arXiv subjects

P. Reig

Publications and source records attributed to P. Reig.

At least 73 records · Page 4Linked to original sources

arXiv2009

Characterizing black hole variability with nonlinear methods: the case of the X-ray Nova 4U 1543-47

We investigate the possible nonlinear variability properties of the black hole X-ray nova 4U1543-47 to complement the temporal studies based on linear techniques, and to search for signs of nonlinearity in Galactic black hole (GBH) light curves. First, we apply the weighted scaling index method (WSIM) to characterize the X-ray variability properties of 4U1543-47 in different spectral states during the 2002 outburst. Second, we use surrogate data to investigate whether the variability is nonlinear in any of the different spectral states. The main findings can be summarized as follows. The mean weighted scaling index appears to be able to parametrize uniquely the temporal variability properties of this GBH: the 3 different spectral states of the 2002 outburst of 4U1543-47 are characterized by different and well constrained values. The search for nonlinearity reveals that the variability is linear in all light curves with the notable exception of the very high state. Our results imply that we can use the WSIM to assign a single number, namely the mean weighted scaling index, to a light curve, and in this way discriminate among the different spectral states of a source. The detection of nonlinearity in the VHS, that is characterized by the presence of most prominent QPOs, suggests that intrinsically linear models which have been proposed to account for the low frequency QPOs in GBHs may be ruled out (abridged).

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arXiv2009

Correlated optical/X-ray variability in the high-mass X-ray binary SAX J2103.5+4545

SAX J2103.5+4545 is the Be/X-ray binary with the shortest orbital period. It shows extended bright and faint X-ray states that last for a few hundred days. The main objective of this work is to investigate the relationship between the X-ray and optical variability and to characterise the spectral and timing properties of the bright and faint states. We have found a correlation between the spectral and temporal parameters that fit the energy and power spectra. Softer energy spectra correspond to softer power spectra. That is to say, when the energy spectrum is soft the power at high frequencies is suppressed. We also present the results of our monitoring of the Halpha line of the optical counterpart since its discovery in 2003. There is a correlation between the strength and shape of the Halpha line, originated in the circumstellar envelope of the massive companion and the X-ray emission from the vicinity of the neutron star. Halpha emission, indicative of an equatorial disc around the B-type star, is detected whenever the source is bright in X-rays. When the disc is absent, the X-ray emission decreases significantly. The long-term variability of SAX J2103.5+4545 is characterised by fast episodes of disc loss and subsequent reformation. The time scales for the loss and reformation of the disc (about 2 years) are the fastest among Be/X-ray binaries.

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arXiv2009

Spectroscopy of the bright optical counterparts of X-ray sources in the direction of M 31. II

A recent survey of the Local Group spiral galaxy M 31 with XMM-Newton yielded a large number of X-ray sources. This is the second in a series of papers with the aim of identifying the optical counterparts of these X-ray sources. We have obtained optical spectra for 21 bright optical counterparts of 20 X-ray sources in the direction of M 31, using the 1.3-m Skinakas telescope in Crete, Greece. For 17 of the 20 X-ray sources, we have identified the optical counterpart as a normal late type star (of type F or later) in the foreground (i.e. in the Milky Way). For two more sources there were two possible optical counterparts in each case, while two more objects have X-ray properties that are not compatible with the spectral characteristics of late type non-flaring stars.

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arXiv2008

Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54

The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pulsations have not been detected in the time range 0.001-1000 s. A cyclotron line at ~30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Our new long and uninterrupted RXTE observations allow us to search for long (~1 hr) pulsations for the first time. We have discovered 5560-s pulsations in the light curve of 4U 2206+54. Initially detected in RXTE data, these pulsations are also present in INTEGRAL and EXOSAT observations. The average X-ray luminosity in the energy range 2-10 keV is 1.5 x 10^{35} erg s^{-1} with a ratio Fmax/Fmin ~ 5. This ratio implies an eccentricity of ~0.4, somewhat higher than previously suggested. The source also shows a soft excess at low energies. If the soft excess is modelled with a blackbody component, then the size and temperature of the emitting region agrees with its interpretation in terms of a hot spot on the neutron star surface. The source displays variability on time scales of days, presumably due to changes in the mass accretion rate as the neutron star moves around the optical companion in a moderately eccentric orbit.

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arXiv2008

Rapid spectral and timing variability of Be/X-ray binaries during type II outbursts

We have investigated the spectral and timing variability of four accreting X-ray pulsars with Be-type companions during major X-ray outbursts. Different spectral states were defined according to the value of the X-ray colours and flux. Transient Be/X-ray binaries exhibit two branches in their colour-colour and colour-intensity diagrams: the horizontal branch corresponds to a low-intensity state and shows the larger fractional rms, similar to the the island state in atolls and horizontal branch in Z sources; the diagonal branch corresponds to a high-intensity state, in which the source spends about 75% of the total duration of the outburst. Despite the complexity of the power spectra due to the peaks of the pulse period and its harmonics, the aperiodic variability of Be/X-ray binaries can be described with a relatively low number of Lorentzian components. Some of these components can be associated with the same type of noise as that seen in low-mass X-ray binaries, although the characteristic frequencies are about one order of magnitude lower. The pattern traced by V 0332+53 results in a Z shaped track, similar to the low-mass Z sources, without the flaring branch. In contrast, the horizontal branch in 4U 0115+63, KS 1947+300 and EXO 2030+375 corresponds to a low/soft state, not seen in other types of X-ray binaries. The noise at very low frequencies follows a power law in V 0332+53 (like in LMXB Z) and it is flat-topped in 4U 0115+63, KS 1947+300 and EXO 2030+375 (like in LMXB atoll). V 0332+53 shows a noise component coupled with the periodic variability that it is not seen in any of the other three sources.

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arXiv2008

A jet model for Galactic black-hole X-ray sources: Some constraining correlations

Some recent observational results impose significant constraints on all the models that have been proposed to explain the Galactic black-hole X-ray sources in the hard state. In particular, it has been found that during the hard state of Cyg X-1 the power-law photon number spectral index is correlated with the average time lag between hard and soft X-rays. Furthermore, the peak frequencies of the four Lorentzians that fit the observed power spectra are correlated with both the photon index and the time lag. We performed Monte Carlo simulations of Compton upscattering of soft, accretion-disk photons in the jet and computed the time lag between hard and soft photons and the power-law index of the resulting photon number spectra. We demonstrate that our jet model naturally explains the above correlations, with no additional requirements and no additional parameters.

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arXiv2008

The very massive X-ray bright binary system Wack 2134 (= WR 21a)

From the radial velocities of the N IV 4058 and He II 4686 emission lines, and the N V 4604-20 absorption lines, determined in digital spectra, we report the discovery that the X-ray bright emission line star Wack 2134 (= WR 21a) is a spectroscopic binary system with an orbital period of $31.673\pm0.002$ days. With this period, the N IV and He II emission and N V absorption lines, which originate in the atmosphere of the primary component, define a rather eccentric binary orbit (e=0.64$\pm$0.03). The radial velocity variations of the N V absorptions have a lower amplitude than those of the He II emission. Such a behaviour of the emission line radial velocities could be due to distortions produced by a superimposed absorption component from the companion. High resolution echelle spectra observed during the quadrature phases of the binary show H and He II absorptions of both components with a radial velocity difference of about 541 km/s. From this difference, we infer quite high values of the minimum masses, of about 87Mo and 53Mo for the primary and secondary components, respectively, if the radial velocity variations of the He II emission represent the true orbit of the primary. No He I absorption lines are observed in our spectra. Thus, the secondary component in the Wack2134 binary system appears to be an early O type star. From the presence of H, He II and N V absorptions, and N IV and C IV emissions, in the spectrum of the primary component, it most clearly resembles those of Of/WNLha type stars.

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arXiv2008

INTEGRAL long-term monitoring of the Supergiant Fast X-ray Transient XTE J1739-302

In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are X-ray binary systems with a compact companion orbiting a supergiant star which show very short and bright outbursts in a series of activity periods overimposed on longer quiescent periods. Only very recently the first attempts to model the behaviour of these sources have been published, some of them within the framework of accretion from clumpy stellar winds.Our goal is to analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of the clumpy structure of the supergiant wind. We have used INTEGRAL and RXTE/PCA observations in order to obtain broad band (1-200 keV) spectra and light curves of XTE J1739-302 and investigate its X-ray spectrum and temporal variability. We have found that XTE J1739-302 follows a much more complex behaviour than expected. Far from presenting a regular variability pattern, XTE J1739-302 shows periods of high, intermediate, and low flaring activity.

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arXiv2008

Bright flares from the X-ray pulsar SWIFT J1626.6-5156

We have performed a timing and spectral analysis of the X-ray pulsar SWIFT J1626.6-5156 during a major X-ray outburst in order to unveil its nature and investigate its flaring activity. Epoch- and pulse-folding techniques were used to derive the spin period. Time-average and pulse-phase spectroscopy were employed to study the spectral variability in the flare and out-of-flare states and energy variations with pulse phase. Power spectra were obtained to investigate the periodic and aperiodic variability. Two large flares, with a duration of ~450 seconds were observed on 24 and 25 December 2005. During the flares, the X-ray intensity increased by a factor of 3.5, while the peak-to-peak pulsed amplitude increased from 45% to 70%. A third, smaller flare of duration ~180 s was observed on 27 December 2005. The flares seen in SWIFT J1626.6-5156 constitute the shortest events of this kind ever reported in a high-mass X-ray binary. In addition to the flaring activity, strong X-ray pulsations with Pspin=15.3714+-0.0003 s characterise the X-ray emission in SWIFT J1626.6-5156. After the major outburst, the light curve exhibits strong long-term variations modulated with a 45-day period. We relate this modulation to the orbital period of the system or to a harmonic. Power density spectra show, in addition to the harmonic components of the pulsation, strong band-limited noise with an integrated 0.01-100 Hz fractional rms of around 40% that increased to 64% during the flares. A weak QPO (fractional rms 4.7%) with characteristic frequency of 1 Hz was detected in the non-flare emission. The timing (short X-ray pulsations, long orbital period) and spectral (power-law with cut off energy and neutral iron line) properties of SWIFT J1626.6-5156 are characteristic of Be/X-ray binaries.

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arXiv2007

Towards an understanding of the Of?p star HD 191612: optical spectroscopy

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe-O8fp). The Balmer and HeI lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. Metal lines and HeII absorptions (including many selective emission lines but excluding He II 4686A emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with P(orb) = 1542d, e=0.45. We conduct a model-atmosphere analysis of the primary, and find that the system is consistent with a O8: giant with a B1: main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying `clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.

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arXiv2007

Fourier resolved spectroscopy of 4U 1728-34: New Insights into Spectral and Temporal Properties of Low-Mass X-ray Binaries

Using archival RXTE data we derive the 2-16 keV Fourier-resolved spectra of the Atoll source 4U 1728-34 in a sequence of its timing states as its low QPO frequency spans the range between 6 and 94 Hz. The increase in the QPO frequency accompanies a spectral transition of the source from its island to its banana states. The banana-states' Fourier-resolved spectra are well fitted by a single blackbody component with $kT \sim 2-3$ keV depending on the source position in the color -- color diagram and the Fourier frequency, thus indicating that this spectral component is responsible for the source variability on these timescales. This result is in approximate agreement with similar behavior exhibited by the Z sources, suggesting that, as in that case, the boundary layer -- the likely source of the thermal component -- is supported by radiation pressure. Furthermore, it is found that the iron line at $\sim$6.6 keV, clearly present in the averaged spectra, not apparent within the limitations of our measurements in the frequency-resolved spectra irrespective of the frequency range. This would indicate that this spectral component exhibits little variability on time scales comprising the interval $10^{-2}-10^2$ seconds. In the island state the single blackbody model proved inadequate, particularly notable in our lowest frequency band ($0.008-0.8$ Hz). An absorbed powerlaw or an additive blackbody plus hard powerlaw model was required to obtain a satisfactory fit. Statistics do not allow unambiguous discrimination between these possible scenarios.

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arXiv2007

On the neutron star-disc interaction in Be/X-ray binaries

We have investigated the long-term X-ray variability, defined as the root-mean-square (rms) of the ASM RXTE light curves, of a set of galactic Be/X-ray binaries and searched for correlations with system parameters, such as the spin period of the neutron star and the orbital period and eccentricity of the binary. We find that systems with larger rms are those harbouring fast rotating neutron stars, low eccentric and narrow orbits. These relationships can be explained as the result of the truncation of the circumstellar disc. We also present an updated version of the Halpha equivalent width-orbital period diagram, including sources in the SMC. This diagram provides strong observational evidence of the interaction of neutron star with the circumstellar envelope of its massive companion.

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arXiv2007

A jet model for black-hole X-ray sources

A jet model for Galactic black-hole X-ray binaries will be presented that appears to explain several observational characteristics. In particular, it explains the energy spectrum from radio to hard X-rays, the time-lags as a function of Fourier frequency, the increase of the variability amplitude (QPO and high frequency) with increasing photon energy, and the narrowing of the autocorrelation function with increasing photon energy. On the other hand, there are additional observational constraints that no model has tried to explain yet. It is important that we all try to address these constraints if we are to make any progress in understanding black-hole X-ray sources

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arXiv2007

M31N 2005-09c: a fast FeII nova in the disk of M31

Classical novae are quite frequent in M~31. However, very few spectra of M31 novae have been studied to date, especially during the early decline phase. Our aim is to study the photometric and spectral evolution of a M31 nova event close to outburst. We present photometric and spectroscopic observations of M31N 2005-09c, a classical nova in the disk of M31, using the 1.3m telescope of the Skinakas Observatory in Crete (Greece), starting on the 28th September, i.e. about 5 days after outburst, and ending on the 5th October 2005, i.e. about 12 days after outburst. We also have supplementary photometric observations from the La Sagra Observatory in Northern Andalucia, Spain, on September 29 and 30, October 3, 6 and 9 and November 1, 2005. The wavelength range covered by the spectra is from 3565 A to 8365 A. The spectra are of high S/N allowing the study of the evolution of the equivalent widths of the Balmer lines, as well as the identification of non-Balmer lines. The nova displays a typical early decline spectrum that is characterized by many weak FeII multiplet emissions. It is classified as a P$_{fe}$ nova. From the nova light curve, we have also derived its speed class, t=14+-2.5 days. As the nova evolved the Balmer lines became stronger and narrower. The early decline of the expansion velocity of the nova follows a power law in time with an exponent of \~-0.2.

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arXiv2006

The Be/X-ray transient 4U 0115+63/V635 Cassiopeiae. III. Quasi-cyclic variability

4u 0115+63 is one of the most active and best studied Be/X-ray transients. Previous studies of 4u0115+63 have led to the suggestion that it undergoes relatively fast quasi-cyclic activity. However, due to the lack of good coverage of the observations, the variability time scales are uncertain. Our objective is to investigate the long-term behaviour of 4u 0115+63 to confirm its quasi-cyclic nature and to explain its correlated optical/IR and X-ray variability. We have performed optical/IR photometric observations and optical spectroscopic observations of 4u 0115+63 over the last decade with unprecedented coverage. We have focused on the Halpha line variability and the long-term changes of the photometric magnitudes and colours and investigated these changes in correlation with the X-ray activity of the source. results The optical and infrared emission is characterised by cyclic changes with a period of ~ 5 years. This long-term variability is attributed to the state of the circumstellar disc around the Be star companion. Each cycle involves a low state when the disc is very weak or absent and the associated low amplitude variability is orbitally modulated and a high state when a perturbed disc precesses, giving rise to fast and large amplitude photometric changes. X-ray outbursts in 4u 0115+63 come in pairs, i.e., two in every cycle. However, sometimes the second outburst is missing. Our results can be explained within the framework of the decretion disc model. The neutron star acts as the perturbing body, truncating and distorting the disc. The first outburst would occur before the disc is strongly perturbed. The second outburst leads to the dispersal of the disc and marks the end of the perturbed phase.

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arXiv2006

Fourier resolved spectroscopy of 4U 1543-47 during the 2002 outburst

We have obtained Fourier-resolved spectra of the black-hole binary 4U 1543-47 in the canonical states (high/soft, very high, intermediate and low/hard) observed in this source during the decay of an outburst that took place in 2002. Our objective is to investigate the variability of the spectral components generally used to describe the energy spectra of black-hole systems, namely a disk component, a power-law component attributed to Comptonization by a hot corona and the contribution of the iron line due to reprocessing of the high energy (E > 7 keV) radiation. We find that i) the disk component is not variable on time scales shorter than ~100 seconds, ii) the reprocessing emission as manifest by the variability of the Fe Kalpha line responds to the primary radiation variations down to time scales of ~70 ms in the high and very-high states, but longer than 2 s in the low state, iii) the low-frequency QPOs are associated with variations of the X-ray power law spectral component and not to the disk component and iv) the spectra corresponding to the highest Fourier frequency are the hardest (show the flatter spectra) at a given spectral state. These results question models that explain the observed power spectra as due to modulations of the accretion rate alone, as such models do not provide any apparent reason for a Fourier frequency dependence of the power law spectral indices.

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arXiv2006

Spectroscopy of the brightest optical counterparts of X-ray sources in the direction of M~31 and M~33

Recent surveys of the Local Group spiral Galaxies M31 and M33 with XMM-Newton yielded a large number of X-ray sources. As part of the effort to identify and classify the objects responsible for this X-ray emission, we have obtained optical spectra of the brightest optical counterparts of the identified X-ray sources, using the 1.3m Skinakas Telescope. Most of these objects are foreground star candidates. The purpose of the present study is to confirm this identification and to explore the compatibility between the optical spectral classification and the observed X-ray properties of the sources. We have obtained optical spectra for the 14 brightest optical counterparts of X-ray sources identified by XMM-Newton in the direction of M31 and for 21 optical counterparts in the direction of M33, using the 1.3m Skinakas telescope in Crete, Greece. All of the M31 sources and all but one of the M33 sources were confirmed to be foreground stars, of spectral types between A and M. One of the stars is a late M dwarf with H-alpha emission, a flare star, also displaying strong X-ray variability. One of the M~33 sources (lying within the D25 ellipse) corresponds to a previously known background galaxy, LEDA 5899.

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arXiv2006

A 0535+26: Back in business

In May/June 2005, after 10 years of inactivity, the Be/X-ray binary system A 0535+26 underwent a major X-ray outburst. In this paper data are presented from 10 years of optical, IR and X-ray monitoring showing the behaviour of the system during the quiescent epoch and the lead up to the new outburst. The results show the system going through a period when the Be star in the system had a minimal circumstellar disk and then a dramatic disk recovery leading, presumably, to the latest flare up of X-ray emission. The data are interpreted in terms of the state of the disk and its interaction with the neutron star companion.

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