EDP Sciences Journals List
Advanced Search
Subscriber Authentication Point
Home All issues Volume 495/ 1 Article

Free access article

Issue A&A
Volume 495, Number 1, February III 2009
Page(s) L1 - L4
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200811396
Published online 20 January 2009

A&A 495, L1-L4 (2009)
DOI: 10.1051/0004-6361:200811396

Letter

Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst

R. Cornelisse1, P. D'Avanzo2, T. Muñoz-Darias1, S. Campana2, J. Casares1, P. A. Charles3, 4, D. Steeghs5, 6, G. Israel7, and L. Stella7

1  Instituto de Astrofisica de Canarias, Calle via Lactea S/N, 3805 La Laguna, Spain
    e-mail: corneli@iac.es
2  INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
3  South Africa Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
4  School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
5  Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
6  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
7  INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Rome), Italy

Received 21 November 2008 / Accepted 15 December 2008

Abstract
Aims. We obtained phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its outburst in 2008 to find a signature of the donor star, constrain its radial velocity semi-amplitude (K2), and derive estimates for the pulsar mass.
Methods. Using Doppler images of the Bowen region, we find a significant ($\ge$8$\sigma$) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure $K_{\rm em}$ = 248$\pm$20 km s-1 (1$\sigma$ confidence), which represents a strict lower limit to K2. Also, the Doppler map of He II $\lambda$4686 shows the characteristic signature of the accretion disc, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disc that are expected in very low mass-ratio interacting binaries.
Results. The lower limit on K2 leads to a lower limit on the mass function of f(M1) $\ge$ 0.10 $M_\odot$. Applying the maximum K-correction gives 228 < K2 < 322 km s-1 and a mass ratio of 0.051 < q < 0.072.
Conclusions. Despite the limited S/N of the data, we were able to detect a signature of the donor star in SAX J1808.4-3658, although future observations during a new outburst are still needed to confirm this. If the derived $K_{\rm em}$ is correct, the largest uncertainty in determining of the mass of the neutron star in SAX J1808.4-3658 using dynamical studies lies with the poorly known inclination.


Key words: accretion, accretion disks -- X-rays: binaries -- stars: individual: SAX J1808.4-3658



© ESO 2009

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.
  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.