arXiv subjects

D. Altamirano

Publications and source records attributed to D. Altamirano.

139 records · Page 8Linked to original sources

arXiv2010

Discovery of a 205.89 Hz accreting-millisecond X-ray pulsar in the globular cluster NGC 6440

We report the discovery of the second accreting millisecond X-ray pulsar (AMXP) in the globular cluster NGC 6440. Pulsations with a frequency of 205.89 Hz were detected with the Rossi X-Ray Timing Explorer on August 30th, October 1st and October 28th, 2009, during the decays of ~4 day outbursts of a newly X-ray transient source in NGC 6440. By studying the Doppler shift of the pulsation frequency, we find that the system is an ultra-compact binary with an orbital period of 57.3 minutes and a projected semi-major axis of 6.22 light-milliseconds. Based on the mass function, we estimate a lower limit to the mass of the companion to be 0.0067 M_sun (assuming a 1.4 M_sun neutron star). This new pulsar shows the shortest outburst recurrence time among AMXPs (~1 month). If this behavior does not cease, this AMXP has the potential to be one of the best sources in which to study how the binary system and the neutron star spin evolve. Furthermore, the characteristics of this new source indicate that there might exist a population of AMXPs undergoing weak outbursts which are undetected by current all-sky X-ray monitors. NGC 6440 is the only globular cluster to host two known AMXPs, while no AMXPs have been detected in any other globular cluster.

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arXiv2010

Fast infrared variability from a relativistic jet in GX 339-4

We present the discovery of fast infrared/X-ray correlated variability in the black-hole transient GX 339-4. The source was observed with sub-second time resolution simultaneously with VLT/ISAAC and RXTE/PCA in August 2008, during its persistent low-flux highly variable hard state. The data show a strong correlated variability, with the infrared emission lagging the X-ray emission by 100 ms. The short time delay and the nearly symmetric cross-correlation function, together with the measured brightness temperature of ~2.5 x 10^6 K, indicate that the bright and highly variable infrared emission most likely comes from a jet near the black hole. Under standard assumptions about jet physics, the measured time delay can provide us a lower limit of Gamma > 2 for the Lorentz factor of the jet. This suggests that jets from stellar-mass black holes are at least mildly relativistic near their launching region. We discuss implications for future applications of this technique.

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arXiv2009

Type I X-ray bursts, burst oscillations and kHz quasi-periodic oscillations in the neutron star system IGR J17191-2821

We present a detailed study of the X-ray energy and power spectral properties of the neutron star transient IGR J17191-2821. We discovered four instances of pairs of simultaneous kilohertz quasi-periodic oscillations (kHz QPOs). The frequency difference between these kHz QPOs is between 315 Hz and 362 Hz. We also report on the detection of five thermonuclear type-I X-ray bursts and the discovery of burst oscillations at ~294 Hz during three of them. Finally, we report on a faint and short outburst precursor, which occurred about two months before the main outburst. Our results on the broadband spectral and variability properties allow us to firmly establish the atoll source nature of IGR J17191-2821.

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arXiv2009

Broad relativistic iron emission line observed in SAX J1808.4-3658

During the September-October 2008 outburst of the accreting millisecond pulsar SAX J1808.4-3658, the source was observed by both Suzaku and XMM-Newton approximately 1 day apart. Spectral analysis reveals a broad relativistic Fe K-alpha emission line which is present in both data-sets, as has recently been reported for other neutron star low-mass X-ray binaries. The properties of the Fe K line observed during each observation are very similar. From modeling the Fe line, we determine the inner accretion disk radius to be 13.2 +/- 2.5 GM/c^2. The inner disk radius measured from the Fe K line suggests that the accretion disk is not very receded in the island state. If the inner disk (as measured by the Fe line) is truncated at the magnetospheric radius this implies a magnetic field strength of ~3E8 G at the magnetic poles, consistent with other independent estimates.

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arXiv2008

Intermittent accreting millisecond pulsars: light houses with broken lamps?

Intermittent accreting millisecond X-ray pulsars are an exciting new type of sources. Their pulsations appear and disappear either on timescales of hundreds of seconds or on timescales of days. The study of these sources add new observational constraints to present models that explain the presence or not of pulsations in neutron star LMXBs. In this paper we present preliminary results on spectral and aperiodic variability studies of all intermittent AMSPs, with a particular focus on the comparison between pulsating and non pulsating periods.

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arXiv2008

A parsec scale X-ray extended structure from the X-ray binary Circinus X-1

We present the results of the analysis of two Chandra observations of Circinus X-1 performed in 2007, for a total exposure time of ~50 ks. The source was observed with the High Resolution Camera during a long X-ray low-flux state of the source. Cir X-1 is an accreting neutron-star binary system that exhibits ultra-relativistic arcsec-scale radio jets and an extended arcmin-scale radio nebula. Furthermore, a recent paper has shown an X-ray excess on arcmin-scale prominent on the side of the receding radio jet. In our images we clearly detect X-ray structures both on the side of the receding and the approaching radio jet. The X-ray emission is consistent with being from synchrotron origin. Our detection is consistent with neutron-star binaries being as efficient as black-hole binaries in producing X-ray outflows, despite their shallower gravitational potential.

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arXiv2008

Discovery of kilohertz quasi-periodic oscillations and state transitions in the low-mass X-ray binary 1E~1724-3045 (Terzan 2)

We have studied the rapid X-ray time variability in 99 pointed observations with the Rossi X-ray Timing Explorer (RXTE)'s Proportional Counter Array of the low-mass X-ray binary 1E~1724--3045 which includes, for the first time, observations of this source in its island and banana states, confirming the atoll nature of this source. We report the discovery of kilohertz quasi-periodic oscillations (kHz QPOs). Although we have 5 detections of the lower kHz QPO and one detection of the upper kHz QPO, in none of the observations we detect both QPOs simultaneously. By comparing the dependence of the rms amplitude with energy of kHz QPOs in different atoll sources, we conclude that this information cannot be use to unambiguously identify the kilohertz QPOs as was previously thought. We find that Terzan~2 in its different states shows timing behavior similar to that seen in other neutron-star low mass X-ray binaries (LMXBs). We studied the flux transitions observed between February 2004 and October 2005 and conclude that they are due to changes in the accretion rate.

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arXiv2008

X-ray time variability across the atoll source states of 4U 1636--53

We have studied the rapid X-ray time variability in 149 pointed observations with the \textit{Rossi X-ray Timing Explorer} (RXTE)'s Proportional Counter Array of the atoll source 4U~1636--53 in the banana state and, for the first time with RXTE, in the island state. We compare the frequencies of the variability components of 4U~1636--53 with those in other atoll and Z-sources and find that 4U~1636--53 follows the universal scheme of correlations previously found for other atoll sources at (sometimes much) lower luminosities. Our results on the hectohertz QPO suggest that the mechanism that sets its frequency differs from that for the other components, while the amplitude setting mechanism is common. A previously proposed interpretation of the narrow low-frequency QPO frequencies in different sources in terms of harmonic mode switching is not supported by our data, nor by some previous data on other sources and the frequency range that this QPO covers is found not to be related to spin, angular momentum or luminosity.

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arXiv2008

Intermittent millisecond X-ray pulsations from the neutron-star X-ray transient SAX J1748.9-2021 in the globular cluster NGC 6440

We report on intermittent X-ray pulsations with a frequency of 442.36 Hz from the neutron-star X-ray binary SAX J1748.9--2021 in the globular cluster NGC 6440. The pulsations were seen during both 2001 and 2005 outbursts of the source, but only intermittently, appearing and disappearing on timescales of hundreds of seconds. We find a suggestive relation between the occurrence of type-I X-ray bursts and the appearance of the pulsations but the relation is not strict. This behavior is very similar to that of the intermittent accreting millisecond X-ray pulsar HETE J1900.1--2455. The reason for the intermittence of the pulsations remains unclear. However it is now evident that a strict division between pulsating and non-pulsating does not exist. By studying the Doppler shift of the pulsation frequency we determine an orbit with a period of 8.7 hrs and an projected semi major axis of 0.39 lightsec. The companion star might be a main--sequence or a slightly evolved star with a mass of $\sim$1 $M_\odot$. Therefore, SAX J1748.9--2021 has a longer period and may have a more massive companion star than all the other accreting millisecond X-ray pulsars except for Aql X-1.

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arXiv2007

Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I X-ray Bursts

Millihertz quasi-periodic oscillations reported in three neutron-star low mass X-ray binaries have been suggested to be a mode of marginally stable nuclear burning on the neutron star surface. In this Letter, we show that close to the transition between the island and the banana state, 4U~1636--53 shows mHz QPOs whose frequency systematically decreases with time until the oscillations disappear and a Type I X-ray burst occurs. There is a strong correlation between the QPO frequency $ν$ and the occurrence of X-ray bursts: when $ν\gtrsim9$ mHz no bursts occur, while $ν\lesssim9$ mHz does allow the occurrence of bursts. The mHz QPO frequency constitutes the first identified observable that can be used to predict the occurrence of X-ray bursts. If a systematic frequency drift occurs, then a burst happens within a few kilo-seconds after $ν$ drops below 9 mHz. This observational result confirms that the mHz QPO phenomenon is intimately related with the processes that lead to a thermonuclear burst.

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arXiv2007

Discovery of coherent millisecond X-ray pulsations in Aql X-1

We report the discovery of an episode of coherent millisecond X-ray pulsation in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for slightly more than 150 seconds, during which the pulse frequency is consistent with being constant. No X-ray burst or other evidence of thermonuclear burning activity is seen in correspondence with the pulsation, which can thus be identified as occurring in the persistent emission. The pulsation frequency is 550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from burst oscillations in this source. Hence we identify this frequency with the neutron star spin frequency. The pulsed fraction is strongly energy dependent, ranging from <1% (3-5 keV) to >10% (16-30 keV). We discuss possible physical interpretations and their consequences for our understanding of the lack of pulsation in most neutron star low-mass X-ray binaries. If interpreted as accretion-powered pulsation, Aql X-1 might play a key role in understanding the differences between pulsating and non-pulsating sources.

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arXiv2007

kHz QPO pairs expose the neutron star of Circinus X-1

We discovered kHz QPOs in 80 archived RXTE observations from the peculiar low-mass X-ray binary (LMXB) Circinus X-1. In 11 cases these appear in pairs in the frequency range of ~230 Hz to ~500 Hz for the upper kHz QPO and ~56 Hz to ~225 Hz for the lower kHz QPO. Their correlation with each other, which is similar to that of frequencies of kHz QPO pairs in other LMXBs containing a neutron star, and their variation by a factor two confirm that the central object is a neutron star. These are the lowest frequencies of kHz QPO pairs discovered so far and extend the above correlation over a frequency range of factor four. In this new frequency range the frequency difference of the two kHz QPOs increases monotonically by more than ~170 Hz with increasing kHz QPO frequency, challenging theoretical models.

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arXiv2006

Discovery of twin kHz QPOs in the peculiar X-ray binary Circinus X-1

We report the discovery with the RXTE/PCA of twin kHz QPOs in Cir X-1. Eleven cases of simultaneous double QPOs occurred, with significances of up to 6.3 and 5.5 sigma and centroid frequencies ranging between approximately 56-225 and 230-500 Hz for the two QPO peaks, respectively, i.e., for the most part at frequencies well below those of other sources. The QPO properties clearly indicate that these double peaks are the kHz QPOs known from low magnetic field neutron stars, and not black-hole high-frequency QPOs, confirming that Cir X-1 is a neutron star. The kHz QPO peak separation varies over a wide range, \~175-340 Hz, and increases with QPO frequency. This is contrary to what is seen in other sources but agrees with predictions of the relativistic precession model and Alfven wave models; beat-frequency models require modification to accommodate this. In other observations single kHz QPOs can be seen down to frequencies as low as ~12 Hz, as well as a strong low-frequency (LF) QPO between 1 and 30 Hz. The relations between the frequencies of the kHz QPOs and the LF QPO are in good agreement with those found previously in Z sources, confirming that Cir X-1 may be a peculiar Z source. We suggest that the low frequencies of the kHz QPOs in Cir X-1 and to a lesser extent in (other) Z sources might be due to a relatively stronger radial inflow to the neutron star than in other kHz QPO sources.

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