bursts in this source, all took place when 4U 1636-53 occupied a relatively narrow region in the colour-colour diagram, corresponding to a relatively high (inferred) mass-accretion rate. No model presently available is able to explain the multiple-peaked bursts.

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arXiv2009

Discovery of a broad iron line in the black-hole candidate Swift J1753.5-0127, and the disk emission in the low/hard state revisited

We analyzed simultaneous archival XMM-Newton and RXTE observations of the X-ray binary and black hole candidate Swift J1753.5-0127. In a previous analysis of the same data a soft thermal component was found in the X-ray spectrum, and the presence of an accretion disk extending close to the innermost stable circular orbit was proposed. This is in contrast with the standard picture in which the accretion disk is truncated at large radii in the low/hard state. We tested a number of spectral models and we found that several of them fit the observed spectra without the need of a soft disk-like component. This result implies that the classical paradigm of a truncated accretion disk in the low/hard state can not be ruled out by these data. We further discovered a broad iron emission line between 6 and 7 keV in these data. From fits to the line profile we found an inner disk radius that ranges between ~6-16 gravitational radii, which can be in fact much larger, up to ~250 gravitational radii, depending on the model used to fit the continuum and the line. We discuss the implications of these results in the context of a fully or partially truncated accretion disk.

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arXiv2007

Non-Detection of Gravitationally Redshifted Absorption Lines in the X-ray Burst Spectra of GS 1826-24

During a 200 ks observation with the XMM-Newton Reflection Grating Spectrometer, we detected 16 type-I X-ray bursts from GS 1826-24. We combined the burst spectra in an attempt to measure the gravitational redshifts from the surface of the neutron star. We divided the composite GS 1826-24 burst spectrum into three groups based on the blackbody temperature during the bursts. The spectra do not show any obvious discrete absorption lines. We compare our observations with those of EXO 0748-676.

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arXiv2007

Is there a link between the neutron-star spin and the frequency of the kilohertz quasi-periodic oscillations?

There is a general consensus that the frequencies of the kilohertz Quasi-Periodic Oscillations (kHz QPOs) in neutron-star low-mass X-ray binaries are directly linked to the spin of the neutron star. The root of this idea is the apparent clustering of the ratio of the frequency difference of the kHz QPOs and the neutron-star spin frequency, $Δν/ν_s$, at around 0.5 and 1 in ten systems for which these two quantities have been measured. Here we reexamine all available data of sources for which there exist measurements of two simultaneous kHz QPOs and spin frequencies, and we advance the possibility that $Δν$ and $ν_s$ are not related to each other. We discuss ways in which this possibility could be tested with current and future observations.

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arXiv2007

Extended Air Shower Simulations Based on EPOS

We discuss air shower simulations based on the EPOS hadronic interaction model. A remarkable feature is the fact that the number of produced muons is considerably larger compared to other interaction models. We show that this is due to an improved treatment of baryon-antibaryon production.

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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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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

On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries

I study the behaviour of the maximum rms fractional amplitude, $r_{\rm max}$ and the maximum coherence, $Q_{\rm max}$, of the kilohertz quasi-periodic oscillations (kHz QPOs) in a dozen low-mass X-ray binaries. I find that: (i) The maximum rms amplitudes of the lower and the upper kHz QPO, $r^{\ell}_{\rm max}$ and $r^{\rm u}_{\rm max}$, respectively, decrease more or less exponentially with increasing luminosity of the source; (ii) the maximum coherence of the lower kHz QPO, $Q^{\ell}_{\rm max}$, first increases and then decreases exponentially with luminosity, at a faster rate than both $r^{\ell}_{\rm max}$ and $r^{\rm u}_{\rm max}$; (iii) the maximum coherence of the upper kHz QPO, $Q^{\rm u}_{\rm max}$, is more or less independent of luminosity; and (iv) $r_{\rm max}$ and $Q_{\rm max}$ show the opposite behaviour with hardness of the source, consistent with the fact that there is a general anticorrelation between luminosity and spectral hardness in these sources. Both $r_{\rm max}$ and $Q_{\rm max}$ in the sample of sources, and the rms amplitude and coherence of the kHz QPOs in individual sources show a similar behaviour with hardness. This similarity argues against the interpretation that the drop of coherence and rms amplitude of the lower kHz QPO at high QPO frequencies in individual sources is a signature of the innermost stable circular orbit around a neutron star. I discuss possible interpretations of these results in terms of the modulation mechanisms that may be responsible for the observed variability.

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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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arXiv2004

The distribution of kHz QPO frequencies in Sco X-1

The frequencies of kHz Quasi-Periodic Oscillations in the bright Low-Mass X-ray Binary Sco X-1 have been reported to appear preferentially in a 3:2 ratio. We show that a proper statistical analysis of those RXTE data upon which the claim is based indicated a 2.4 sigma deviation from a constant distribution in ratios. The analysis of a much larger RXTE/PCA dataset shows that there is no sharp concentration around a 3:2 ratio, but that the ratios are broadly distributed over the range 820-1150 Hz.

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arXiv2004

The continued spectral evolution of the neutron star RX J0720.4-3125

We observed the isolated neutron star RX J720.4-3125 with Chandra's Low Energy Transmission Grating Spectrometer, following the XMM-Newton discovery of long term spectral evolution of this source. The new observation shows that the spectrum of RX J720.4-3125 has continued to change in the course of 5 months. It has remained hard, similar to the last XMM-Newton observation, but the strong depression observed with XMM-Newton at long wavelengths has disappeared. Contrary to the XMM-Newton observations, the new Chandra observation shows that the flux increase at short wavelengths and the decrease at long wavelengths do not necessarily occur simultaneously.

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