The optical counterpart to SAX J1808.4-3658: observations in quiescence

Abstract

We report the first extensive set of optical photometric observations of the counterpart to SAX J1808.4-3658 (V4580 Sagittarius) in quiescence. The source was detected at V~21, 5 magnitudes fainter than at the peak of its 1998 outburst. However, a comparable ~6% semi-amplitude 2hr modulation of its flux is revealed. This has the same phasing and approximately sinusoidal modulation as seen during outburst and with photometric minimum when the pulsar is behind the companion. The lack of a double-humped morphology rules out an ellipsoidal origin, implying that the bulk of the optical flux does not arise from the companion. Moreover, applying crude modelling to the disc and X-ray irradiated face of the donor shows that the internal energy release of a remnant disc (with mass transfer driven by gravitational radiation) is sufficient to explain most of the optical emission, and with the modulation due to the varying contribution of the heated star's face. We note that this model is also consistent with the much lower X-ray to optical flux ratio in quiescence versus outburst, and with the phasing of the optical modulation.Comment: 7 pages, 5 figures, accepted version with very minor changes (after ref's report), to appear in MNRA

The optical counterparts of accreting millisecond X-ray pulsars during quiescence

“…This behaviour, reminiscent of what observed for SAX J1808.4−3658 (Homer et al 2001;Campana et al 2004) and IGR J00291+5934 (D'Avanzo et al 2007), suggests a companion star heated by the compact object. This is at variance with the "classical" ellipsoidal modulation observed in wider LMXTs.…”

“…On the other hand, with the significant exception of SAX J1808.4−3658 and IGR J00291+5934, their optical/NIR quiescent counterparts are only poorly known. The optical light curve of SAX J1808.4−3658 in outburst and quiescence shows variability modulated at the orbital period, in antiphase with the X-ray light curve (Giles et al 1999;Homer et al 2001;Campana et al 2004;Deloye et al 2008;Wang et al 2009). This is unlike other quiescent transients that normally show a double-humped morphology, due to an ellipsoidal modulation, and indicates that the companion star is exposed to some irradiation.…”

The Optical Counterpart of the Accreting Millisecond Pulsar SAX J1808.4−3658 in Outburst: Constraints on the Binary Inclination

“…The optical intensity of this source faded as the X-ray source declined, and a 2 hr orbital modulation was marginally detected in the optical flux (Giles, Hill, & Greenhill 1999). This 2 hr optical modu-lation was subsequently confirmed in observations during quiescence (Homer et al 2001). The optical counterpart has been designated V4580 Sagittarii (Kazarovets, Samus, & Durlevich 2000).…”

The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

“…In this case the magnetosphere should be devoid of matter and the radio pulsar mechanism should turn on with a strong pulsar wind preventing further accretion (see e.g., Stella et al 1994;Burderi et al 2001). This is what is currently thought to occur in the radio-pulsar phase of the three transitional pulsars recently discovered (Archibald et al 2009;Papitto et al 2013;Bassa et al 2014;Patruno et al 2014;Roy et al 2014Roy et al , 2015Stappers et al 2014) and in the quiescence phase of SAX J1808.4−3658 (Homer et al 2001;Burderi et al 2003;). The fact that in SAX J1808.4 −3658 the X-ray luminosity increases by three orders of magnitude right after reaching the luminosity minima on a very fast timescale of 1-2 days (see Figures 2 and 3) suggests that the radio pulsar mechanism does not turn on, although a very rapid switch cannot be excluded at the moment.…”

“…The exact trigger of the reflare is uncertain-they appear spontaneously in the simulations of Dubus et al (2001), although they do not resemble the observed reflares and they are seen by Hameury et al (2000) where reflares are caused by an increased irradiation of the donor star that causes a superoutburst (so called because their duration is much larger than that of normal outbursts) followed by reflares. The donor star in SAX J1808.4−3658 is observed to be strongly irradiated during quiescence (Homer et al 2001;Burderi et al 2003;) and indeed there are suggestions that it is losing a large amount of mass (di Salvo et al 2008; see however Hartman et al 2008 for criticisms of this strong mass loss scenario in SAX J1808.4 −3658 ). Another suggestion for the reflare trigger comes from the mass reservoir model of Osaki et al (2001), where reflares are triggered also after superoutbursts as long as the effective viscosity of the disk (parametrized by the α parameter) remains large through the entire sequence of reflares.…”