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

Remon Cornelisse, Paolo D'Avanzo, T. Muñoz-Darias, S Campana, Jorge Casares, P. A. Charles, D. Steeghs, G Israel, L Stella

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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 (K22), and derive estimates for the pulsar mass. Methods. Using Doppler images of the Bowen region, we find a significant (≤8σ) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure = 24820 kms (1 confidence), which represents a strict lower limit to. Also, the Doppler map of HeII 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 leads to a lower limit on the mass function of 0.10. Applying the maximum K-correction gives 228≤K2 notation= notation= 322 kms and a mass ratio of 0.051 formula notation= notation=0.072.Conclusions. Despite the limited 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 is correct, the largest uncertainty in determining of the mass of the neutron star in SAXJ1808.4-3658 using dynamical studies lies with the poorly known inclination.

Original languageEnglish
JournalAstronomy & Astrophysics
Volume495
Issue number1
DOIs
Publication statusPublished - Feb 2009
Externally publishedYes

Keywords

  • Accretion, accretion disks
  • Stars: individual: SAXJ1808.4-3658
  • X-rays: binaries

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