Irradiation of the secondary star in X-ray Nova Scorpii 1994 (=GRO J1655−40)

T. Shahbaz 2 w P. Groot 1 S. N. Phillips 2 J. Casares 0 P. A. Charles 2 J. van Paradijs 1 3 0 Instituto de Astrofsica de Canarias 38200 La Laguna , Tenerife, Spain 1 Astronomical Institute 'Anton Pannekoek', University of Amsterdam and Center for High Energy Astrophysics , Kruislaan, 403, 1098 SJ Amsterdam, the Netherlands 2 University of Oxford, Department of Physics, Nuclear Physics Laboratory , Keble Road, Oxford , OX1 3RH 3 Physics Department , UAH, Hunstville, Alabama 35899, USA A B S T R A C T We have obtained intermediate resolution optical spectra of the black-hole candidate Nova Sco 1994 in 1996 June, when the source was in an X-ray/optical active state R , 15:05 : We measure the radial velocity curve of the secondary star and obtain a semi-amplitude of 279 ^ 10 km s21; a value which is 30 per cent larger than the value obtained when the source is in quiescence. Our large value for K2 is consistent with 60297 per cent of the secondary star's surface being heated, compared with 35 per cent, which is what one would expect if only the inner face of the secondary star were irradiated. Effects such as irradiation-induced flows on the secondary star may be important in explaining the observed large value for K2. I N T R O D U C T I O N The soft X-ray transients, a subclass of the low-mass X-ray binaries distinguished by their X-ray outbursts, have proved to be an ideal hunting ground for stellar-mass black hole candidates (Tanaka & Shibazaki 1996). The system Nova Sco 1994 ( GRO J165540) is particularly interesting, since as well as being a source of superluminal jets (Zhang et al. 1994; Harmon et al. 1995), its optical brightness and partial eclipse features mean that it is one of the few systems which has yielded a reliable estimate for the mass of the collapsed star. Nova Sco 1994 was discovered on 1994 July 27 with BATSE (Burst and Transient Source Experiment) on board the Compton Gamma Ray Observatory (Zhang et al. 1994). It has been studied extensively during the past few years in X-rays and at optical and radio wavelengths (Bailyn et al. 1995a,b; Zhang et al. 1995; van der Hooft et al. 1998). Strong evidence that the compact object in Nova Sco 1994 is a black hole was presented by Bailyn et al. (1995b) who initially established a spectroscopic period of 2:601 ^ 0:027 days; classified the secondary as an F2F6 iv type star and suggested a mass function f M 3:16 ^ 0:15 M(: An improved value of f M 3:24 ^ 0:09 M( was presented by Orosz & Bailyn (1997) using both quiescent and outburst data, derived from a radial velocity semi-amplitude of 228:2 ^ 2:2 km s21: Shahbaz et al. (1999) using only quiescent data, determined the true radial velocity semi-amplitude K2 215:5 ^ 2:4 km s21 which gives a Spectroscopy O B S E R VAT I O N S A N D D ATA R E D U C T I O N Intermediate resolution optical spectra of Nova Sco 1994 were obtained on 1996 June 2024 with the 1.54-m Danish Telescope at the European Southern Observatory (ESO) in Chile using the Danish Faint Object Spectrograph and Camera (DFOSC). We used grating #8 which gave a dispersion of 1.26 A per pixel and a wavelength coverage from 58658336 A . The Loral 2048 2048 CCD (charge-coupled device) was used, binned by a factor of 2 in the spatial direction in order to reduce the readout noise, but not binned in the dispersion direction. The seeing during the observations was poor and variable (see section 2.2) so we used a slit width of 2.5 arcsec on the first night and then 2.0 arcsec for the other nights. This resulted in spectral resolutions of 7.6 and 5.5 A for the first and other nights respectively. Wavelength calibration was performed using a CuAr arc. A total of 47 spectra were taken, each having exposure times of 1800 s (see Table 1 for details). The data reduction and analysis was performed using the Starlink figaro package, the pamela routines of K. Horne and the molly package of T. R. Marsh. Removal of the individual bias signal was achieved through subtraction of a median bias frame. Small scale pixel-to-pixel sensitivity variations were removed with a flat-field frame prepared from observations of a tungsten lamp. One-dimensional spectra were extracted using the optimalextraction algorithm of Horne (1986) and calibration of the wavelength scale was achieved using fifth order polynomial fits which gave an rms scatter of 0.03 A . The stability of the final calibration was verified with the OH skyline at 6300.3 A , the position of which was accurate to within 0.1 A . Photometry Using the same setup as for the spectroscopy, we also obtained # of spectra Seeing (00) 0.7220.834 0.1000.225 0.5010.592 0.8830.964 0.2700.353 limited Bessell r-band images of Nova Sco 1994 every night. The data were debiased using a median bias frame, but not flat-fielded, as none were taken. These images were used to estimate the seeing each night (see Table 1). We applied aperture photometry to Nova Sco 1994 and several nearby comparison stars within the field of view. Johnson V- and R-band magnitudes of these comparison stars were made available to us by J. Orosz. We determined the relative magnitude of Nova Sco 1994 with respect to three stars having a range of colours [ V 2 R 0:49; 0.77 and 1.23]. Assuming that the colour correction between the two filter systems is small (,0.05 mag; similar to the accuracy of our photometry) and that Nova Sco 1994 has a colour in the same range as the comparison stars used, we estimate R , 15:05 for Nova Sco 1994. T H E S P E C T R A O F N O VA S C O 1 9 9 4 In Fig. 1 we show the variance-weighted average and also the nightly averages of the Nova Sco 1994 spectra. A strong Ha emission line (mean equivalent width of 7:5 ^ 0:06 A and a much weaker He i 6678 A (equivalent width of 0:4 ^ 0:03 A can be seen. In Table 2 we list the Ha equivalent width for the nightly averages. The Fe i absorption blend at 6485, 6496, 6499 and 6502 A is also visible. These features are used to determine the radial velocity of the secondary star (see Section 4). The 6613 A diffuse interstellar band is also present. The emission lines in Nova Sco 1994 are double peaked, which is presumably a consequence of the system being at high inclination. We can compare the observed peak-to-peak half separation of the Ha emission line (which arises from the accretion disc) with the projected velocity of the outer disc edge. In a binary system with a mass ratio .0.25 it is generally assumed that the accretion disc cannot grow larger than the tidal truncation radius, rd (Paczynski 1977; Whitehurst 1988; Osaki, Hirose & Ichikawa 1993), which is approximately given by rd 0:60a= 1 q for 0:03 , q , 1; where a is the binary separation (Warner 1995). Given the system parameters Porb 2:621 68 d; q , 0:39; i , 698; M1 , 6:7 M(; see Shahbaz et al. 1999) the minimum value for the projected velocity of the accretion disc rim is ,394 km s21. The observed peak-to-peak half separation of the Ha emission line (see Fig. 1) in late June 1 (...truncated)