M. Armas Padilla

University of Amsterdam, Amsterdamo, North Holland, Netherlands

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Publications (23)33.95 Total impact

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    ABSTRACT: We search the literature for reports on the spectral properties of neutron-star low-mass X-ray binaries when they have accretion luminosities between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron-star systems behave in a similar manner and possibly even follow a universal relation. When comparing the neutron-star systems with black-hole systems, it is clear that most black-hole binaries have significantly harder spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data points, there are indications that these spectral differences also extend to the 1E35 - 1E36 erg/s range. We note, however, that the system in our sample which has the hardest spectra is in fact a neutron-star system, demonstrating that above a luminosity of 1E35 erg/s the separation between neutron-star and black-hole systems is not as clear as below. This observed difference between the neutron-star binaries and black-hole ones suggests that the spectral properties (between 0.5-10 keV) at 1E34 - 1E35 erg/s can be used to tentatively determine the nature of the accretor in unclassified X-ray binaries. We discuss our results in the context of properties of the accretion flow at low luminosities and we suggest that the observed spectral differences likely arise from the neutron-star surface becoming dominantly visible in the X-ray spectra. We also suggest that both the thermal component and the non-thermal component might be caused by low-level accretion onto the neutron-star surface for luminosities below a few times 1E34 erg/s.
    09/2014;
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    ABSTRACT: Swift J1357.2-0933 is the first confirmed very faint black hole X-ray transient and has a short orbital period of 2.8 hr. We observed Swift J1357.2-0933 for ~50 ks with XMM-Newton in 2013 July during its quiescent state. The source is clearly detected at a 0.5-10 keV flux of ~3x10^-15 erg cm-2 s-1. If the source is located at a distance of 1.5 kpc (as suggested in the literature), this would imply a luminosity of ~8x10^29 erg s-1, making it the faintest detected quiescent black hole LMXB. This would also imply that there is no indication of a reversal in the quiescence X-ray luminosity versus orbital period diagram down to 2.8 hr, as has been predicted theoretically and recently supported by the detection of the 2.4 hr orbital period black hole MAXI J1659-152 at a 0.5-10 keV X-ray luminosity of ~ 1.2 x 10^31 erg s-1. However, there is considerable uncertainty in the distance of Swift J1357.2-0933 and it may be as distant as 6 kpc. In this case, its quiescent luminosity would be Lx ~ 1.3 x 10^31 erg s-1, i.e., similar to MAXI J1659-152 and hence it would support the existence of such a bifurcation period. We also detected the source in optical at r' ~22.3 mag with the Liverpool telescope, simultaneously to our X-ray observation. The X-ray/optical luminosity ratio of Swift J1357.2-0933 agrees with the expected value for a black hole at this range of quiescent X-ray luminosities.
    04/2014;
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    ABSTRACT: We report on the X-ray spectral (using XMM-Newton data) and timing behavior (using XMM-Newton and Rossi X-ray Timing Explorer [RXTE] data) of the very faint X-ray transient and black hole system Swift J1357.2-0933 during its 2011 outburst. The XMM-Newton X-ray spectrum of this source can be adequately fitted with a soft thermal component with a temperature of ~0.22 keV (using a disc model) and a hard, non-thermal component with a photon index of ~1.6 when using a simple power-law model. In addition, an edge at ~ 0.73 keV is needed likely due to interstellar absorption. During the first RXTE observation we find a 6 mHz quasi-periodic oscillation (QPO) which is not present during any of the later RXTE observations or during the XMM-Newton observation which was taken 3 days after the first RXTE observation. The nature of this QPO is not clear but it could be related to a similar QPO seen in the black hole system H 1743-322 and to the so-called 1 Hz QPO seen in the dipping neutron-star X-ray binaries (although this later identification is quite speculative). The observed QPO has similar frequencies as the optical dips seen previously in this source during its 2011 outburst but we cannot conclusively determine that they are due to the same underlying physical mechanism. Besides the QPO, we detect strong band-limited noise in the power-density spectra of the source (as calculated from both the RXTE and the XMM-Newton data) with characteristic frequencies and strengths very similar to other black hole X-ray transients when they are at low X-ray luminosities. We discuss the spectral and timing properties of the source in the context of the proposed very high inclination of this source. We conclude that all the phenomena seen from the source cannot, as yet, be straightforwardly explained neither by an edge-on configuration nor by any other inclination configuration of the orbit.
    08/2013; 439(4).
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    M. Armas Padilla, R. Wijnands, N. Degenaar
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    ABSTRACT: A growing group of low-mass X-ray binaries are found to be accreting at very-faint X-ray luminosities of <1E36 erg/s (2-10 keV). Once such system is the new X-ray transient IGR J17494-3030. We present Swift and XMM-Newton observations obtained during its 2012 discovery outburst. The Swift observations trace the peak of the outburst, which reached a luminosity of ~7 E35 (D/8 kpc)^2 erg/s (2-10 keV). The XMM-Newton data were obtained when the outburst had decayed to an intensity of ~ 8 E34 (D/8 kpc)^2 erg/s. The spectrum can be described by a power-law with an index of ~1.7 and requires an additional soft component with a black-body temperature of ~0.37 keV (contributing ~20% to the total unabsorbed flux in the 0.5-10 keV band). Given the similarities with high-quality spectra of very-faint neutron star low-mass X-ray binaries, we suggest that the compact primary in IGR J17494-3030 is a neutron star. Interestingly, the source intensity decreased rapidly during the ~12 hr XMM-Newton observation, which was accompanied by a decrease in inferred temperature. We interpret the soft spectral component as arising from the neutron star surface due to low-level accretion, and propose that the observed decline in intensity was the result of a decrease in the mass-accretion rate onto the neutron star.
    Monthly Notices of the Royal Astronomical Society 07/2013; 436(1). · 5.52 Impact Factor
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    ABSTRACT: We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745-26, starting only 19 min after the discovery of the source. We observe the system peaking at i' ˜ 17.6 on day six (MJD 561 92) to then decay at a rate of ˜0.04 mag d-1. We show that the optical peak occurs at least 3 d later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 mag, which favours an orbital period ≲21 h and a companion star with a spectral type later than ˜A0. Spectroscopic observations taken with the Gran Telescopio de Canarias 10.4 m telescope reveal a broad (full width at half-maximum ˜1100 km s-1), double-peaked Hα emission line from which we constrain the radial velocity semi-amplitude of the donor to be K2 > 250 km s-1. The breadth of the line and the observed optical and X-ray fluxes suggest that Swift J1745-26 is a new black hole candidate located closer than ˜7 kpc.
    Monthly Notices of the Royal Astronomical Society 06/2013; 432(2):1133-1137. · 5.52 Impact Factor
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    M. Armas Padilla, N. Degenaar, R. Wijnands
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    ABSTRACT: AX J1754.2-2754, 1RXS J171824.2-402934 and 1RXH J173523.7-354013 are three persistent neutron star low-mass X-ray binaries that display a 2--10 keV accretion luminosity Lx of only (1-10)x1E34 erg s-1 (i.e., only ~0.005-0.05 % of the Eddington limit). The phenomenology of accreting neutron stars which accrete at such low accretion rates is not yet well known and the reason why they have such low accretion rates is also not clear. Therefore, we have obtained XMM-Newton data of these three sources and here we report our analysis of the high-quality X-ray spectra we have obtained for them. We find that AX J1754.2-2754 has Lx~1E35 erg s-1, while the other two have X-ray luminosities about an order of magnitude lower. However, all sources have a similar, relatively soft, spectrum with a photon index of 2.3-2.5, when the spectrum is fitted with an absorbed power-law model. This model fits the data of AX J1754.2-2754 adequately, but it cannot fit the data obtained for 1RXS J171824.2-402934 and 1RXH J173523.7-354013. For those sources a clear soft thermal component is needed to fit their spectra. This soft component contributes 40% - 50% to the 0.5-10 keV flux of the sources. When including this additional spectral component, the power-law photon indices are significantly lower. It can be excluded that a similar component with similar contributions to the 2-10 keV X-ray flux is present for AX J1754.2-2754, indicating that the soft spectrum of this source is mostly due to the fact that the power-law component itself is not hard. We note that we cannot excluded that weaker soft component is present in the spectrum of this source which only contributes up to ~25% to the 0.5-10 keV X-ray flux. We discuss our results in the context of what is known of accreting neutron stars at very low accretion rate.
    Monthly Notices of the Royal Astronomical Society 03/2013; 434(2). · 5.52 Impact Factor
  • Montserrat Armas Padilla, Rudy Wijnands, Nathalie Degenaar, Dave Russell
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    ABSTRACT: In the last decade a new population of X-ray transients has been discovered. They show anomalously low peak luminosities (2-10 keV) of 1E34 to 1E36 ergs/s. A large fraction of them is expected to harbor accreting neutron stars and black holes in binaries systems.These very faint X-ray binaries provide a new regime to study accretion onto compact objects, and therefore they can improve our understanding of accretion physics and binary evolution models. We report the study of the 2011 outburst evolution of the newly discovered black hole candidate X-ray binary Swift J1357.2-0933. We analyzed the Swift X-ray telescope and Ultraviolet/Optical telescope (UVOT) data taken during the outburst. The low column density towards the source and its proximity (~1.5 kpc) provides an exceptional opportunity to study the X-ray spectrum and the optical counterpart in unprecedented detail and search for features such as X-ray bursts, X-ray pulsations and eclipses/dips in the X-ray lightcurve. Its peak luminosity of ~1E35 ergs/s classifies the source as a very faint X-ray transient. If the black hole nature is confirmed, Swift J1357.2-0933 would be the first established black hole very-faint X-ray binary.
    09/2012;
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    M. Armas Padilla, N. Degenaar, D. M. Russell, R. Wijnands
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    ABSTRACT: We report our multiwavelength study of the 2011 outburst evolution of the newly discovered black hole candidate X-ray binary Swift J1357.2-0933. We analysed the Swift X-ray telescope and Ultraviolet/Optical telescope (UVOT) data taken during the ~7 months duration of the outburst. It displayed a 2-10 keV X-ray peak luminosity of ~1E35(D/1.5 kpc)^2 erg s-1 which classifies the source as a very faint X-ray transient. We found that the X-ray spectrum at the peak was consistent with the source being in the hard state, but it softened with decreasing luminosity, a common behaviour of black holes at low luminosities or returning to quiescence from the hard state. The correlations between the simultaneous X-ray and ultraviolet/optical data suggest a system with a black hole accreting from a viscous disc that is not irradiated. The UVOT filters provide the opportunity to study these correlations up to ultraviolet wavelengths a regime so far unexplored. If the black hole nature is confirmed, Swift J1357.2-0933 would be one of the very few established black hole very-faint X-ray transients.
    Monthly Notices of the Royal Astronomical Society 07/2012; · 5.52 Impact Factor
  • Montserrat Armas Padilla, R. Wijnands, N. Degenaar
    [Show abstract] [Hide abstract]
    ABSTRACT: In the last decade a new population of X-ray transients has been discovered. They show anomalously low peak luminosities (2-10 keV) of 1E34 to 1E36 ergs/s. A large fraction of them is expected to harbor accreting neutron stars and black holes in binaries systems. These very faint X-ray binaries provide a new regime to study accretion onto compact objects, and therefore they can improve our understanding of accretion physics and binary evolution models. Swift J1357.2-0933 is a very-faint X-ray transient recently discovered during outburst in January of 2011, which nature is still unknown. The low column density towards the source and its proximity (∼1.5 kpc) provide an exceptional opportunity to study the X-ray spectrum of a very-faint X-ray transient in unprecedented detail and search for features such as X-ray bursts, X-ray pulsations and eclipses/dips in the X-ray lightcurve. This provides the opportunity to gain insight into the nature of this system and allows for a deeper understanding the peculiar population of very-faint X-ray transients in general. We present our initial results from multi-satellite X-ray observations obtained during the outburst of this unique source.
    09/2011;
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    ABSTRACT: The X-ray transient XTE J1719-291 was discovered with RXTE/PCA during its outburst in 2008 March, which lasted at least 46 days. Its 2-10 keV peak luminosity is 7E35 erg/s assuming a distance of 8 kpc, which classifies the system as a very faint X-ray transient. The outburst was monitored with Swift, RXTE, Chandra and XMM-Newton. We analysed the X-ray spectral evolution during the outburst. We fitted the overall data with a simple power-law model corrected for absorption and found that the spectrum softened with decreasing luminosity. However, the XMM-Newton spectrum can not be fitted with a simple one-component model, but it can be fitted with a thermal component (black body or disc black body) plus power-law model affected by absorption. Therefore, the softening of the X-ray spectrum with decreasing X-ray luminosity might be due to a change in photon index or alternatively it might be due to a change in the properties of the soft component. Assuming that the system is an X-ray binary, we estimated a long-term time-averaged mass accretion rate of ~ 7.7E-13 M_sun/yr for a neutron star as compact object and ~ 3.7E10-13 M_sun/yr in the case of a black hole. Although no conclusive evidence is available about the nature of the accretor, based on the X-ray/optical luminosity ratio we tentatively suggest that a neutron star is present in this system.
    Monthly Notices of the Royal Astronomical Society 04/2011; 417(1). · 5.52 Impact Factor
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    ABSTRACT: Following reports of renewed activity of the transient neutron star low-mass X-ray binary SAX J1806.5-2215 (ATel #3193), we obtained a ~1 ks Swift/XRT pointing of the source field on 2011 March 1. The observation was carried out in the PC mode and reveals one relatively bright X-ray source within the XRT field of view. For this object, we find an astrometrically corrected X-ray position (using the method described by Evans et al.
    The Astronomer's Telegram. 03/2011;
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    ABSTRACT: We report on the discovery and the timing analysis of the first eclipsing accretion-powered millisecond X-ray pulsar (AMXP): SWIFT J1749.4-2807. The neutron star rotates at a frequency of ~517.9 Hz and is in a binary system with an orbital period of 8.8 hr and a projected semimajor axis of ~1.90 lt-s. Assuming a neutron star between 0.8 and 2.2 M ☉ and using the mass function of the system and the eclipse half-angle, we constrain the mass of the companion and the inclination of the system to be in the ~0.46-0.81 M ☉ and ~ 744-773 range, respectively. To date, this is the tightest constraint on the orbital inclination of any AMXP. As in other AMXPs, the pulse profile shows harmonic content up to the third overtone. However, this is the first AMXP to show a first overtone with rms amplitudes between ~6% and ~23%, which is the strongest ever seen and which can be more than two times stronger than the fundamental. The fact that SWIFT J1749.4-2807 is an eclipsing system that shows uncommonly strong harmonic content suggests that it might be the best source to date to set constraints on neutron star properties including compactness and geometry.
    The Astrophysical Journal Letters 12/2010; 727(1):L18. · 6.35 Impact Factor
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    ABSTRACT: We have been monitoring the position of the very faint X-ray transient XTE J1728-295 (= IGR J17285-2922, ATel #2823, #2824, #2825) with the Faulkes Telescopes North and South throughout its outburst. Data were acquired on thirteen dates from 1st September until 28th October, mostly in I- band and R-band. Of the original seven stars detected with the Faulkes Telescope South within and around the 3.5 arcsec Swift XRT error circle, only star 3, a blue star, is consistent with the refined 0.6 arcsec Chandra position (ATel #2827, #2869).
    The Astronomer's Telegram. 11/2010;
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    ABSTRACT: We report a Swift follow-up observation of the very faint X-ray transient source XTE J1728-295 (also IGR J17285-2922, ATel #2825) currently in the decaying phase of its recent outburst (ATel #2823, #2824, #2869). The observation was performed on 2010-10-29 07:07:35 UTC for a duration of 2 ks, and was observed in Photon Counting (PC) mode. Preliminary results show that there is no pile-up and the average count rate for this observation is about 0.5 c/s.
    The Astronomer's Telegram. 10/2010;
  • M. Armas Padilla, N. Degenaar, R. Kaur, R. Wijnands, Y. Yang
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    ABSTRACT: After the re-brightening of the new very faint X-ray source XMMSL1 J171900.4-353217 [ATel #2607, #2722], an additional observation has been carried out using the XRT aboard Swift on July 13th 2010. In this last pointing ~960 seconds only two counts were present in the source extraction region demonstrating that we did not detect the source in our observation. We have calculated the upper limit on the flux with HEARSAC tool WebPIMMs (v4.1a) assuming an absorbed powerlaw model with an hydrogen column of 3.4E22 cm-2 (see ATel #2627) and a photon index of 2.11 (see ATel #2722).
    The Astronomer's Telegram. 07/2010;
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    ABSTRACT: We report on the timing analysis of the first eclipsing accretion-powered millisecond X-ray pulsar (AMXP): SWIFT J1749.4-2807. The neutron star rotates at a frequency of ~517.9 Hz and is in a binary system with an orbital period of 8.8 hrs and a projected semi-major axis of ~1.90 lt-s. Based on the mass function and the eclipse half-angle, we constrain the inclination of the system to be between ~76 and ~80 deg. This is to date the tightest constraint on the orbital inclination of any AMXP. We also estimate the mass of the companion to be in the 0.6-0.8 Msun range. As in other AMXPs, the pulse profile shows harmonic content up to the 3rd overtone. However, this is the first AMXP to show a 1st overtone with rms amplitudes between 5 and 25%, which is the strongest ever seen, and which can be more than two times stronger than the fundamental. The fact that SWIFT J1749.4-2807 is an eclipsing system which shows uncommonly strong harmonic content suggests that it might be the best source to date to set constraints on neutron star properties including compactness and geometry. Comment: 5 pages, 4 Figures, Submitted to ApJ Letters
    05/2010;
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    ABSTRACT: We analyzed seven, target ID 31686, Swift follow-up observations of the neutron-star X-ray transient Swfit J1749.4-2807 (Wijnands et al. 2009) currently in outburst and which was found to be an accreting millisecond X-ray pulsar (ATel #2565). The observations span from April 11 to April 20.
    The Astronomer's Telegram. 03/2010; 2579:1.
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    ABSTRACT: On the 28th of December, MAXI/GSC detected an X-ray outburst from the globular cluster NGC6440 (Atel #2360), which has continued to the present.
    Journal of Leukocyte Biology - J LEUKOCYTE BIOL. 01/2010;
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    ABSTRACT: We report on RXTE observations of the ongoing outburst of the accreting millisecond X-ray pulsar SAX J1748.9-2021 (Altamirano et al. 2008, 674, 45A, see ATEL #2360 and #2407). Since the first detection of intermittent pulsations on January 27th, 2010, observations have been carried out on a daily basis. The source remained between 120 and 160 mCrab (2-16 keV - see ATEL #2407) until February 3rd, when the intensity started to decrease.
    Journal of Leukocyte Biology - J LEUKOCYTE BIOL. 01/2010;
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    ABSTRACT: The newly identified black hole candidate X-ray binary MAXI J1659-152 (ATel #2873, #2881, GCN #11307), originally designated GRB100925A (GCN #11296) has now been detected at radio (ATel #2874), sub-mm (GCN #11304), optical (GCN #11298, #11301, #11302, #11305, #11306, #11307, #11308, #11314) and X-ray (GCN #11296, ATel #2875, #2877) wavelengths. Here we report a brightening of the optical counterpart since the outburst began and short-timescale (minutes) variability characteristic of a black hole transient in outburst.
    Journal of Leukocyte Biology - J LEUKOCYTE BIOL. 01/2010;