Open AccessThe optical counterpart to SAX J1808.4−3658: observations in quiescence
Author(s) -
Homer L.,
Charles P.A.,
Chakrabarty D.,
Van Zyl L.
Publication year - 2001
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2001.04567.x
Subject(s) - physics , astrophysics , flux (metallurgy) , modulation (music) , pulsar , amplitude , astronomy , millisecond pulsar , radiation , optics , materials science , acoustics , metallurgy
We report the first extensive set of optical photometric observations of the counterpart to SAX J1808.4−3658 (V4580 Sagittarii) in quiescence. The source was detected at V∼21 , 5 mag fainter than at the peak of its 1998 outburst. However, a comparable ∼6 per cent semi‐amplitude 2‐h 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 because of the varying contribution of the heated face of the star. 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.