arXiv subjects

Mariano Mendez

Publications and source records attributed to Mariano Mendez.

At least 73 records · Page 4Linked to original sources

arXiv2004

Long-term variability in the X-ray emission of RX J0720.4-3125

We detect a gradual, long-term change in the shape of the X-ray spectrum of the isolated neutron star RX J0720.4-3125, such that the spectrum of the source can no longer be described as a blackbody spectrum. The change is accompanied by an energy-dependent change in the pulse profile. If the X-ray emission is influenced by the magnetic field of the pulsar, these changes in spectral shape may point to precession of the neutron star.

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arXiv2003

A transient variable 6 Hz QPO from GX 339-4

We report the results of an observation with the Rossi X-ray Timing Explorer of the black hole candidate GX 339-4 during its 2002/2003 outburst. This observation took place during a spectral transition from the hard to the soft state. A strong (6% rms) transient quasi-periodic oscillation (QPO) appears suddenly in the power density spectrum during this observation. The QPO centroid is ~6 Hz, but it varies significantly between 5 and 7 Hz with a characteristic time scale of ~10 seconds, correlated with the 2-30 keV count rate. The appearance of the QPO is related to spectral hardening of the flux, due to a change in the relative contribution of the soft and hard spectral components. We compare this peculiar behavior with results from other systems that show similar low frequency QPO peaks, and discuss the results in terms of possible theoretical models for QPO production.

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arXiv2003

Chandra Observations of the Faintest Low-Mass X-ray Binaries

There exists a group of persistently faint galactic X-ray sources that, based on their location in the galaxy, high L_x/L_opt, association with X-ray bursts, and absence of low frequency X-ray pulsations, are thought to be low-mass X-ray binaries (LMXBs). We present results from Chandra observations for eight of these systems: 4U 1708-408, 2S 1711-339, KS 1739-304, SLX 1735-269, GRS 1736-297, SLX 1746-331, 1E 1746.7-3224, and 4U 1812-12. Locations for all sources, excluding GRS 1736-297, SLX 1746-331, and KS 1739-304 (which were not detected) were improved to 0.6" error circles (90% confidence). Our observations support earlier findings of transient behavior of GRS 1736-297, KS 1739-304, SLX 1746-331, and 2S 1711-339 (which we detect in one of two observations). Energy spectra for 4U 1708-408, 2S 1711-339, SLX 1735-269, 1E 1746.7-3224, and 4U 1812-12 are hard, with power law indices typically 1.4-2.1, which are consistent with typical faint LMXB spectra.

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arXiv2003

The atoll source states of 4U 1608-52

We have studied the atoll source 4U 1608-52 using a large data set obtained with the Rossi X-ray Timing Explorer. We find that the timing properties of 4U 1608-52 are almost exactly identical to those of the atoll sources 4U 0614+09 and 4U 1728-34 despite the fact that contrary to these sources 4U 1608-52 is a transient covering two orders of magnitude in luminosity. The frequencies of the variability components of these three sources follow a universal scheme when plotted versus the frequency of the upper kilohertz QPO, suggesting a very similar accretion flow configuration. If we plot the Z sources on this scheme only the lower kilohertz QPO and HBO follow identical relations. Using the mutual relations between the frequencies of the variability components we tested several models; the transition layer model, the sonic point beat frequency model, and the relativistic precession model. None of these models described the data satisfactory. Recently, it has been suggested that the atoll sources (among them 4U 1608-52) trace out similar three-branch patterns as the Z sources in the color-color diagram. We have studied the relation between the power spectral properties and the position of 4U 1608-52 in the color-color diagram and conclude that the timing behavior is not consistent with the idea that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram along which the timing properties vary gradually, as Z sources do.

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arXiv2002

Iron Emission Lines from Extended X-ray Jets in SS 433: Reheating of Atomic Nuclei

Powerful relativistic jets are among the most ubiquitous and energetic observational consequences of accretion around supermassive black holes in active galactic nuclei and neutron stars and stellar-mass black holes in x-ray binary (XRB) systems. But despite more than three decades of study, the structure and composition of these jets remain unknown. Here we present spatially resolved x-ray spectroscopy of arc second-scale x-ray jets from the XRB SS 433 analized with the Chandra advanced charge-coupled device imaging spectrometer. These observations reveal evidence for a hot continuum and Doppler-shifted iron emission lines from spatially resolved regions. Apparently, in situ reheating of the baryonic component of the jets takes place in a flow that moves with relativistic bulk velocity even more than 100 days after launch from the binary core.

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arXiv2002

Fast X-ray Oscillations and General Relativity effects in Neutron star systems

Kilohertz quasi-periodic oscillations (kHz QPOs) are probably caused by matter in Keplerian orbit at some preferred radius in the accretion disc around a compact star. In a given source, QPO frequencies can drift by a few hundred Hz following changes of the inner disc radius; but the disc cannot move closer to the star than the radius of the innermost stable circular orbit (ISCO) predicted by general realtivity, hence the kHz QPO frequencies must be limited by some maximum frequency.

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arXiv2002

First observation of a transition between "parallel tracks" in the kHz QPO frequency vs. intensity diagram

Contrary to theoretical expectations, observations with the Rossi X-ray Timing Explorer (RXTE) show that in X-ray binaries timing properties are not uniquely correlated with X-ray luminosity. For instance, although the frequencies of the kilohertz quasi-periodic oscillations (kHz QPOs) correlate with X-ray flux on short (~few hours) time scales, on time scales longer than a day the QPO appears at more or less the same frequency, whereas the luminosity may be a factor of a few different. The result is a set of almost parallel tracks in a QPO frequency vs. X-ray flux plot. Despite the "parallel tracks" are a common phenomenon among kHz QPO sources, until now, after five years of observations with RXTE, not a single transition between two of these tracks had been seen. Here I present the first detection of such a transition, in 4U 1636-53.

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arXiv2002

Kilohertz quasi-periodic oscillations and strong field gravity in X-ray binaries

In the past five years observations with the Rossi X-ray Timing Explorer have revealed fast quasi-periodic oscillations in the X-ray flux of about 20 X-ray binaries. Thought to originate close to the surface of a neutron star, these oscillations provide unique information about the strong gravitational field in which they are produced.

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arXiv2002

The XEUS Mission

XEUS, the X-ray Evolving Universe Spectroscopy mission, is at present an ESA-ISAS initiative for the study of the evolution of the hot Universe in the post-Chandra/XMM-Newton era. The key science objectives of XEUS are: Search for the origin, and subsequent study of growth, of the first massive black holes in the early Universe; assessment of the formation of the first gravitationally bound dark matter dominated systems and their evolution; study of the evolution of metal synthesis up till the present epoch; characterization of the true intergalactic medium. To reach these ambitious science goals the two salient characteristics of the XEUS observatory entail: (1) Its effective spectroscopic grasp, combining a sensitive area > 20 m^2 below 2 keV with a spectral resolution better than 2 eV. This allows significant detection of the most prominent X-ray emission lines (e.g. O-VII, Si-XIII and Fe-XXV) in cosmologically distant sources against the sky background; (2) Its angular resolving power, between 2 and 5 arc seconds, to minimize source confusion as well as noise due to the galactic X-ray foreground emission. To accommodate these instrument requirements a mission concept has been developed featuring an X-ray telescope of 50-m focal length, comprising two laser-locked (separate) mirror and detector spacecraft's. The telescope is injected in a low earth orbit with an inclination commensurate with the ISS. At present an on-orbit growth of the mirror spacecraft is foreseen with the aid of the ISS, raising the mirror diameter from 4.5 to 10 m. The detector spacecraft will be replaced at 5 year intervals after run-out of consumables with an associated upgrade of the focal plane package.

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arXiv2002

KiloHertz quasi-periodic oscillations difference frequency exceeds inferred spin frequency in 4U 1636-53

Recent observations of the low-mass X-ray binary 4U 1636-53 with the Rossi X-ray Timing Explorer show, for the first time, a kiloHertz quasi-periodic oscillation (kHz QPO) peak separation that exceeds the neutron star spin frequency as inferred from burst oscillations. This strongly challenges the sonic-point beat frequency model for the kHz QPOs found in low-mass X-ray binaries. We detect two simultaneous kHz QPOs with a frequency separation of 323.3+-4.3 Hz in an average Fourier power spectrum of observations obtained in September 2001 and January 2002. The lower kHz QPO frequency varied between 644 Hz and 769 Hz. In previous observations of this source the peak separation frequency was ~250 Hz when the lower kHz QPO frequency was ~900 Hz. Burst oscillations occur in 4U 1636-53 at ~581 Hz and possibly at half that frequency (290.5 Hz). This is the first source where the peak separation frequency is observed to change from less than (half) the burst oscillation frequency to more than that. This observation contradicts all previously formulated implementations of the sonic-point beat frequency model except those where the disk in 4U 1636-53 switches from prograde to retrograde.

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arXiv2002

XMM-Newton observations of the neutron star X-ray transient KS 1731-260 in quiescence

We report on XMM-Newton observations performed on 2001 September 13-14 of the neutron star X-ray transient KS 1731-260 in quiescence. The source was detected at an unabsorbed 0.5-10 keV flux of only 4 - 8 x 10^{-14} erg/s, depending on the model used to fit the data, which for a distance of 7 kpc implies a 0.5-10 keV X-ray luminosity of approximately 2 - 5 x 10^{32} erg/s. The September 2001 quiescent flux of KS 1731-260 is lower than that observed during the Chandra observation in March 2001. In the cooling neutron star model for the quiescent X-ray emission of neutron star X-ray transients, this decrease in the quiescent flux implies that the crust of the neutron star in KS 1731-260 cooled down rapidly between the two epochs, indicating that the crust has a high conductivity. Furthermore, enhanced cooling in the neutron star core is also favored by our results.

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arXiv2002

A Polarimetric Survey for Dust in 47 Tucanae (NGC 104)

We present linear polarization in the V band for 77 stars in the field of the globular cluster 47 Tucanae (NGC 104), and for 14 bright-star-free regions, located along an elliptical isophotal contour of the cluster, as well as UBVRI measurements for the cluster nucleus. The observations show variable foreground polarization that, once removed, leaves marginally significant polarization residuals for the non-variable bright red giants. Although these residuals are small there is, however, a systematic trend in the sense that the larger ones are seen towards the south of the cluster (in a direction opposite to that of the cluster proper motion). In contrast, most of the variable stars do show significant intrinsic polarization. The behavior of the star-free regions is similar to that of the non-variable stars and sets an upper limit to the possible existence of a global pattern of scattered (and polarized) intra-cluster light in the V band. In turn, the multicolor observations of the cluster nucleus cannot be fitted with a Serkowski law and exhibit a polarization excess both in U and B. This polarization could be explained as a combination of the foreground interstellar component and another one arising in dust located in the nucleus and illuminated by a bright blue post-AGB star (at 48" from the cluster center). An inspection of a set of archive HST WFPC2 images reveals the presence of a number of dark patches in the innermost regions of the cluster. A prominent patch (some 5" by 3" in size) located at 12" from the cluster center and with a position angle (N to E) of 120 degrees, has a slightly different polarization, compared to that of the cluster nucleus, and appears as a good candidate to be identified as a dust globule within the cluster.

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arXiv2001

RXTE observations of the neutron star low-mass X-ray binary GX 17+2: correlated X-ray spectral and timing behavior

We have analyzed ~600 ks of Rossi X-ray Timing Explorer data of the neutron star low-mass X-ray binary and Z source GX 17+2. A study was performed of the properties of the noise components and quasi-periodic oscillations (QPOs) as a function of the spectral properties, with the main goal to study the relation between the frequencies of the horizontal branch and upper kHz QPOs. It was found that when the upper kHz QPO frequency is below 1030 Hz these frequencies correlate, whereas above 1030 Hz they anti-correlate. GX 17+2 is the first source in which this is observed. We also found that the frequency difference of the high frequency QPOs was not constant and that the quality factors (Q values) of the HBO, its second harmonic, and the kHz QPOs are similar, and vary almost hand in hand by a factor of more than three. Observations of the normal branch oscillations during two type I X-ray bursts showed that their absolute amplitude decreased as the flux from the neutron star became stronger. We discuss these and other findings in terms of models that have been proposed for these phenomena. We also compare the behavior of GX 17+2 and other Z sources with that of black hole sources and consider the possibility that the mass accretion rate might not be driving force behind all spectral and variability changes.

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arXiv2001

A detailed study of the 5 Hz quasi-periodic oscillations in the bright X-ray transient and black-hole candidate GRS 1739-278

We present a detailed study of the 5 Hz quasi-periodic oscillation (QPO) recently discovered in the bright X-ray transient and black-hole candidate GRS 1739-278 (Borozdin & Trudolyubov 2000) during a Rossi X-ray Timing Explorer observation taken on 1996 March 31. In total 6.6 ksec of on-source data were obtained, divided in two data sets of 3.4 and 3.2 ksec which were separated by 2.6 ksec. The 5 Hz QPO was only present during the second data set. The QPO increased in strength from below 2% rms amplitude for photon energies below 4 keV to ~5% rms amplitude for energies above 10 keV. The soft QPO photons (below 5 keV) lagged the hard ones (above 10 keV) by almost 1.5 radian. Besides the QPO fundamental, its first overtone was detected. The strength of the overtone increased with photon energy (from <2% rms below 5 keV to ~8% rms above 10 keV). Although the limited statistics did not allow for an accurate determination of the lags of the first overtone, indications are that also for this QPO the soft photons lagged the hard ones. When the 5 Hz QPO was not detected (i.e., during the first part of the observation), a broad noise component was found for photon energies below 10 keV but it became almost a true QPO (with a Q value of ~1.9) above that energy, with a frequency of ~3 Hz. Its hard photons preceded the soft ones in a way reminiscent of the 5 Hz QPO, strongly suggesting that both features are physically related. We discuss our finding in the frame work of low-frequency QPOs and their properties in BHCs.

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arXiv2001

The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity

We study the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces out a set of nearly parallel lines in a frequency vs. X-ray count rate diagram. We find that between two of these tracks, for similar QPO frequency, the source count rate can differ by up to a factor of ~ 4, whereas at the same time the rms amplitude of the kHz QPOs is only a factor of ~ 1.1 different. We also find that, for 4U 1608-52 and Aql X-1, the rms spectrum of the kHz QPOs does not depend upon which track the source occupies in the frequency vs. X-ray count rate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 are inconsistent with scenarios in which the properties of the kHz QPOs are only determined by the mass accretion rate through the disk, whereas X-ray count rate also depends upon other sources of energy that do not affect the QPOs.

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arXiv2001

The erratic luminosity behavior of SAX J1808.4-3658 during its 2000 outburst

We report on the highly variable and erratic long-term X-ray luminosity behavior of the only known accretion-driven millisecond X-ray pulsar SAX J1808.4-3658 during its 2000 outburst, as observed with RXTE. The maximum observed luminosity is ~2.5 x 10^35 erg s^-1 (3-25 keV; for a distance of 2.5 kpc), which is a factor of ~10 lower than that observed during the 1996 and 1998 outbursts. Due to solar constraints, the source could not be observed for several months with RXTE before 21 Jan. 2000. Therefore, the exact moment of the outburst onset is unknown and the peak luminosity could have been significantly higher. On some occasions SAX J1808.4-3658 was observed with luminosities of ~10^35 erg s^-1, but on other occasions it could not be detected with RXTE resulting in upper limits of a few times 10^33 erg s^-1 (3-25 keV). The non-detections of the source during its 2000 outburst obtained with BeppoSAX demonstrate that its luminosity was at times <10^32 erg s^-1 (0.5-10 keV). However, only a few days after these BeppoSAX observations, we detected the source again with RXTE at high luminosities, giving a factor of >1000 of luminosity swings in this system on time scales of days. The last detection of SAX J1808.4-3658 with RXTE was on 2000 May 13, almost 4 months after the first detection. Due to the lack of sensitivity and observations during the 1996 and 1998 outbursts, it cannot be excluded that after those outbursts the source remained active for months and that the source behavior during the 2000 outburst is not unique. Long duration activity at low luminosities has been observed in other transients (both neutron stars and black holes), although not with such extreme variability which might point to a different origin for this behavior for the pulsar.

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arXiv2001

The High-Frequency QPOs in GRS 1915+105

We analyzed two 1996 observations of the Galactic Microquasar GRS 1915+105 taken with the Proportional Counter Array on board the Rossi X-ray Timing Explorer. We focus on the properties of the high-frequency QPO as a function of the `dip' oscillations, a factor of 1000 slower. In one observation, 1996 May 5th, we find that the energy spectrum of the QPO at ~65 Hz changes drastically between the high-intervals and the dips, hardening during the dips to the point that the QPO peak is no longer detected at low energies. In the second observation, 1996 May 14th, although it has similar overall characteristics, the 65 Hz QPO is not seen, but another broader QPO peak appears at 27 Hz, only during the dips. This peak is too weak to obtain reliable information about its spectrum. Our results indicate that the presence/absence of high-frequency features in this enigmatic source are intimately linked to the slower oscillations and variations that happen on longer time scales.

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arXiv2001

Discovery of a kHz QPO in 2S 0918-549

We report the discovery of a kilohertz quasi-periodic oscillation (kHz QPO) in the low-mass X-ray binary (LMXB) 2S 0918-549. The kHz QPO has a frequency of 1156+-9 Hz, a FWHM of 70+-20 Hz, and a fractional rms amplitude of 18%+-2%. We also detected for the first time a type I X-ray burst in this source. The compact object must therefore be a neutron star. Finally, we were able to classify the source as an atoll source exhibiting the full complement of spectral/timing states of this class (island and banana). From the peak burst flux an upper limit on the persistent luminosity can be derived of 0.5% of the Eddington luminosity, making 2S 0918-549 one of the least luminous LMXBs showing a kHz QPO. We compare the fractional rms amplitudes of the upper kHz QPO across the ensemble of LMXBs. We find a strong anticorrelation with luminosity. In LMXBs with luminosities ~100 times lower than those of Z-sources, the fractional rms amplitude is a factor ~10 larger.

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