Issue |
A&A
Volume 404, Number 3, June IV 2003
|
|
---|---|---|
Page(s) | L43 - L46 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361:20030669 | |
Published online | 06 June 2003 |
Letter to the Editor
The optical counterpart to SAX J1808.4–3658 in quiescence: Evidence of an active radio pulsar?
1
Osservatorio Astronomico di Monteporzio, via Frascati 33, 00040 Roma, Italy
2
Astronomical Institute “Anton Pannekoek”, University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, NL 1098 SJ Amsterdam, The Netherlands e-mail: disalvo@astro.uva.nl
3
Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
Corresponding author: L. Burderi, burderi@coma.mporzio.astro.it
Received:
12
March
2003
Accepted:
26
April
2003
The optical counterpart of the binary millisecond X-ray pulsar
SAX J1808.4–3658 during quiescence was detected at mag by
Homer et al. (2001). This star shows a 6% semi-amplitude
sinusoidal modulation of its flux at the orbital period of the system.
It was proposed that the modulation arises from X-ray
irradiation of the intrinsically faint companion by
a remnant accretion disk, and that the bulk of the optical emission
arises from viscous dissipation in the disk.
The serious difficulty in this scenario lies in the estimate of the
irradiating luminosity required to match the observational data, that
is a factor
higher than the quiescent X-ray
luminosity of this source.
To overcome this problem, we propose an alternative scenario, in
which the irradiation is due to the release of rotational energy by
the fast spinning neutron star, switched on, as magneto-dipole rotator
(radio pulsar), during quiescence. Our computations indicate that the
optical magnitudes are fully consistent with this hypothesis. In this case
the observed optical luminosity may be the first evidence that a radio
pulsar is active in this system in quiescence, a key phase for understanding
the evolution of this class of objects.
Key words: accretion discs / stars: individual: SAX J1808.4–3658 / stars: neutron / X-rays: stars / X-rays: binaries / X-rays: general
© ESO, 2003
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