Refereed publications:
- Discovery of burst
oscillations in the intermittent accretion-powered millisecond pulsar
HETE J1900.1-2455
Watts A., Altamirano D., Linares M., Patruno A., Casella P., Cavecchi Y., Degenaar N., Rea N., Soleri P., van der Klis M., Wijnands R.
ApJ-Letters, submitted. (2009)
Abstract: We report the discovery of burst oscillations from the intermittent accretion-powered millisecond pulsar (AMP) HETE J1900.1-2455, with a frequency approximately 1 Hz below the known spin frequency. The burst oscillation properties are far more similar to those of the non-AMPs and Aql X-1 (an intermittent AMP with a far lower duty cycle), than those of the AMPs SAX J1808.4-3658 and XTE J1814-338. We discuss the implications for models of the burst oscillation and intermittency mechanisms.
- SAX J1808.4-3658: high
resolution spectroscopy and decrease of pulsed fraction at low energies
Patruno A., Rea N., Altamirano D., Linares M., Wijnands R., van der Klis M.
MNRAS-Letters, in press. (2009)
Abstract: XMM-Newton observed the accreting millisecond pulsar SAX J1808.4-3658 during its 2008 outburst. We present timing and spectral analyses of this observation, in particular the first pulse profile study below 2 keV, and the high-resolution spectral analysis of this source during the outburst. Combined spectral and pulse profile analyses suggest the presence of a strong unpulsed source below 2 keV that strongly reduces the pulsed fraction and a hard pulsed component that generates markedly double peaked profiles at higher energies. We also studied the high-resolution grating spectrum of SAX J1808.4-3658, and found several absorption edges and Oxygen absorption lines with whom we infer, in a model independent way, the interstellar column densities of several elements in the direction of SAX J1808.4-3658.
- Broad Relativistic Iron
Emission Line Observed in SAX J1808.4–3658
Cackett E. M., Altamirano D., Patruno A., Miller J. M., Reynolds M., Linares M., Wijnands R.
ApJ-Letters, 694, 21. (2009)
Abstract: During the 2008 September-October 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 Kα 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 hard state. If the inner disk (as measured by the Fe line) is truncated at the magnetospheric radius this implies a magnetic field strength of ~3 × 108 G at the magnetic poles, consistent with other independent estimates.
- The Swift capture of a
long X-ray burst from XTE J1701-407
Linares M., Watts A. L., Wijnands R., Soleri P., Degenaar N., Curran P. A., Starling R. L. C., van der Klis M.
MNRAS-Letters, 392, 11. (2009)
Abstract: XTE J1701-407 is a new transient X-ray source discovered on June 8th, 2008. More than one month later it showed a rare type of thermonuclear explosion: a long type I X-ray burst. We report herein the results of our study of the spectral and flux evolution during this burst, as well as the analysis of the outburst in which it took place. We find an upper limit on the distance to the source of 6.1 kpc by considering the maximum luminosity reached by the burst. We measure a total fluence of 3.5*10^{-6} erg/cm^2 throughout the ~20 minutes burst duration and a fluence of 2.6*10^{-3} erg/cm^2 during the first two months of the outburst. We show that the flux decay is best fitted by a power law (index ~1.6) along the tail of the burst. Finally, we discuss the implications of the long burst properties, and the presence of a second and shorter burst detected by Swift ten days later, for the composition of the accreted material and the heating of the burning layer.
- Timing and Spectral
Properties of the Accreting Millisecond Pulsar SWIFT J1756.9-2508
Linares M., Wijnands R., van der Klis M., Krimm H., Markwardt C. B., Chakrabarty D.
ApJ, 677, 1, 515-519. (2008)
Abstract: SWIFT J1756.9-2508 is one of the few accreting millisecond pulsars (AMPs) discovered to date. We report here the results of our analysis of its aperiodic X-ray variability, as measured with the Rossi X-Ray Timing Explorer during the 2007 outburst of the source. We detect strong (~35%) flat-topped broadband noise throughout the outburst with low characteristic frequencies (~0.1 Hz). This makes SWIFT J1756.9-2508 similar to the rest of AMPs and to other low-luminosity accreting neutron stars when they are in their hard states, and enables us to classify this AMP as an atoll source in the extreme island state. We also find a hard tail in its energy spectrum extending up to 100 keV, fully consistent with such source and state classification.
- An Accreting Millisecond
Pulsar with Black Hole-like X-Ray Variability: IGR J00291+5934
Linares M., van der Klis M., Wijnands R.
ApJ, 660,1, 595-604. (2007)
Abstract: IGR J00291+5934 is one of the seven accreting millisecond pulsars (AMPs) discovered so far. We report on the aperiodic timing and color analysis of its X-ray flux, using all the RXTE observations of the 2004 outburst. Flat-top noise and two harmonically related quasi-periodic oscillations, all of them at very low frequencies (0.01-0.1 Hz), were present in the power spectra during most of the outburst as well as a very high fractional variability (~50%). These properties are atypical not only for AMPs but also for neutron star low-mass X-ray binaries (LMXBs) in general. There are instead some remarkable similarities with the variability observed in black hole systems, reinforcing the connections between these two types of LMXBs, as well as some interesting differences. We note finally that the results of this paper are difficult to reconcile with interpretations where any break frequency of power density spectra scales inversely with the mass of the central object at an accuracy sufficient to distinguish between the masses of neutron stars and black holes in LMXBs.
- Cooling of the
quasi-persistent neutron star X-ray transients KS 1731-260 and MXB
1659-29
Cackett E. M., Wijnands R., Linares M., Miller J. M., Homan J., Lewin W. H. G.
MNRAS, 372, 1, 479-488. (2006)
Abstract: We present Chandra and XMM-Newton X-ray observations that monitor the neutron star cooling of the quasi-persistent neutron star X-ray transients KS 1731-260 and MXB 1659-29 for approximately 4 yr after these sources returned to quiescence from prolonged outbursts. In both sources the outbursts were long enough to significantly heat the neutron star crust out of thermal equilibrium with the core. We analyse the X-ray spectra by fitting absorbed neutron star atmosphere models to the observations. The results of our analysis strengthen the preliminary findings of Wijnands et al. that in both sources the neutron star crust cools down very rapidly suggesting it has a high heat conductivity and that the neutron star core requires enhanced core cooling processes. Importantly, we now detect the flattening of the cooling in both sources as the crust returns to thermal equilibrium with the core. We measure the thermal equilbrium flux and temperature in both sources by fitting a curve that decays exponentially to a constant level. The cooling curves cannot be fit with just a simple exponential decay without the constant offset. We find the constant bolometric flux and effective temperature components to be (9.2 +/- 0.9) × 10-14ergcm-2 s-1 and 70.0 +/- 1.6 eV in KS 1731-260 and (1.7 +/- 0.3) × 10-14ergcm-2 s-1 and 51.6 +/- 1.4 eV in MXB 1659-29. We note that these values are dependent on the assumed distance to the sources and the column density which was tied between the observations due to the low number of photons in the latter observations. However, importantly, the shape of the cooling curves is independent of the distance assumed. In addition, we find that the crust of KS 1731-260 cools faster than that of MXB 1659-29 by a factor of ~2, likely due to different crustal properties. This is the first time that the cooling of a neutron star crust into thermal equilibrium with the core has been observed in such detail.
- Discovery of Kilohertz
Quasi-periodic Oscillations and Shifted Frequency Correlations in the
Accreting Millisecond Pulsar XTE J1807-294
Linares M., van der Klis M., Altamirano D., Markwardt C. B.
ApJ, 634, 2, 1250-1260. (2005)
Abstract: We report the discovery of twin kilohertz quasi-periodic oscillations (kHz QPOs) in the X-ray flux of XTE J1807-294, the fourth accreting millisecond pulsar (AMP). This is the second AMP exhibiting twin kHz QPOs. In contrast to the first case, SAX J1808.4-3658, the frequency separation Δν between these kHz QPOs is consistent with the pulse frequency (190.6 Hz), not with half that value, confirming for the first time from pulsation measurements the inference, based on burst oscillations, that ``slow rotators'' (spin frequency less than 400 Hz) have Δν approximately equal to the spin frequency. While the QPOs move in frequency together over a range of more than 200 Hz, Δν remains constant with an average value of 205+/-6 Hz. Variability components were found in the 5-130 Hz range, similar to those seen in other LMXBs. The correlations between the QPO and noise frequencies are also similar to those in other sources, but shifted by a factor of 1.59 in kHz QPO frequencies, similar to the factor of 1.45 shift found for SAX J1808.4-3658. Our results argue in favor of a spin-related formation mechanism for twin kHz QPOs and against a spin-related cause of the shift in the frequency correlations.
Non-refereed publications:
- Discovery of burst
oscillations near the spin frequency in the intermittent accreting
millisecond pulsar HETE J1900.1-2455
Watts, A.; Altamirano, D.; Casella, P.; Cavecchi, Y.; Degenaar, N.; Linares, M.; Patruno, A.; Rea, N.; Soleri, P.; van der Klis, M.; Wijnands, R.;
The Astronomer's Telegram, #2004. (2009)
- Discovery of the shortest
recurrence time between thermonuclear X-ray bursts
Linares, M.; Watts, A.; Altamirano, D.; Patruno, A.; Casella, P.; Rea, N.; Soleri, P.; van der Klis, M.; Wijnands, R.; Belloni, T.; Homan, J.; Mendez, M.
The Astronomer's Telegram, #1979. (2009)
- Broad relativistic iron
line observed in SAX J1808.4-3658 by Suzaku
Cackett, E.; Altamirano, D.; Miller, J.; Reynolds, J.; Blum, J.; Linares, M.; Patruno, A.; Wijnands, R.
The Astronomer's Telegram, #1844. (2008)
- Swift detects increased
activity of the neutron star transient SAX J1750.8-2900
Linares, M.; Degenaar, N.; Wijnands, R.; Altamirano, D.
The Astronomer's Telegram, #1777. (2008)
- Radio non-detection of
SAX J1808.4-3658
Tudose, V.; Tzioumis, T.; Maitra, D.; Linares, M.; Patruno, A.; Russell, D.; Casella, P.; Altamirano, D.; Wijnands, R.
The Astronomer's Telegram, #1748. (2008)
- Near-IR and Optical
Observations of the Ongoing Outburst of SAX J1808.4-3658
Maitra, D.; Buxton, M.; Tourtellotte, S.; Altamirano, D.; Patruno, A.; Casella, P.; Linares, M.; Tudose, V.; Russell, D.; Wijnands, R.; Bailyn, C.
The Astronomer's Telegram, #1733. (2008)
- Renewed optical and X-ray
activity in IGR J00291+5934
Lewis, F..; Linares, M.; Russell, D. M.; Wijnands, R.; Roche, P.
The Astronomer's Telegram, #1726. (2008)
- The disk jet coupling in
Aql X-1
Tudose, V.; Fender, R.; Linares, M.; Maitra, D.
PoS Conference Proceedings, in press. "VII Microquasar workshop: Microquasars and beyond", Foca, Turkey (2008)
- Improved position of the
new X-ray transient XTE J1637-498
Wijnands, R.; Linares, M.; Degenaar, N.; Markwardt, C. B.
The Astronomer's Telegram, #1700. (2008)
- Timing the accretion flow
around accreting millisecond
pulsars
Linares, M.
AIP Conference Proceedings, in press. "A decade of accreting millisecond X-ray pulsars", Amsterdam (2008)
- A WSRT radio observation
of IGR J00291+5934 in outburst
Linares, M.; Tudose, V.; Migliari, S.
The Astronomer's Telegram, #1667. (2008)
- Faulkes Telescope
monitoring of the current outburst of IGR J00291+5934
Russell, D. M.; Lewis, F..; Linares, M.; Roche, P.; Maitra, D.
The Astronomer's Telegram, #1666. (2008)
- SAX J1750.8-2900 is
returning to quiescence
Linares, M.; Degenaar, N.; Altamirano, D.; Wijnands, R.; van der Klis, M.; Markwardt, C.
The Astronomer's Telegram, #1662. (2008)
- The cooling tail of a
long X-ray burst from XTE J1701-407
Linares, M.; Soleri, P.; Curran, P.; Wijnands, R.; Degenaar, N.; van der Klis, M.; Starling, R.; Markwardt, C.
The Astronomer's Telegram, #1618. (2008)
- The neutron star
transient SAX J1810.8-2609 going back to quiescence
Linares, M.; Degenaar, N.; Wijnands, R.
The Astronomer's Telegram, #1260. (2007)
- XTE J1751-305 going back
to its quiescent state
Linares, M.; Wijnands, R.; van der Klis, M.
The Astronomer's Telegram, #1055. (2007)
- Accreting millisecond
pulsars: one on each hand
Linares M., van der Klis M., Wijnands R.
AIP Conference Proceedings, 924, 629-634. "The Multicolored landscape of compact objects and their explosive origins", Cefalu (2007)
- A Swift/XRT observation
of Aql X-1 during its recent outburst
Wijnands, R.; Maitra, D.; Bailyn, C.; Linares, M.
The Astronomer's Telegram, #871. (2006)
- Pointed RXTE observations
of SAX J1808.4-3658
Wijnands, R.; Wolt, M. Klein; Linares, M.; Klis, M. Van Der; Chakrabarty, D.; Morgan, E. H.; Markwardt, C. B.
The Astronomer's Telegram, #507. (2005)
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