The faint neutron star soft X-ray transient SAX J1810.8–2609 in quiescence

P. G. Jonker 2 R. Wijnands 1 M. van der Klis 0 0 Astronomical Institute Anton Pannekoek, University of Amsterdam , Kruislaan 403, 1098 SJ Amsterdam , the Netherlands 1 School of Physics and Astronomy, University of St Andrews , North Haugh, St Andrews, Fife KY16 9SS 2 Institute of Astronomy , Madingley Road, Cambridge CB3 0HA A B S T R A C T We present the analysis of a 35-ksec-long Chandra observation of the neutron star soft X-ray transient (SXT) SAX J1810.8-2609. We detect three sources in the field of view. The position of one of them is consistent with the location of the ROSAT error circle of SAX J1810.8-2609. The accurate Chandra position of that source coincides with the position of the proposed optical counterpart, strengthening the identification as the counterpart. We detected the neutron star SXT system in quiescence at an unabsorbed luminosity of 1 1032 erg s1 (assuming a distance of 4.9 kpc). This luminosity is at the low end of quiescent luminosities found in other neutron star SXTs. This renders support to the existence of a group of faint soft X-ray transients of which the accreting millisecond X-ray pulsar SAX J1808.4-3658 is the most prominent member. The quiescent spectrum of SAX J1810.8-2609 is well-fitted with an absorbed power law with a photon index of 3.3 0.5. With a value of 3.3 1021 cm2, the galactic absorption is consistent with the value derived in outburst. Because the spectra of quiescent neutron star SXTs are often fitted with an absorbed blackbody or neutron star atmosphere plus power-law model, we also fitted the spectrum using those fitting functions. Both models provide a good fit to the data. If cooling of the neutron star core and/or crust is responsible for the soft part of the spectrum, the time-averaged mass accretion rate must have been very low (5.7 1013 M yr1; assuming standard core cooling only) or the neutron star must be massive. We also discuss the possibility that the thermal spectral component in neutron stars in quiescence is produced by residual accretion. 1 I N T R O D U C T I O N Low-mass X-ray binaries are binary systems in which a 1 M star transfers matter to a neutron star or a black hole. A large fraction of the low-mass X-ray binaries is transient; these systems form the so-called soft X-ray transients (SXTs). The characterizing property of SXTs is that the accretion rate drops several orders of magnitude when the source returns to quiescence (van Paradijs & Verbunt 1984). SXTs have been and are being studied extensively with previous and present X-ray satellites; for an overview of SXTs, see Chen, Shrader & Livio (1997). Using data obtained with the BeppoSAX satellite, Heise et al. (1999) and int Zand (2001) suggested that 10 bursting neutron stars form a separate class of faint SXTs; the outburst peak luminosity is low (typically 1036.5 erg s1). Later, King (2000) argued that a class of faint SXT could be explained evolutionarily. King (2000) argues that faint SXTs should have evolved beyond the period minimum of 80 min to orbital periods of 80120 min. Using the Chandra and XMMNewton satellites, it is possible to study these systems even when they are in quiescence (cf. Wijnands et al. 2001; Campana et al. 2002; Rutledge et al. 2002b). A large fraction of the neutron star SXT quiescent X-ray luminosity is often ascribed to cooling of the hot neutron star core and/or crust (cf. van Paradijs et al. 1987; Brown, Bildsten & Rutledge 1998). Assuming that the thermal spectral component is coming from the neutron star surface, in principle neutron star parameters such as the mass and radius can be derived when using a neutron star atmosphere model to fit the spectrum (Rutledge et al. 2001a,b; Heinke et al. 2003a). The quiescent luminosity of the neutron star SXT and accreting millisecond X-ray pulsar SAX J1808.43658 was found to be very low (LX = 5 1031 erg s1; Campana et al. 2002). This is nearly as low as the luminosities found for several quiescent black hole candidate (BHC) SXTs with a short orbital period (Kong et al. 2002). Such a low neutron star luminosity could result if the neutron star core and crust are relatively cool (Colpi et al. 2001). This in turn could hint at a massive neutron star (M NS > 1.7 M ; Colpi et al. 2001), or a very low time-averaged mass accretion rate (Brown et al. 1998). Several other emission mechanisms have been proposed to explain the quiescent luminosity of SXTs as well. First, in quiescence mass accretion may be ongoing at a low level (Zampieri et al. 1995, possibly via an advection-dominated accretion flow (ADAF), e.g. Menou et al. 1999). Secondly, shocks formed by the propeller mechanism preventing accretion on to the neutron star may produce X-rays (Illarionov & Sunyaev 1975; Stella, White & Rosner 1986). Thirdly, the switch-on of a radio pulsar mechanism may produce the observed X-rays (Campana & Stella 2000). Finally, leaking of matter through the magnetosphere especially at high latitudes in an ADAF- (...truncated)