arXiv subjects

J. Casares

Publications and source records attributed to J. Casares.

At least 145 records · Page 8Linked to original sources

arXiv2005

A possible black hole in the gamma-ray microquasar LS 5039

The population of high energy and very high energy gamma-ray sources, detected with EGRET and the new generation of ground-based Cherenkov telescopes, conforms a reduced but physically important sample. Most of these sources are extragalactic (e.g., blazars), while among the galactic ones there are pulsars and SN remnants. The microquasar LS 5039, previously proposed to be associated with an EGRET source by Paredes et al. (2000), has recently been detected at TeV energies, confirming that microquasars should be regarded as a class of high energy gamma-ray sources. To model and understand how the energetic photons are produced and escape from LS 5039 it is crucial to unveil the nature of the compact object, which remains unknown. Here we present new intermediate-dispersion spectroscopy of this source which, combined with values reported in the literature, provides an orbital period of 3.90603+/-0.00017 d, a mass function f(M)=0.0053+/-0.0009 M_sun, and an eccentricity e=0.35+/-0.04. Atmosphere model fitting to the spectrum of the optical companion, together with our new distance estimate of d=2.5+/-0.1 kpc, yields R_opt=9.3+0.7-0.6 R_sun, log (L_opt/L_sun)=5.26+/-0.06, and M_opt=22.9+3.4-2.9 M_sun. These, combined with our dynamical solution and the assumption of pseudo-synchronization, yield an inclination i=24.9+/-2.8 degree and a compact object mass M_X=3.7+1.3-1.0 M_sun. This is above neutron star masses for most of the standard equations of state and, therefore, we propose that the compact object in LS 5039 is a black hole. We finally discuss about the implications of our orbital solution and new parameters of the binary system on the CNO products, the accretion/ejection energetic balance, the SN explosion scenario, and the behaviour of the TeV emission with the new orbital period.

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arXiv2005

Doppler tomography of the transient X-ray binary Centaurus X-4 in quiescence

We present ESO-NTT low resolution spectroscopy of the neutron star X-ray transient Cen X-4 in quiescence over a complete orbital cycle. Our data reveal the presence of a K3-7 V companion which contributes 63% to the 5600-6900A flux and orbits the neutron star with a velocity semi-amplitude of K_2=145.8 +/- 1.0 km s^{-1}. This, combined with a previous determination of the inclination angle and mass ratio, yields a neutron star and companion mass of M_1=1.5 +/- 1.0 M_Sun and M_2=0.31 +/- 0.27 M_Sun, respectively. The mass donor is thus undermassive for the inferred spectral type indicating it is probably evolved, in agreement with previous studies. Doppler tomography of the H_alpha line shows prominent emission located on the companion and a slightly asymmetric accretion disc distribution similar to that seen in systems with precessing eccentric discs. Strong H_alpha emission from the companion can be explained by X-ray irradiation from the primary. No evidence is found for a hot spot in H_alpha, whereas one is revealed via Doppler tomography of the HeI lines. This can be interpreted as the hot spot and outer regions of the disc being at a higher temperature than in other systems.

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arXiv2005

The "K-Correction" for Irradiated Emission Lines in LMXBs: Evidence for a Massive Neutron Star in X1822-371 (V691 CrA)

We study the K-correction for the case of emission lines formed in the X-ray illuminated atmosphere of a Roche lobe filling star. We compute the K-correction as function of the mass ratio 'q' and the disc flaring angle 'alpha' using a compact binary code where the companion's Roche lobe is divided into 10^5 resolution elements. We also study the effect of the inclination angle in the results. We apply our model to the case of the neutron star low-mass X-ray binary X1822-371 (V691 CrA), where a K-emission velocity K_em=300 +-8 km/s has been measured by Casares et al. (2003). Our numerical results, combined with previous determination of system parameters, yields 1.61Msun < M_NS < 2.32Msun and 0.44Msun < M_2 < 0.56Msun for the two binary components(i. e. 0.24 < q < 0.27), which provide a compelling evidence for a massive neutron star in this system. We also discuss the implications of these masses into the evolutionary history of the binary.

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arXiv2005

ULTRACAM observations of the black hole X-ray transient XTE J1118+480 in quiescence

We present high time-resolution multicolour observations of the quiescent soft X-ray transient XTE J1118+480 obtained with ULTRACAM. Superimposed on the double-humped continuum g' and i'-band lightcurves are rapid flare events which typically last a few minutes. The power density spectrum of the lightcurves can be described by a broken power-law model with a break frequency at ~2 mHz or a power-law model plus a broad quasi-periodic oscillation (QPO) at ~2 mHz. In the context of the cellular-automaton we estimate the size of the quiescent advection-dominated flow (ADAF) region to be ~10^4 Schwarzschild radii, similar to that observed in other quiescent black hole X-ray transients, suggesting the same underlying physics. The similarites between the low/hard and quiescent state PDS suggest a similar origin for the optical and X-ray variability, most likely from regions at/near the ADAF.

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arXiv2005

Multiband Echo Tomography of Sco X-1

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 and a red continuum at lambda ~ 6000 Ang were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with simultaneous RXTE 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 while the continuum lightcurves have shorter delays consistent with reprocessing in the accretion disc

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arXiv2005

Time-Delayed transfer functions simulations for LMXBs

Recent works (Steeghs & Casares 2002, Casares et al. 2003, Hynes et al. 2003) have demonstrated that Bowen flourescence is a very efficient tracer of the companion star in LMXBs. We present a numerical code to simulate time-delayed transfer functions in LMXBs, specific to the case of reprocessing in emission lines. The code is also able to obtain geometrical and binary parameters by fitting observed (X-ray + optical) light curves using simulated annealing methods. In this work we present the geometrical model for the companion star and the analytical model for the disc and show synthetic time-delay transfer functions for different orbital phases and system parameters.

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arXiv2005

Orbital Parameters of the Microquasar LSI +61 303

New optical spectroscopy of the HMXB microquasar LSI +61 303 is presented. Eccentric orbital fits to our radial velocity measurements yield updated orbital parameters in good agreement with previous work. Our orbital solution indicates that the periastron passage occurs at radio phase 0.23 and the X-ray/radio outbursts are triggered 2.5--4 days after the compact star passage. The spectrum of the optical star is consistent with a B0 V spectral type and contributes ~65 percent of the total light, the remaining due to emission by a circumstellar disc. We also measure the projected rotational velocity to be v sin i = 113 km/s.

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arXiv2005

Echo Tomography of Sco X-1 using Bowen Fluorescence Lines

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.

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arXiv2005

On the kinematics of the neutron star low mass X-ray binary Cen X-4

We present the first determination of the proper motion of the neutron star low mass X-ray binary {Cen X-4} measured from relative astrometry of the secondary star using optical images at different epochs. We determine the Galactic space velocity components of the system and find them to be significantly different from the mean values that characterize the kinematics of stars belonging to the halo, and the thin and the thick disc of the Galaxy. The high metallicity of the secondary star of the system rules out a halo origin and indicates that the system probably originated in the Galactic disc. A statistical analysis of the galactocentric motion revealed that this binary moves in a highly eccentric ($e\simeq 0.85\pm0.1$) orbit with an inclination of $\simeq 110^\circ$ to the Galactic plane. The large Galactic space velocity components strongly support that a high natal kick as a result of a supernova explosion could have propelled the system into such an orbit from a birth place in the Galactic disc. The high Li abundance in the secondary, comparable to that of stars in star forming regions and young stellar clusters like the Pleiades, may suggest a relatively recent formation of the system. Following the orbit backwards in time, we found that the system could have been in the inner regions of the Galactic disc $\sim$100--200 Myr ago. The neutron star might have formed at that moment. However, we cannot rule out the possibility that the system formed at a much earlier time if a Li production mechanism exists in this LMXB.

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arXiv2004

The massive neutron star or low-mass black hole in 2S0921-630

We report on optical spectroscopy of the eclipsing Halo LMXB 2S0921-630, that reveals the absorption line radial velocity curve of the K0III secondary star with a semi-amplitude K_2=92.89 +/- 3.84 km/s, a systemic velocity $γ$=34.9 +/- 3.3 \kms and an orbital period P_orb of 9.0035 +/- 0.0029 day (1-sigma). Given the quality of the data, we find no evidence for the effects of X-ray irradiation. Using the previously determined rotational broadening of the mass donor, and applying conservative limits on the orbital inclination, we constrain the compact object mass to be 2.0-4.3 Msolar (1-sigma), ruling out a canonical neutron star at the 99% level. Since the nature of the compact object is unclear, this mass range implies that the compact object is either a low-mass black hole with a mass slightly higher than the maximum neutron star mass (2.9 Msolar) or a massive neutron star. If the compact object is a black hole, it confirms the prediction of the existence of low-mass black holes, while if the object is a massive neutron star its high mass severely constrains the equation of state of nuclear matter.

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arXiv2004

Indirect evidence for an active radio pulsar in the quiescent state of the transient ms pulsar SAX J1808.4-3658

Millisecond radio pulsars are neutron stars that have been spun-up by the transfer of angular momentum during the low-mass X-ray binary phase. The transition from an accretion-powered to a rotation-powered pulsar takes place on evolutionary timescales at the end of the accretion process, however it may also occur sporadically in systems undergoing transient X-ray activity. We have obtained the first optical spectrum of the low mass transient X-ray pulsar SAX J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X-ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ~100 larger than directly observed, must be present in the system. This most likely derives from a rotation-powered neutron star that resumes activity during quiescence.

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arXiv2004

Evidence for a Black Hole in the X-ray transient GS 1354-64 (=BW Cir)

We present the first radial velocity curve of the companion star to BW Cir which demonstrates the presence of a black hole in this X-ray transient which recorded outbursts in 1987 and 1997 (and possibly 1971-2). We identify weak absorption features corresponding to a G0-5III donor star, strongly veiled by a residual accretion disc which contributes 61-65 % of the total light at lambda 6300. The Doppler motions of these features trace an orbit of P=2.54448 days (or its 1-yr alias of P=2.56358 days) and velocity semi-amplitude K2=279 +- 5 km/s (or K2=292 +- 5 km/s). Both solutions are equally possible. The mass function implied by the shorter period solution is f(M)= 5.75 +- 0.30 Msun which, combined with the rotational broadening of the tidally locked companion (V sin i =71 +- 4 km/s), yields a compact object mass of M1 sin3 i = 7.34 +- 0.46 Msun. This is substantially above the mass of a neutron star under any standard equation of state of nuclear matter. The companion star is probably a G subgiant which has evolved off the main sequence in order to fill its Roche lobe. Remarkably, a distance of > 27 kpc is inferred by the companion's luminosity and this is supported by the large observed systemic velocity (gamma = 103 +- 4 km/s) which requires such a distance in order to be consistent with the Galactic rotation curve.

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arXiv2004

Optical polarimetric observations of the microquasar LS 5039

We present the first optical polarimetric observations of the runaway microquasar LS 5039. Our results reveal the presence of a large amount (~5%) of polarized emission towards this binary system. By combining polarimetric and spectroscopic observations of some stars in the field together with available statistical information on the galactic interstellar polarization of the region, we have estimated and subtracted the contribution of the interstellar polarization in this direction. As a result, we obtain an intrinsic polarization of ~3% for the object, much higher than what would be expected from jet emission in the optical domain. We suggest that the polarized light originates by electron Thomson scattering in the stellar envelope of the companion star. This allows us to constrain the size of the region where the polarized emission originates, as well as estimating the scattering electronic density and the wind velocity at such distance.

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arXiv2004

Optical spectroscopy of flares from the black hole X-ray transient A0620-00 in quiescence

We present a time-resolved spectrophotometric study of the optical variability in the quiescent soft X-ray transient A0620-00. Superimposed on the double-humped continuum lightcurve are the well known flare events which last tens of minutes. Some of the flare events that appear in the continuum lightcurve are also present in the emission line lightcurves. From the Balmer line flux and variations, we find that the persistent emission is optically thin. During the flare event at phase 1.15 the Balmer decrement dropped suggesting either a significant increase in temperature or that the flares are more optically thick than the continuum. The data suggests that there are two HI emitting regions, the accretion disc and the accretion stream/disc region, with different Balmer decrements. The orbital modulation of H$α$ with the continuum suggests that the steeper decrement is most likely associated with the stream/disc impact region. We construct Doppler images of the H$α$ and H$β$ emission lines. Apart from showing enhanced blurred emission at the region where the stream impacts the accretion disc, the maps also show significant extended structure from the opposite side of the disc. The trailed spectra show characteristic S-wave features that can be interpreted in the context of an eccentric accretion disc (abridged).

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arXiv2004

Correlated X-ray and Optical Variability in V404 Cyg in Quiescence

We report simultaneous X-ray and optical observations of V404 Cyg in quiescence. The X-ray flux varied dramatically by a factor of >20 during a 60ks observation. X-ray variations were well correlated with those in Halpha, although the latter include an approximately constant component as well. Correlations can also be seen with the optical continuum, although these are less clear. We see no large lag between X-ray and optical line variations; this implies they are causally connected on short timescales. As in previous observations, Halpha flares exhibit a double-peaked profile suggesting emission distributed across the accretion disk. The peak separation is consistent with material extending outwards to at least the circularization radius. The prompt response in the entire Halpha line confirms that the variability is powered by X-ray (and/or EUV) irradiation.

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arXiv2004

The bright optical afterglow of the long GRB 001007

We present optical follow up observations of the long GRB 001007 between 6.14 hours and ~468 days after the event. An unusually bright optical afterglow (OA) was seen to decline following a steep power law decay with index alpha = -2.03 +/- 0.11, possibly indicating a break in the light curve at t - to < 3.5 days, as found in other bursts. Upper limits imposed by the LOTIS alerting system 6.14 hours after the gamma ray event provide tentative (1.2 sigma) evidence for a break in the optical light curve. The spectral index beta of the OA yields -1.24 +/- 0.57. These values may be explained both by several fireball jet models and by the cannonball model. Fireball spherical expansion models are not favoured. Late epoch deep imaging revealed the presence of a complex host galaxy system, composed of at least two objects located 1.2" (1.7 sigma) and 1.9" (2.7 sigma) from the afterglow position.

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arXiv2004

Long-term optical/infrared variability in the quiescent X-ray transient V404 Cyg

We present the results of optical and infrared photometry of the quiescent X-ray transient V404 Cyg during the period 1992-2003. The ellipsoidal modulations extracted from the most complete databases (years 1992, 1998 and 2001) show unequal maxima and minima with relative strength varying from year to year although their peak to peak amplitudes remain roughly constant at 0.24+-0.01 magnitudes. Fast optical variations superimposed on the secondary star's double-humped ellipsoidal modulation were detected every year with a mean amplitude of ~0.07 mags. We have not found significant changes in the activity during this decade which indicates that this variability is probably not connected to the 1989 outburst. We have found periodicities in the 1998 and 2001 data near the 6 hr quasi-periodicity observed in 1992, although we interpret it as consequence of the appearance of a flare event almost every night. Significant variability is also present in the I and near infrared (J and K_short) bands and this decreases slightly or remains approximately constant at longer wavelengths. A cross correlation analysis shows that both the R and I emission are simultaneous down to 40 s.

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arXiv2004

Chemical Abundances in the Secondary Star in the Black Hole Binary A0620-00

Using a high resolution spectrum of the secondary star in the black hole binary A0620-00, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have used a chi^2 minimization procedure to explore a grid of 800 000 LTE synthetic spectra computed for a plausible range of both stellar and veiling parameters. Adopting the best model parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni and Al. The Fe abundance of the star is [Fe/H]=0.14 +- 0.20. Except for Ca, we found the other elements moderately over-abundant as compared with stars in the solar neighborhood of similar iron content. Taking into account the small orbital separation, the mass transfer rate and the mass of the convection zone of the secondary star, a comparison with element yields in supernova explosion models suggests a possible explosive event with a mass cut comparable to the current mass of the compact object. We have also analyzed the Li abundance, which is unusually high for a star of this spectral type and relatively low mass.

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