Publications (22)13.84 Total impact
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Article: Searching for flickering statistics in T CrB
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ABSTRACT: We analyze $V$-band photometry of the aperiodic variability in T CrB. By applying a simple idea of angular momentum transport in the accretion disc, we have developed a method to simulate the statistical distribution of flare durations with the assumption that the aperiodic variability is produced by turbulent elements in the disc. Both cumulative histograms with Kolmogorov-Smirnov tests, and power density spectra are used to compare the observed data and simulations. The input parameters of the model $R_{\rm in}$ and $\alpha$ are correlated on a certain interval and the most probable values are an inner disc radius of $R_{\rm in} \simeq 4 \times 10^9$ cm and a viscosity of $\alpha \simeq 0.9$. The disc is then weakly truncated. We find that the majority of turbulent events producing flickering activity are concentrated in the inner parts of the accretion disc. Comment: 9 pages, 10 figures, accepted for publication in MNRAS11/2009; -
Article: SY Cancri, a case for unstable mass transfer?
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ABSTRACT: Intermediate resolution (0.5-1 Angs) optical spectroscopy of the cataclysmic variable SY Cnc reveals the spectrum of the donor star. Our data enable us to resolve the orbital motion of the donor and provide a new orbital solution, binary mass ratio and spectral classification. We find that the donor star has spectral type G8+-2 V and orbits the white dwarf with P=0.3823753 +- 0.0000003 day, K2=88.0 +-2.9 km/s and V sin i=75.5 +- 6.5 km/s. Our values are significantly different from previous works and lead to q=M2/M1=1.18 +- 0.14. This is one of the highest mass ratios known in a CV and is very robust because it is based on resolving the rotational broadening over a large number of metallic absorption lines. The donor could be a slightly evolved main-sequence or descendant from a massive star which underwent an episode of thermal-timescale mass transfer. Comment: accepted for publication in MNRAS, 5 pages, 5 figures07/2009; -
Article: SDSS unveils a population of intrinsically faint cataclysmic variables at the minimum orbital period
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ABSTRACT: We discuss the properties of 137 cataclysmic variables (CVs) which are included in the Sloan Digital Sky Survey (SDSS) spectroscopic data base, and for which accurate orbital periods have been measured. 92 of these systems are new discoveries from SDSS and were followed-up in more detail over the past few years. 45 systems were previously identified as CVs because of the detection of optical outbursts and/or X-ray emission, and subsequently re-identified from the SDSS spectroscopy. The period distribution of the SDSS CVs differs dramatically from that of all the previously known CVs, in particular it contains a significant accumulation of systems in the orbital period range 80--86 min. We identify this feature as the elusive "period minimum spike" predicted by CV population models, which resolves a long-standing discrepancy between compact binary evolution theory and observations. We show that this spike is almost entirely due to the large number of CVs with very low accretion activity identified by SDSS. The optical spectra of these systems are dominated by emission from the white dwarf photosphere, and display little or no spectroscopic signature from the donor stars, suggesting very low-mass companion stars. We determine the average absolute magnitude of these low-luminosity CVs at the period minimum to be =11.6+-0.7. Comparison of the SDSS CV sample to the CVs found in the Hamburg Quasar Survey and the Palomar Green Survey suggests that the depth of SDSS is the key ingredient resulting in the discovery of a large number of intrinsically faint short-period systems. Comment: MNRAS in press, 19 pages05/2009; -
Article: The magnetic SW Sextantis star RX J1643.7+3402
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ABSTRACT: We present time-resolved spectroscopy and circular spectropolarimetry of the SW Sex star RX J1643.7+3402. We find significant polarisation levels exhibiting a variability at a period of 19.38 +- 0.39 min. In addition, emission-line flaring is found predominantly at twice the polarimetric period. These two findings are strong evidences in favour of the presence of a magnetic white dwarf in the system. We interpret the measured periodicities in the context of our magnetic accretion model for SW Sex stars. In contrast with LS Pegasi -the first SW Sex star discovered to have modulated circular polarisation- the polarisation in RX J1643.7+3402 is suggested to vary at 2(omega - Omega), while the emission lines flare at (omega - Omega). However, a 2omega/omega interpretation cannot be ruled out. Together with LS Peg and V795 Her, RX J1643.7+3402 is the third SW Sex star known to exhibit modulated circular polarisation. Comment: 7 pages, 4 figures, accepted for publication in MNRAS02/2009; -
Article: Dynamical constraints on the neutron star mass in EXO 0748-676
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ABSTRACT: We present VLT intermediate resolution spectroscopy of UY Vol, the optical counterpart of the LMXB X-ray burster EXO 0748-676. By using Doppler tomography we detect narrow components within the broad He II 4542 A, 4686 A and 5412 A emission lines. The phase, velocity and narrowness of these lines are consistent with their arising from the irradiated hemisphere of the donor star, as has been observed in a number of LMXBs. Under this assumption we provide the first dynamical constraints on the stellar masses in this system. In particular, we measure K_2>K_em = 300 +/- 10 km/s. Using this value we derive 1 M_sun < M_1 < 2.4 M_sun and 0.11 < q < 0.28. We find M_1 > 1.5 M_sun for the case of a main sequence companion star. Our results are consistent with the presence of a massive neutron star as has been suggested by Ozel (2006), although we cannot discard the canonical value of ~1.4 M_sun. Comment: To be published in MNRAS Letters. 5 pages01/2009; -
Article: On the masses and evolutionary status of the black hole binary GX 339-4. A twin system of XTE J1550-564?
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ABSTRACT: We apply the K-correction to the black hole LMXB GX 339-4 which implies Mx > 6 Msun by only assuming that the companion is more massive than ~0.17 Msun, the lower limit allowed by applying a 'stripped-giant' model. This evolutionary model successfully reproduces the observed properties of the system. We obtain a maximum mass for the companion of M2 < 1.1 Msun and an upper limit to the mass ratio of q(=M2/Mx)< 0.125. The high X-ray activity displayed by the source suggests a relatively large mass transfer rate which, according to the model, results in M2 > 0.3 Msun and Mx > 7 Msun. We have also applied this scenario to the black hole binary XTE J1550-564, which has a similar orbital period but the donor is detected spectroscopically. The model successfully reproduces the observed stellar parameters. Comment: 5 pages, 2 tables, 1 figues. Accepted for publication in MNRAS01/2008; -
Article: Bowen blend echo-tomography of low mass X-ray binaries
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ABSTRACT: We present simultaneous high time resolution (1-10 Hz) X-ray and optical observations of the persistent LMXBs Sco X-1 and V801 Ara(=4U 1636-536). In the case of Sco X-1 we find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of ~11-16s which is consistent with reprocessing in the donor star. We also present the detection of three correlated X-ray/optical bursts in V801 ara. Although this latter project is still in progress our preliminary results obtained by subtracting the Continuum light-curve from the Bowen/HeII data provide evidence of orbital phase dependent echoes from the companion star.10/2007; -
Article: Echoes from the companion star in Sco X-1
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ABSTRACT: We present simultaneous X-ray (RXTE) and optical (ULTRACAM) narrow band (Bowen blend/HeII and nearby continuum) observations of Sco X-1 at 2-10 Hz time resolution. We find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of ~11-16s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ~0.5 when Sco X-1 is at the top of the Flaring Branch. Evidence of echoes is also seen at the bottom of the Flaring Branch but with time-lags of 5-10s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.07/2007; -
Article: SW Sextantis stars: the dominant population of CVs with orbital periods between 3-4 hours
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ABSTRACT: [Abridged] We present time-series optical photometry of five new CVs identified by the Hamburg Quasar Survey. The eclipses observed in HS 0129+2933, HS 0220+0603, and HS 0455+8315 provided very accurate orbital periods of 3.35129827(65), 3.58098501(34), and 3.56937674(26) h, respectively. HS 0805+3822 shows grazing eclipses and has a likely orbital period of 3.2169(2) h. Time-resolved optical spectroscopy of the new CVs (with the exception of HS 0805+3822) is also presented. Radial velocity studies provided an orbital period of 3.55 h for HS 1813+6122, which allowed us to identify the observed photometric signal at 3.39 h as a negative superhump wave. The spectroscopic behaviour clearly identifies these new CVs as new SW Sextantis stars. These new additions increase the number of known SW Sex stars to 35. Almost 40 per cent of the total SW Sex population do not show eclipses, invalidating the requirement of eclipses as a defining characteristic of the class and the models based on a high orbital inclination geometry alone. On the other hand, the predominance of orbital periods in the narrow 3-4.5 h range is becoming more pronounced. In fact, almost half the CVs which populate the 3-4.5 h period interval are definite members of the class. These statistics are confirmed by our results from the Hamburg Quasar Survey CVs. Remarkably, 54 per cent of the Hamburg nova-like variables have been identified as SW Sex stars with orbital periods in the 3-4.5 h range. The observation of this pile-up of systems close to the upper boundary of the period gap is difficult to reconcile with the standard theory of CV evolution, as the SW Sex stars are believed to have the highest mass transfer rates among CVs. Finally, we review the full range of common properties that the SW Sex stars exhibit. Comment: MNRAS, in press, 17 pages and 13 figures. Corrected object id typo in abstract: HS 0855+3822 should be HS 0805+382204/2007; -
Article: A search for evidence of irradiation in Centaurus X-4 during quiescence
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ABSTRACT: We present a study of the neutron star X-Ray Transient Cen X-4. Our aim is to look for any evidence of irradiation of the companion with a detailed analysis of its radial velocity curve, relative contribution of the donor star and Doppler tomography of the main emission lines. To improve our study all our data are compared with a set of simulations that consider different physical parameters of the system, like the disc aperture angle and the mass ratio. We conclude that neither the radial velocity curve nor the orbital variation of the relative donor's contribution to the total flux are affected by irradiation. On the other hand, we do see emission from the donor star at H${\alpha}$ and HeI 5876 which we tentatively attribute to irradiation effects. In particular, the H${\alpha}$ emission from the companion is clearly asymmetric and we suggest is produced by irradiation from the hot-spot. Finally, from the velocity of the HeI 5876 spot we constrain the disc opening angle to alpha=7-14 deg. Comment: 4 pages, 5 figures, accepted for publication in A&A as a RN09/2006; -
Article: A ZZ Ceti white dwarf in SDSS J133941.11+484727.5
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ABSTRACT: We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSSJ133941.11+484727.5 (SDSS1339) which has been discovered in the Sloan Digital Sky Survey Data Release 4. The orbital period determined from radial velocity studies is 82.524(24)min, close to the observed period minimum. The optical spectrum of SDSS1339 is dominated to 90% by emission from the white dwarf. The spectrum can be successfully reproduced by a three-component model (white dwarf, disc, secondary) with Twd=12500K for a fixed log g=8.0, d=170pc, and a spectral type of the secondary later than M8. The mass transfer rate corresponding to the optical luminosity of the accretion disc is very low,~1.7x10^-13Msun/yr. Optical photometry reveals a coherent variability at 641s with an amplitude of 0.025mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photometric variation with a period of either 320min or 344min and an amplitude of 0.025mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photometric signals have been found in the CVs SDSSJ123813.73-033933.0, SDSSJ204817.85-061044.8, GW Lib and FS Aur, but so far no working model for this behaviour is available. Comment: MNRAS, in press, 8 pages, 10 figures, some figures downgraded to meet the file size constraint of arxiv.org10/2005; -
Article: Echo Tomography of Sco X-1 using Bowen Fluorescence Lines
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ABSTRACT: We present preliminary results of a simultaneous X-ray/optical campaign of the prototypical LMXB Sco X-1 at 1-10 Hz time resolution. Lightcurves of the high excitation Bowen/HeII emission lines were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with X-ray lightcurves. We find evidence for correlated variability, in particular when Sco X-1 enters the Flaring Branch. The Bowen/HeII lightcurves lag the X-ray lightcurves with a light travel time which is consistent with reprocessing in the companion star. Comment: Contribution presented at the conference "Interacting Binaries: Accretion, Evolution and Outcomes", held in Cefalu, Sicily (Italy) in July 2004. To be published by AIP (American Institute of Physics), eds. L. A. Antonelli, L. Burderi, F. D'Antona, T. Di Salvo, G.L. Israel, L. Piersanti, O. Straniero, A. Tornambe. 6 pages, 5 figures03/2005; -
Article: Rotational Broadening and Doppler Tomography of the Quiescent X-Ray Nova Centaurus X-4
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ABSTRACT: We present high and intermediate resolution spectroscopy of the X-ray nova Centaurus X-4 during its quiescent phase. Our analysis of the absorption features supports a K3-K5V spectral classification for the companion star, which contributes approximately 75 % of the total flux at Halpha. Using the high resolution spectra we have measured the secondary star's rotational broadening to be V_rot*sin(i) = 43 +/- 6 km/s and determined a binary mass ratio of q=0.17 +/- 0.06. Combining our results for K_2 and q with the published limits for the binary inclination, we constrain the mass of the compact object and the secondary star to the ranges 0.49 < M_1 < 2.49 Msun and 0.04 < M_2 < 0.58 Msun. A Doppler image of the Halpha line shows emission coming from the secondary star, but no hotspot is present. We discuss the possible origins of this emission. Comment: 10 pages, 4 figures, accepted by MNRAS03/2002; -
Article: Detection of variable circular polarization in the SW Sex star V795 Herculis
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ABSTRACT: We report the detection of modulated circular polarization in V795 Her. The degree of polarization increases with wavelength and is modulated with a period of 19.54 min, which is very close to the reported optical QPO period. The modulation has a peak-to-peak amplitude of 0.12% in the U-band. The estimated magnetic field intensity is in the range 2-7 MG.11/2001; -
Article: V348 Puppis: a new SW Sex star in the period gap
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ABSTRACT: We present time-resolved optical spectroscopy and photometry of the nova-like cataclysmic variable V348 Puppis. The system displays the same spectroscopic behaviour as SW Sex stars, so we classify V348 Pup as a new member of the class. V348 Pup is the second SW Sex system (the first is V795 Herculis) which lies in the period gap. The spectra exhibit enhanced HeII 4686 emission, reminiscent of magnetic cataclysmic variables. The study of this emission line gives a primary velocity semi-amplitude of K1 ~= 100 km/s. We have also derived the system parameters, obtaining: M1 ~= 0.65 Msun, M2 ~= 0.20 Msun (q ~= 0.31), i ~= 80 deg and K2 ~= 323 km/s. The spectroscopic behaviour of V348 Pup is very similar to that of V795 Her, with the exception that V348 Pup shows deep eclipses. We have computed the ``0.5-absorption'' spectrum of both systems, obtaining spectra which resemble the absorption spectrum of a B0 V star. We propose that absorption in SW Sex systems can be produced by a vertically extended atmosphere which forms where the gas stream re-impacts the system, either at the accretion disc or at the white dwarf's magnetosphere (assuming a magnetic scenario). Comment: 6 pages, 10 figures, accepted for publication in MNRAS09/2001; -
Article: Evidence of magnetic accretion in an SW Sex star: discovery of variable circular polarization in LS Pegasi
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ABSTRACT: We report on the discovery of variable circular polarization in the SW Sex star LS Pegasi. The observed modulation has an amplitude of ~0.3 % and a period of 29.6 minutes, which we assume as the spin period of the magnetic white dwarf. We also detected periodic flaring in the blue wing of Hbeta, with a period of 33.5 minutes. The difference between both frequencies is just the orbital frequency, so we relate the 33.5-min modulation to the beat between the orbital and spin period. We propose a new accretion scenario in SW Sex stars, based on the shock of the disk-overflown gas stream against the white dwarf's magnetosphere, which extends to the corotation radius. From this geometry, we estimate a magnetic field strength of B(1) ~ 5-15 MG. Our results indicate that magnetic accretion plays an important role in SW Sex stars and we suggest that these systems are probably Intermediate Polars with the highest mass accretion rates. Comment: Accepted by ApJ Letters. LaTeX, 14 pages, 3 PostScript figures11/2000; -
Article: Long-term photometry of WX Arietis: evidence for eclipses and dips
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ABSTRACT: We present R-band photometry of the SW Sex-type cataclysmic variable WX Arietis made in October 1995 and August 1998-February 1999. Contrary to previous results, we find that WX Ari is an eclipsing system with an orbital inclination of ~72 deg. The R-band light curves display highly variable, shallow eclipses ~0.15-mag deep and ~40 min long. The observed eclipse depth suggests a partial eclipse of the accretion disc. The light curves also show a wide dip in brightness centred at orbital phase ~0.75 and a hump close to the opposite phase at ~0.2. The observed dip may be explained by the probable vertical thickening of the outer rim of the accretion disc downstream from the bright spot. We also demonstrate that the disc brightness in all SW Sex systems is nearly the same. This implies that the orbital inclination of these systems is only a function of eclipse depth.12/1999; -
Article: Infrared Light Curves and Spectroscopic Classification of the Algol System UX HER
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ABSTRACT: We present the first light curves in the infrared J, H, and K bands of UX Her and analyze them together with published light curves in B and V in order to determine the physical parameters of the system. Based on the spectra of UX Her and standard stars in the region 8440-8870 Å, we classify the primary component of the binary as a type A2-3 V star. From the absolute radii of the stars that we derive from the light-curve analysis, we find that none of the components of the binary are filling their Roche lobes. Considering the contributions of the secondary star to the total light given by the light-curve solutions, and the measured B, V, J, H, and K magnitudes at maxima and during the eclipses, we find good agreement with those predicted from stellar atmosphere models for effective temperatures of the stellar components: Teff,1≍9000 K and Teff.2˜4500 K, with the system at a distance d≍280 pc.The Astronomical Journal 02/1997; 113:1122. · 4.03 Impact Factor -
Article: A coordinated campaign of the intermediate polar AE aqr. 1: The system parameters
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ABSTRACT: We report intermediate resolution spectroscopy (50 km/s) of the magnetic Cataclysmic Variable AE Aqr performed as part of the World Astronomy Days (WAD) campaign. Our analysis of the absorption features supports a K4 classification for the companion star, which contributes more than 95% of the total flux in the range 600-700 nm. We find that the companion star fills its Roche lobe since we detect a 10% modulation in the rotational broadening (V sin i) of the absorption fines, due to the changing geometry. Model fitting to the V sin i(phi) curve enables us to measure directly the system inclination (i = 58 +/- 6 deg) and the gravity darkening coefficient (beta = 0.08 +/- 0.01). Improved orbital parameters are also presented, including an updated mass ratio (q = K1/K2 = 0.630 +/- 0.012) based upon our K2 value and the spin-pulse delay. An independent determination of q is also provided by correcting the phase-averaged V sin i for non-spherical effects. The need for these two values to agree constrain the limb-darkening coefficient to be epsilon approximately equal to 0.40 (for beta = 0.08). The component masses derived from our values of K2, q and i are M1 = 0.79 +/- 0.16 solar mass units and M2 = 0.50 +/- 0.10 solar mass units. In addition, we do not see spectral evidence of irradiation effects.Monthly Notices of the Royal Astronomical Society 08/1996; 282:182-190. · 4.90 Impact Factor -
Article: V795 Her: an SW Sex star in the period gap?
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ABSTRACT: We present optical photometry and spectroscopy of the nova-like variable V795 Her. The emission lines exhibit an extremely complex structure consisting of several time-variable emission and absorption components. We confirm the spectroscopic modulation of 0.1082d which we attribute to the orbital period of the system. The equivalent widths (EWs) of all the emission lines are strongly modulated on this period, showing a pronounced minimum around phase 0.5. We associate this minimum with the transit of an absorption feature which drives dramatic line changes, seen as transient double peaks and PCygni type profiles. The absorption depth grows with the excitation level in Balmer and HeI lines, but both the HeII lambda4686 and the metallic lines (e.g. CII lambda4267 and the Bowen blend) are entirely in emission. The centroid of the Hα emission (which is contaminated very little by the absorption feature) is delayed by 83 deg with respect to the high-excitation lines. We assume that the latter trace the orbital motion of the primary, enabling us to define an absolute zero-phase. High-velocity S-waves are clearly observed in all Balmer lines, with amplitudes of about 1750 km s^-1 and maximum redshift at about phase 0. However, our high-resolution spectra suggest that these can be separated into two phased components, with gamma-velocities of ~+/-750 km s^-1 and K~1000 km s^-1, probably formed in accretion columns on the white dwarf. In addition, the HI cores exhibit the presence of low-velocity (K=268 km s^-1) S-waves, roughly phased with their high-amplitude partners. On the other hand, our R-band photometry is dominated by flickering with no evidence for modulation with the orbital period nor the previously reported 0.1165-d periodicity. Based on the spectral properties, we propose that V795 Her is a new SW Sex star, observed at intermediate inclination. The complex behaviour of V795 Her can be qualitatively explained invoking an intermediate polar scenario with a synchronously rotating white dwarf.Monthly Notices of the Royal Astronomical Society 12/1995; 278:219-235. · 4.90 Impact Factor
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Institutions
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2007–2008
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Universidad de La Laguna
- Departamento de Astrofísica
La Laguna, Canary Islands, Spain -
The University of Sheffield
- Department of Physics and Astronomy
Sheffield, ENG, United Kingdom
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1999
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Instituto de Astrofísica de Canarias
La Laguna, Canary Islands, Spain
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