Pablo Reig

Publications (17)10.92 Total impact

• Article: The Be X-ray Binary Outburst Zoo

  Peter Kretschmar, Elisa Nespoli, Pablo Reig, Friedrich Anders
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  ABSTRACT: Be X-ray binaries are among the best known transient high-energy sources. Their outbursts are commonly classified into a simple scheme of 'normal' and 'giant' outbursts, but a closer look shows that actual outbursts do not always follow this simple scheme. Recent data show a variety of properties, like pre-flares, shifts of the outburst peaks with respect to the periastron, multi-peaked outbursts etc. We present results from a systematic study of a large number of outbursts monitored by various space missions, comparing outburst properties and their relation to system parameters and current theoretical understanding.
  02/2013;
• Article: Patterns of variability in Be/X-ray pulsars during giant outbursts

  Pablo Reig, Elisa Nespoli
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  ABSTRACT: The discovery of source states in the X-ray emission of black-hole binaries and neutron-star low-mass X-ray binaries constituted a major step forward in the understanding of the physics of accretion onto compact objects. While there are numerous studies on the correlated timing and spectral variability of these systems, very little work has been done on high-mass X-ray binaries, the third major type of X-ray binaries. The main goal of this work is to investigate whether Be accreting X-ray pulsars display source states and characterise those states through their spectral and timing properties. We have made a systematic study of the power spectra, energy spectra and X-ray hardness-intensity diagrams of nine Be/X-ray pulsars. The evolution of the timing and spectral parameters were monitored through changes over two orders of magnitude in luminosity. We find that Be/X-ray pulsars trace two different branches in the hardness-intensity diagram: the horizontal branch corresponds to a low-intensity state of the source and it is characterised by fast colour and spectral changes and high X-ray variability. The diagonal branch is a high-intensity state that emerges when the X-ray luminosity exceeds a critical limit. The photon index anticorrelates with X-ray flux in the horizontal branch but correlates with it in the diagonal branch. The correlation between QPO frequency and X-ray flux reported in some pulsars is also observed if the peak frequency of the broad-band noise that accounts for the aperiodic variability is used. The two branches may reflect two different accretion modes, depending on whether the luminosity of the source is above or below a critical value. This critical luminosity is mainly determined by the magnetic field strength, hence it differs for different sources.
  12/2012;
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  Available from: ArXiv

  Article: Be/X-ray binaries

  Pablo Reig
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  ABSTRACT: The purpose of this work is to review the observational properties of Be/X-ray binaries. The open questions in Be/X-ray binaries include those related to the Be star companion, that is, the so-called "Be phenomenon", such as, timescales associated to the formation and dissipation of the equatorial disc, mass-ejection mechanisms, V/R variability, and rotation rates; those related to the neutron star, such as, mass determination, accretion physics, and spin period evolution; but also, those that result from the interaction of the two constituents, such as, disc truncation and mass transfer. Until recently, it was thought that the Be stars' disc was not significantly affected by the neutron star. In this review, I present the observational evidence accumulated in recent years on the interaction between the circumstellar disc and the compact companion. The most obvious effect is the tidal truncation of the disc. As a result, the equatorial discs in Be/X-ray binaries are smaller and denser than those around isolated Be stars.
  01/2011;
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  Available from: ArXiv

  Article: Discovery of a QPO in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

  Elisa Nespoli, Pablo Reig
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  ABSTRACT: Our goal is to investigate the X-ray timing and spectral variability of the high-mass X-ray binary 1A 1118-615 during a type-II outburst. We performed a detailed color, spectral and timing analysis of a giant outburst from 1A 1118-615, using RXTE data. Results. We report the discovery of a variable quasi-periodic oscillation (QPO) in the power spectral density of 1A 1118-615, with a centroid frequency of ~0.08 Hz. The centroid frequency of the QPO correlates with the X-ray flux, as expected according to the most accredited models for QPO production. For energies above ~4 keV, the QPO rms variability decreases as the energy increases. Pulse profiles display energy dependence, with a two-peak profile at lower energies, and a single peak at higher energies. From spectral analysis, we confirm the presence of a cyclotron absorption feature at ~60 keV, the highest value measured for an X-ray pulsar. We find that the spectral parameters (photon index, cutoff energy, iron fluorescence line strength) display a marked dependence with flux. We detect two different levels of neutral hydrogen column density, possibly due to the Be companion activity. We report for the first time a correlation between the timing and spectral parameters in an X-ray pulsar. All the correlations found between spectral/timing parameters and X-ray flux are present up to a flux of ~6x10^-9 erg cm^-2 s^-1, when a saturation level is reached. We propose that the saturation observed corresponds to the minimum extent of the neutron star magnetosphere. We estimate the magnetic field of the neutron star from two independent ways, using results from spectral (cyclotron line energy) and timing (QPO frequency) analysis, obtaining consistent values, of ~7-8x10^12 G. Results from the comprehensive spectral and timing analysis are discussed in comparison with other X-ray pulsars. Comment: 9 pages, 11 figures. Accepted for publication in Astronomy and Astrophysics
  11/2010;
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  Available from: arxiv.org

  Article: Discovery of a Be/X-Ray Binary Consistent with the Position of GRO J2058+42

  Colleen A. Wilson, Martin C. Weisskopf, Mark H. Finger, M. J. Coe, Jochen Greiner, Pablo Reig, and Giannis Papamastorakis
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  ABSTRACT: GRO J2058+42 is a 195 s transient X-ray pulsar discovered in 1995 with BATSE. In 1996, RXTE located GRO J2058+42 to a 90% confidence error circle with a 4' radius. On 2004 February 20, the region including the error circle was observed with Chandra ACIS-I. No X-ray sources were detected within the error circle; however, two faint sources were detected in the ACIS-I field of view. We obtained optical observations of the brightest object, CXOU J205847.5+414637, which had about 64 X-ray counts and was just 03 outside the error circle. The optical spectrum contains a strong Hα line and corresponds to an infrared object in the Two Micron All Sky Survey catalog, indicating a Be/X-ray binary system. Pulsations were not detected in the Chandra observations, but similar flux variations and distance estimates suggest that CXOU J205847.5+414637 and GRO J2058+42 are the same object. We present results from the Chandra observation, optical observations, new and previously unreported RXTE observations, and a reanalysis of a ROSAT observation.
  The Astrophysical Journal 12/2008; 622(2):1024. · 6.02 Impact Factor
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  Available from: ArXiv

  Article: Supergiant Fast X-ray Transients and Other Wind Accretors

  Ignacio Negueruela, Jose Miguel Torrejon, Pablo Reig, Marc Ribo, David M. Smith
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  ABSTRACT: Supergiant Fast X-ray Transients are obviously related to persistent Supergiant X-ray Binaries. Any convincing explanation for their behaviour must consistently take into account all types of X-ray sources powered by wind accretion. Here we present a common framework for wind accreting sources, within the context of clumpy wind models, that allows a coherent interpretation of their different behaviours as an immediate consequence of diverse orbital geometries. Comment: 5 pages, 3 figures proceedings of "A population explosion: the nature and evolution of X-ray binaries in diverse environments", conference held in St.Petersburg Beach, Florida (USA) 28 Oct - 2 Nov 2007; R. M. Bandyopadhyay et al. (eds.)
  01/2008;
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  Available from: ArXiv

  Article: Supergiant Fast X-ray Transients: A common behaviour or a class of objects?

  Ignacio Negueruela, David M. Smith, Jose Miguel Torrejon, Pablo Reig
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  ABSTRACT: INTEGRAL monitoring of the Galactic Plane is revealing a growing number of recurrent X-ray transients, characterised by short outbursts with very fast rise times (~ tens of minutes) and typical durations of a few hours. A substantial fraction of these sources are associated with OB supergiants and hence define a new class of massive X-ray binaries, which we call Supergiant Fast X-ray Transients. Characterisation of the astrophysical parameters of their counterparts is underway. So far, we have found a number of late O and early B supergiants of different luminosities at a large range of distances. Nothing in their optical properties sets them apart from classical Supergiant X-ray Binaries. On the other hand, there is now rather concluding evidence that persistent supergiant X-ray binaries also show fast outbursts. This suggests a continuum of behaviours between typical persistent supergiant systems and purely transient systems, but offers very little information about the physical causes of the outbursts.
  05/2007;
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  Available from: arxiv.org

  Article: Supergiant Fast X-ray Transients: A new class of high mass X-ray binaries unveiled by INTEGRAL

  Ignacio Negueruela, David M. Smith, Pablo Reig, Sylvain Chaty, Jose Miguel Torrejon
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  ABSTRACT: INTEGRAL monitoring of the Galactic Plane is revealing a growing number of recurrent X-ray transients, characterised by short outbursts with very fast rise times (~ tens of minutes) and typical durations of a few hours. Here we show that several of these transients are associated with OB supergiants and hence define a new class of massive X-ray binaries which we call Supergiant Fast X-ray Transients (SFXTs). Many other transient X-ray sources display similar X-ray characteristics, suggesting that they belong to the same class. Since they are difficult to detect and their number is growing fast and steadily, they could represent a major class of X-ray binaries.
  12/2005;
• Article: Discovery of a Be/X-Ray Binary Consistent with the Location of GRO J2058+42

  Colleen Wilson, Martin Weisskopf, Mark H. Finger, M. J. Coe, Jochen Greiner, Pablo Reig, Giannis Papamastorakis
  [show abstract] [hide abstract]
  ABSTRACT: GRO J2058+42 is a 195 s transient X-ray pulsar discovered in 1995 with BATSE. In 1996, RXTE located GRO J2058+42 to a 90% confidence error circle with a 4 radius. On 2004 February 20, the region including the error circle was observed with Chandra ACIS-I. No X-ray sources were detected within the error circle; however, two faint sources were detected in the ACIS-I field of view. We obtained optical observations of the brightest object, CXOU J205847.5+414637, which had about 64 X-ray counts and was just 013 outside the error circle. The optical spectrum contains a strong Ha line and corresponds to an inhued object in the Two Micron All Sky Survey catalog, indicating a Be/X-ray binary system. Pulsations were not detected in the Chandra observations, but similar flux variations and distance estimates suggest that CXOU J205847.5+414637 and GRO J2058+42 are the same object. We present results from the Chandra observation, optical observations, new and previously unreported RXTE observations, and a reanalysis of a ROSAT observation.
  05/2005;
• Article: Optical observations of the field around the X-ray pulsars XTE J1858+034 and GRO J2058+42

  Pablo Reig, Tasos Kougentakis, Giannis Papamastorakis
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  ABSTRACT: We report photometric and spectroscopic optical observations of the likely optical counterparts to the 221-s XTE J1858+034 (Remillard & Levine and Takeshima & Corbet, 1998, IAUCirc 6826) and the 196-s high-mass X-ray binary (HMXB) pulsars GRO J2058+42 (Wilson et al. 1996, IAUCirc 6514, Wilson et al. 1998, ApJ, 499, 820). Optical photometric observations of the field around the best-fit ISGRI/IBIS INTEGRAL position R.A.=18h58m43s, Decl.=03d26m06s (Molkov et al, 2004, ATel 274) of XTE J1858+034 using Johnson BVR and a H alpha filter were carried out on the night 20 May 2004 (JD 2,453,146.45) with the 1.3-m telescope of the Skinakas Observatory (Crete, Greece).
  The Astronomer's Telegram. 06/2004; 308:1.
• Article: GRO J2058+42 Observations with Chandra and Discovery of a Likely Optical Counterpart

  Colleen A. Wilson, Martin C. Weisskopf, Mark H. Finger, M. J. Coe, Jochen Greiner, Pablo Reig, Giannis Papamastorakis
  [show abstract] [hide abstract]
  ABSTRACT: GRO J2058+42 is a 195 s transient X-ray pulsar discovered in 1995 with BATSE. In 1996, RXTE located GRO J2058+42 to a 90% confidence error circle with a 4' radius. On 20 February 2004, the region including the error circle was observed with Chandra ACIS-I. No X-ray sources were detected within the error circle, however, 2 faint sources were det.ected in the ACISI field-of-view. We obtained optical observations of the brightest object that had about 64 X-ray counts and was just 0.3' outside the error circle. The optical spectrum contained a strong H alpha line and corresponds to an infrared object in the 2MASS catalog, indicating a Be/X-ray system. Although X-ray pulsations were undetectable due to the low count rate, our evidence is strong that this object is the optical counterpart to GRO J2058+42. We present results from the Chandra observation and from the optical observations.
  02/2004;
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  Available from: arxiv.org

  Article: Energy and time-lag spectra of black-hole systems in the low state

  Pablo Reig, Nikolaos D. Kylafis, Dimitris Giannios
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  ABSTRACT: Most, probably all, accreting binaries that are believed to contain a black-hole emit radio waves when they are in the low/hard state. Whenever this radio emission has been resolved, a jet-like structure has become apparent. We propose that Compton upscattering of low-energy photons in the jet can explain both the energy spectra and the time lags versus Fourier frequency observed in the low/hard state of black-hole systems. The soft photons originate in the inner part of the accretion disk. We have performed Monte Carlo simulations of Compton upscattering in a jet and have found that for a rather wide range of values of the parameters we can obtain power-law high-energy X-ray spectra with photon-number index in the range 1.5 - 2 and power-law time lags versus Fourier frequency with index ~ 0.7. The black-hole source Cyg X-1 in the low/hard state is well described by our model.
  03/2003;
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  Available from: arxiv.org

  Article: Orbital Comptonization in accretion disks around black holes

  Pablo Reig, Nikolaos D. Kylafis, Henk C. Spruit
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  ABSTRACT: We have performed Monte Carlo simulations of Compton upscattering of low-energy photons in an accretion disk around a Schwarzschild black hole. The photons gain energy from the rotational motion of the electrons in the disk. The upscattering occurs near the black hole horizon, where the flow velocity of the electrons approaches the speed of light. We show that this type of bulk-flow Comptonization can produce power-law X-ray spectra similar to the ones observed in black-hole X-ray transients in the high/soft state, i.e., a soft bump dominating the spectrum below ~ 10 keV and a power-law tail with photon index in the range 2-3. In order to reproduce the observed hard to soft flux ratio the disk has to have vertical optical depth above ~ 3 at the last stable orbit. We conclude that the power-law component of the high/soft state of black-hole transients may be due to an intrinsically cool disk extending all the way to the hole, without a separate hot plasma component. Comment: 6 pages, 6 figures
  04/2001;
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  Available from: arxiv.org

  Article: On the nature of the hard X-ray source 4U2206+54

  Ignacio Negueruela, Pablo Reig
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  ABSTRACT: The recent discovery of a ~9.5-d period in the X-ray lightcurve of the massive X-ray binary 4U2206+54 has opened the possibility that it is a Be/X-ray binary with an unusually close orbit, which, together with its low intrinsic luminosity, suggests that the system is actually a Be + WD binary, in which a white dwarf accretes material from the dense circumstellar disc surrounding a classical Be star. In this paper we present new X-ray observations and for the first time high-resolution optical spectroscopy of the source. We show that both the X-ray behaviour and the characteristics of the optical counterpart, BD +53 2790, are more consistent with a neutron star accreting from the wind of an early-type star. The X-ray lightcurve shows irregular flaring and no indications of pulsations, while the very high hydrogen column density supports accretion from a dense wind. BD +53 2790 is shown not to be a classical Be star, as believed until now, but rather a very peculiar late O-type active star, exhibiting emission components in the HeII lines, complex spectral variability and strong wind resonance lines in the ultraviolet. Though many of the characteristics of the spectrum resemble those of the He-rich stars, the absence of HeI variability makes a connection unlikely. The spectrum is compatible with a composite of two stars of similar spectral type, though circumstantial evidence points to a single very peculiar active early-type star. This adds weight to the growing evidence that the traditional subdivisions of supergiant and Be/X-ray binaries fail to cover the whole phenomenology of massive X-ray binaries. Comment: 10 pages, 7 figures, uses new aa.cls; accepted for publication in A&A
  03/2001;
• Article: Discovery of X‐ray pulsations in the Be/X‐ray binary LS 992/RX J0812.4–3114

  Pablo Reig, Paul Roche
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  ABSTRACT: We report on the discovery of X-ray pulsations from the Be/X-ray system LS 992/RX J0812.4–3114 during an RXTE observation. From a timing analysis of the source we obtained a barycentric pulse period of 31.8851±0.0004 s. The pulse profile is highly structured and departs from a pure sinusoidal shape. It shows a sharp dip that may indicate absorption by the accretion flow. The energy spectrum in the range 3–30 keV can be fitted by a power-law model with an exponential cut-off in accordance with other X-ray pulsars. The X-ray luminosity is estimated to be ∼ 1.1× 1036 erg s-1 in the energy range 3–30 keV, assuming a distance of ∼ 9 kpc.
  Monthly Notices of the Royal Astronomical Society 06/1999; 306(1):95 - 99. · 4.90 Impact Factor
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  Available from: ArXiv

  Article: Discovery of two new persistent Be/X-ray pulsar systems

  Pablo Reig, Paul Roche
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  ABSTRACT: We present RXTE observations of two recently identified massive X-ray binaries. RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 are confirmed as accreting Be/X-ray systems following the discovery of X-ray pulsations, with barycentric pulse periods of 202.5$\pm$0.5 s and 860$\pm$2 s respectively. The X-ray spectral analysis shows that the energy spectra of the pulsars can be represented by a power-law, modified at low energy by an absorption component and at high energy by a cut-off. Very weak Fe lines may be present. Both sources appear to display a low cut-off energy when compared to typical X-ray pulsars, low X-ray variability (factor of < 10), and no dependence of the X-ray spectrum with energy. Given the similarity of these X-ray properties with those of the other persistent BeXRB pulsars, 4U0352+309/X Per and RX J0146.9+6121/LS I +61 235, we suggest that RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 are also members of this subclass. Comment: 8 pages 7 figures, accepted for publication in MNRAS
  02/1999;
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  Available from: arxiv.org

  Article: Discovery of X-ray pulsations in the Be/X-ray binary LS992/ RX J0812.4-3114

  Pablo Reig, Paul Roche
  [show abstract] [hide abstract]
  ABSTRACT: We report on the discovery of X-ray pulsations from the Be/X-ray system LS 992/RX J0812.4-3114 during an RXTE observation. From a timing analysis of the source we obtained a barycentric pulse period of 31.8851 \pm 0.0004 s. The pulse profile is highly structured and departs from a pure sinusoidal shape. It shows a sharp dip that may indicate absorption by the accretion flow. The energy spectrum from 3-30 keV can be fitted by a power-law model with an exponential cut-off in accordance with other X-ray pulsars. The X-ray luminosity is estimated to be $\sim 1.1 \times 10^{36} erg/s$ in the energy range 3-30 keV, assuming a distance of $\sim 9 kpc$. Comment: 5 pages, 6 fgures, 2 tables, to appear in MNRAS
  02/1999;