The mass ratio in IGR J19308+0530

IGR J19308+0530: Roche lobe overflow on to a compact object from a donor 1.8 times as massive thanks: Observations made with the HERMES spectrograph at the Mercator Telescope, operated on La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrof’sica de Canarias. HERMES is supported by the Fund for Scientific Research of Flanders, the Research Council of K.U.Leuven and the Fonds National Recherches Scientific (FNRS), Belgium, the Royal Observatory of Belgium, the Observatoire de Genéve, Switzerland and the ThŸringer Landessternwarte Tautenburg, Germany.

E.M. Ratti, T.F.J. van Grunsven, M.A.P. Torres, P.G. Jonker, J. C. A. Miller-Jones, J. W.T. Hessels, H. Van Winckel, M. van der Sluys, G. Nelemans
SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
Department of Astrophysics/IMAPP, Radboud University Nijmegen, Heyendaalseweg 135,6525 AJ, Nijmegen, The Netherlands
Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A.
International Centre for Radio Astronomy Research - Curtin University, GPO Box U1987, Perth, WA 6845, Australia
ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
Astronomical Institute ”Anton Pannekoek,” University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
Nikhef National Institute for Subatomic Physics, Science Park 105, 1098 XG Amsterdam, The Netherlands
email : e.m.ratti@sron.nl
Abstract

We present phase-resolved spectroscopy and photometry of the optical counterpart to the X–ray binary IGR J19308+0530. Ellipsoidal modulations in the lightcurve show that the F-type companion star in the system is Roche-lobe filling. The optical spectra are dominated by absorption features from the donor star, with 10-20% disc contribution to the optical continuum. We measure an orbital period of 14.6620.001 hours, a radial velocity semi-amplitude for the companion star of K91.41.4 and a rotational broadening of . From K and , given that the donor star is filling its Roche lobe, we derive a mass ratio of MM 1.78, which is typically considered to be too large for stable Roche-lobe overflow. Our observations support an inclination of 50 degrees. The accretor in IGR J19308+0530 is most likely a white dwarf, although a neutron star cannot entirely be excluded.

keywords:
stars: individual (IGR J19308+0530) — accretion: accretion discs — stars: binaries — X-rays: binaries

1 introduction

Intermediate-mass X–ray binaries (IMXBs) are binary systems where a compact object - black hole, neutron star (NS) or white dwarf (WD) - is accreting matter from a companion star of spectral type A or F. IMXBs are rarely observed (see, e.g., the catalogue from Liu et al. 2007 and the 2012 version of the RK catalogue Ritter & Kolb 2003111http://physics.open.ac.uk/RKcat/). The majority of accreting WDs in binaries, in fact, belongs to the class of cataclysmic variables (CVs), which have late type secondaries () and hours (Knigge et al., 2011). NSs or BHs, instead, are typically observed in X–ray binaries (XRBs) hosting either a massive O-B donor star (high-mass XRBs, HMXBs) driving accretion via stellar wind, or a late M or K dwarf secondary star (low-mass XRBs, LMXBs) accreting via Roche lobe overflow. The reason for the observed rarity of IMXBs especially among NS and WD systems is that, when the companion is more massive than the accretor but not massive enough to have strong winds, wind accretion proceeds at a very low rate and Roche-lobe accretion is thought to be unstable. For NSs and WDs in IMXBs mass flows from the more massive to the lighter star and angular momentum conservation shrinks the orbit, leading to enhanced mass transfer. The bright X-ray binary phase is therefore intense and short-lived, causing an observational bias towards LMXBs, CVs and HMXBs (Tauris & van den Heuvel, 2006). Nevertheless, IMXBs could be a large fraction of the XRB population and have an important role in understanding their evolution (Podsiadlowski et al., 2001). Cyg X–2 and Her X–1 are though to have started as IMXBs, even though the measured mass ratio is currently .
IGR J19308+0530 was discovered by INTEGRAL (Bird et al., 2006) and observed by Swift (Rodriguez et al., 2008). An association with the star TYC 486-295-1, classified as an F8 star in the survey by McCuskey (1949), was made using the Swift position (Rodriguez et al., 2008). This was confirmed using an accurate Chandra position of the X-ray source (Ratti et al., 2010). Considering typical parameters of an F8 star, Rodriguez et al. (2008) suggested IGR J19308+0530 to be a L/IMXB in quiescence or a CV at a distance of .
Here, we present phase-resolved optical spectroscopy and photometry of IGR J19308+0530, in order to measure the orbital period , the radial velocity semi-amplitude and the projected rotational velocity of the companion star, and the system inclination . In a Roche lobe filling system, and allow us to infer the ratio MM between the mass of the secondary and the primary star in the system (Wade & Horne 1988, see also Gray 1992) and, knowing P and , to solve the system mass function.

2 Observations and data reduction

In total twenty-two high-resolution spectra of IGR J19308+0530 were collected. Observations were made on two nights in 2010 Mar., one night in 2010 Apr. and 9 nights in 2010 Jun. using the fiber spectrograph High Efficiency and Resolution Mercator Echelle Spectrograph (HERMES), mounted at the Mercator telescope in La Palma (Raskin et al., 2011). The typical exposure time was 1200 seconds. The spectra have a dispersion of 0.027 /pixel at 5000 and cover the wavelength range 3770-7230 . The fiber aperture is 25 on the sky, but the presence of a slicer mimics a narrow slit providing a resolution of 85000 irrespective of the seeing. The template star HD185395, of spectral type F4 V (later found to be the closest match to the spectral type of the target, see below) was observed with the same settings for 360 seconds on 2009 Aug. 5. The extraction of the spectra was performed through the dedicated automated data reduction pipeline HermesDRS. For each spectrum we selected two regions for the analysis, one covering the H and H lines (4280-5250 ) and one around the H line (5950-6700 ), which we will refer to as S1 and S2, respectively. These regions were selected as they are rich in stellar lines, with little contamination from interstellar features. A good fit of the continuum was achieved in each region with a polynomial function of order 9. We normalized the spectra dividing by the polynomial fit.

We also performed time-resolved photometry of IGR J19308+0530, with the 80-cm IAC80 telescope at the Observatorio del Teide in Tenerife equipped with the CAMELOT CCD imager. The observations were obtained during part of three nights between 2010 Jul. 30 and Aug. 02, by cycling through the Sloan g, r, i and z-band filters. Three consecutive exposures were obtained in each filter, with integration times ranging from 6 to 24 sec depending on the filter and seeing conditions. Standard stars were not taken due to non-photometric weather. After debiasing and flat-fielding the images using dome flat-field observations (with standard routines in IRAf) the instrumental magnitudes of IGR J19308+0530 and four comparison stars were computed by means of aperture photometry. Differential lightcurves were then obtained for IGR J19308+0530 with respect to the comparison star TYC 486-968-1. We also extracted lightcurves for TYC 486-968-1 using the other comparison stars. No significant variability was detected in the -band (r.m.s. mag), whereas for the other bands a larger scatter was observed due to weather conditions (clouds and Calima). The maximum departure from the mean value was of 0.1 mag. Given that the ellipsoidal modulation in IGR J19308+0530 is small in amplitude we decided to model only the -band lightcurves.

3 Analysis and results


Figure 1: Optimal subtraction of the spectrum of the F4V stellar template HD185395 from the spectrum of IGR J19308+0530. The image shows the part of spectral region S2 used to measure . The interstellar feature at 6280 was masked. The template and target spectrum are offset by 0.4 and 0.2 in y.

3.1 Spectroscopy

The spectra are dominated by absorption features from the secondary star (LABEL:fig:opt_sub). Although no emission line is directly visible, residual emission in the Balmer lines appears when subtracting the spectra one from another, after correcting for the orbital shift of the lines. The emission component is variable in intensity and wavelength and slightly shifted with respect to the absorption line.
Using the package molly developed by T.Marsh, we measured the orbital velocity of the companion star in IGR J19308+0530 by cross-correlating the spectra of the target with that of the template star. As the absorption lines are rotationally broadened in the target spectrum, a broadening of 100 was applied to the template (see below on rotational broadening), improving the cross-correlation. The Balmer lines and interstellar features were masked.
We performed a fit of the velocities versus time with a sine function, with , the systemic radial velocity , , and as free parameters. was constrained to be near the middle of the time span over which the observations were taken, and such that phase 0 is at the inferior conjunction of the companion star. The best-fitting sinusoid provided a of 57.6 for the region S1, and 41.76 for S2 (19 d.o.f.). In both cases, the uncertainties on the parameters were estimated assuming that the sinusoidal model was correct, and we scaled the errors on the velocities to reach a reduced of 1. The values of , and measured in S1 and S2 are consistent at the 1 level, is consistent at the 2 level. The error-weighted average of the parameters gives days, HJD/UTC, K and . Figure 2 shows the radial velocity curve (rvc) folded on the above period. The value of is in the reference frame of the template star used for the cross-correlation, whose systemic radial velocity is  (Wilson, 1953). The systemic radial velocity of IGR J19308+0530 is therefore  .


Figure 2: Radial velocity curve from the absorption lines in the IGR J19308+0530 spectra (region S1) and its best fitting sinusoid (solid line). For comparison we plot circular orbits with 112 (dotted line) and 140 (dashed line), providing and respectively for the observed .

We obtained a set of high signal-to-noise template spectra from the UVES Paranal Observatory Project (UVESPOP, Bagnulo et al. 2003) of A, F and early G stars of luminosity class V, III and IV. We subtracted each template spectrum from the Doppler-corrected average of the IGR J19308+0530 spectra between orbital phase 0.9 and 0.1. The reason for choosing this range in phase is that the oblate shape of the Roche-lobe filling companion star and the possible presence of irradiation from the compact object could cause asymmetries in the line profiles, which are minimised close to phase 0. We performed a test on the residuals of the subtraction: the template resulting in the minimum provides our best estimate for the source spectral type. In particular, we adopted the optimal subtraction procedure implemented in molly, where the templates are multiplied by a factor 0f before the subtraction, representing the fractional contribution of light from the secondary star (1 minus the disc veiling). The factor is found by minimizing the difference between the residuals and a smoothed version of itself. Before doing the subtraction, the UVESPOP spectra were shifted to the rest frame of the average target spectrum, and degraded to match the sampling and line broadening of the latter.
The procedure favors an F4-F6 V companion star, with a disc veiling of %. The same spectral type and veiling are obtained when considering an average of the target spectra around phase 0.5, suggesting little irradiation on the inner face of the companion star.

To measure , we compared the spectrum of the template star with the Doppler-corrected average of the IGR J19308+0530 spectra between phase 0.9 and 0.1. The observed full-width at half-maximum (FWHM) of the absorption lines in the target spectra is determined by the intrinsic line width (expected to be dominated by ), broadened by the instrumental resolution profile and smeared by the motion of the companion star during the integration time of one observation. In order to account for the smearing, we made as many copies of the template spectrum as the number of target spectra we used for the average and we artificially smeared each copy of the template by , where is the duration of one exposure on IGR J19308+0530 and the phase of one of the IGR J19308+0530 spectra we averaged. After that, we averaged the smeared template and broadened the resulting spectrum with different values of . For each , we performed an optimal subtraction of the broadened template from the averaged spectrum of IGR J19308+0530: again the broadening which gives the minimum provides a measure of the actual (Figure 1). A value of 0.5 was assumed for the limb darkening. We masked interstellar features and the lines from the Balmer series.
In order to estimate the uncertainty on , we included this procedure in a Monte Carlo simulation, following Steeghs & Jonker (2007). We copied each target spectrum 500 times, using a bootstrap technique where the input spectrum is resampled by randomly selecting data points from it. The bootstrapping maintains the total number of data points in the spectrum. For each bootstrap copy, one value of is measured as described above. The distribution of obtained from the 500 copies is well described by a Gaussian, whose mean and r.m.s. provides the best-fit and its 1 error. As template and target spectra are acquired with the same instrument, the instrumental resolution profile is not affecting our measurement.
The weighted average of the results from S1 and S2, consistent at the 1 level, is . With and , we calculated the system mass ratio from the relation (Horne et al., 1986), obtaining 1.780.04.

Figure 3: -band CAMELOT lightcurve, folded using the ephemeris from the radial velocity data. The colours indicate the three observing nights (Jul. 30 in red, Jul. 31 in green and Aug. 1 in blue). The drawn line shows a fit with ellipsoidal modulations plus a disc with a bright disc spot.

3.2 Ellipsoidal modulation and system inclination

Figure 3 shows the -band lightcurve for IGR J19308+0530 obtained by phase folding the CAMELOT data on the ephemeris determined in Section 3.1. The lightcurve displays the typical signature of ellipsoidal variation, with two unequal minima, but in addition asymmetric maxima (O’Connell 1951 and Wilsey & Beaky 2009).
We modeled the -band lightcurve using the XRbinary program written by E.L. Robinson. A reasonable fit to the data (reduced , 403 d.o.f.) is obtained with a model assuming an F4V secondary star and including 30% disc contribution to the total light plus a disc hot-spot at phase 0.75. The disc veiling in the model is larger than observed in the spectra, but variability is possible as the photometry was performed one month after the last spectrum was acquired. Similar models with different assumptions about the disc properties (disc radius, height and temperature profile) also give reasonable fits. As we have no indications to single out one preferred set of parameters, we do not provide a formal uncertainty on , but an indicative value of 52 as a guide.

3.3 Upper limits in the radio waveband

We searched for a radio counterpart to IGR J19308+0530 with a Karl G. Jansky Very Large Array observation at 4.6 and 7.9 GHz (120 MHz bandwidth) (proposal ID 10B-238), taken on 2010 Aug. 19 with the array in its most compact D-configuration. The on-source time was 33 minutes and the data were reduced according to the standard procedures within the Common Astronomy Software Application (McMullin et al., 2007) software package, using the calibrator 3C 286 to set the amplitude scale and J1922+1530 to calibrate the amplitude and phase gains for the target source. IGR J19308+0530 was not detected at either frequency. A 3-sigma upper limit to the source flux of 54 Jy/beam was derived
To search for radio pulsations from a potential pulsar in this system, we used the Westerbork Synthesis Radio Telescope and the PuMa2 backend (Karuppusamy et al., 2008). We observed for 1 hr from both 310380 MHz (Obs. ID 11205182, 29 Aug. 2012) and from 13001460 MHz (Obs. ID 11205210, 31 Aug. 2012). The predicted dispersion measure (DM) for a 300 pc distance along this line-of-sight (see Discussion) is only pc (Cordes & Lazio, 2002). For each data set, we used the PRESTO software suite (Ransom, 2001) to search a set of trial DMs up to 30 pc . No obvious radio pulsar signal was detected after an acceleration search. From these observations, we can place conservative flux density limits of mJy and mJy for any pulsar present in the system, assuming it is beamed towards us.

4 Discussion and conclusion

We performed a dynamical study of the system IGR J19308+0530 through optical spectroscopy and photometry. The optical spectra are dominated by the companion star, with no evidence of irradiation, no emission features visible from the accretion flow besides a partial filling in of the Balmer lines and % disc contribution to the continuum. The secondary star is most likely of spectral type F4-6 V. Ellipsoidal modulations are detected on the 14.6 hour orbit. From phase-resolved spectroscopy we measure an extreme value for the binary mass ratio of 1.780.04. The lightcurve modeling provides a reasonable fit to the data with a disc+hot-spot model at . Solving the mass function 0.03  with this inclination, we obtain the following indicative masses: M0.8  and M1.4 . The masses are consistent with a WD accretor and an F4V donor (typical mass of 1.37 , Mamajek’s list 2011222http://www.pas.rochester.edu/ emamajek/
EEM_dwarf_UBVIJHK_colors_Teff.dat
). If the inclination is lower, which we cannot exclude based on these data, the masses will increase, allowing a scenario with a NS primary if . However, in this case the companion star would be over-massive for the spectral type, which is unusual for XRB and CVs.

Assuming an F4 V mass donor with a radius equal to that of the Roche lobe for our best estimated masses, the magnitude of the companion is in the visual band (half a magnitude brighter than for a typical F4 V star, Mamajek 2011). Comparing with the apparent magnitude of IGR J19308+0530 (, converted from in the Tycho catalogue), we estimate a distance range of , for an extinction between  cm (Dickey & Lockman 1990, where is converted into following Güver & Özel 2009).
Combining the systemic radial velocity with the source proper motion reported in the UCAC3 catalogue (Zac10), we computed the Galactic space velocity components of IGR J19308+0530 using the method of Johnson & Soderblom (1987). Assuming that the Local Standard of Rest (LSR) participates in the Galactic rotation at 254 (Reid et al., 2009) and a distance of , the derived peculiar velocity is 45.32.9 . This rules out a large asymmetric kick from a supernova.
The X–ray luminosity of IGR J19308+0530 measured by Chandra in 2007 (Ratti et al., 2010) is , for distance in the above range of and the corresponding . In the same way, UV Swift observations provide at 2500 . A scenario where IGR J19308+0530 is not in full contact and the X–rays are from coronal activity of the companion seems unlikely. First, coronal activity usually does not produce prolonged high energy emission, while the source was discovered from a stack of INTEGRAL observations. Second, coronal activity could explain the observed X–ray luminosity only for the earliest spectral type allowed by the observations combined with very little , as for F stars the ratio between the X–rays and bolometric flux is for (Walter, 1983). Finally, the spectra indicate disc contribution to the continuum, and the emission detected in the Balmer lines shows a slight, variable velocity offset with respect to the radial velocity curve of the companion star which is not expected in case they originate from coronal activity. The emission line component also seems weak for an highly active star. A hot WD of 60000 alone could account for the UV emission, but not for unless thermonuclear burning is happening on its surface. The few intermediate-mass CVs with long orbital period that are known do appear as super soft sources (SSSs), showing soft X–ray spectra possibly due to stable hydrogen burning on the WD (e.g. Kahabka 2006). However, with a luminosity of 10 , SSSs are much more luminous than what we observe from IGR J19308+0530. As the wind mass loss expected for the secondary spectral type is low (  yr, Cranmer & Saar 2011) wind accretion can not account for the observed X–ray luminosity.
We conclude that IGR J19308+0530 is most likely in contact, consistent with the ellipsoidal modulations observed in the lightcurve. This makes the source particularly interesting among the scarcely populated class of IMXBs since it shows Roche lobe overflow at a low accretion rate . This is unusual, as Roche lobe accretion with an intermediate mass companion and such a large mass ratio is typically considered unstable, with an intense and short-lived accretion phase (e.g., Tauris & van den Heuvel 2006). Donor stars of mass around 1.4  have thin or non-existent convective envelopes (Verbunt & Zwaan, 1981) which implies that the instability does not proceed on a dynamical timescale. While a convective envelope star would react to mass loss by expanding, a star with a mostly radiative envelope will shrink, slowing down the mass transfer. However, with a mass ratio of 1.8 the radius of the Roche lobe will reduce faster in response to mass transfer than the radius of the companion, triggering a thermal instability (Tauris & van den Heuvel, 2006). Using the binary stellar-evolution code originally developed by Eggleton (Yakut & Eggleton 2005 and references therein), we modeled evolutionary tracks that allow for periods of stable Roche-lobe accretion onto a WD with masses and orbital periods consistent with our findings, although at higher values than implied by the (see also the appendix in Podsiadlowski et al. 2003). The low can be explained if we are observing a short lived phase of low , or if the mass transfer is non-conservative (a fully non-conservative scenario requires a mass loss rate from the system of /yr). There is currently no direct evidence in favour of or against the presence of mass loss from the system.
From an observational point of view, an overestimate of could be due to uncertainties on the limb darkening. However, even assuming a limb darkening of 0 (instead of 0.5) reduces by only a few , still providing a high of .

Acknowledgments

PGJ and GN acknowledge support from a VIDI grant from the Netherlands Organisation for Scientific Research. We thank T. Marsh for Molly, E.L. Robinson for his XRbinary code and F. Verbunt for useful discussion. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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