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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 (K_2), and derive estimates on the pulsar mass. methods: Using Doppler images of the Bowen region we find a significant (>8sigma) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure K_em=248+/-20 km/s (1sigma confidence) which represents a strict lower limit to K_2. Also, the Doppler map of He II lambda4686 shows the characteristic signature of the accretion disk, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disk that are expected in very low mass ratio interacting binaries. results: The lower-limit on K_2 leads to a lower-limit on the mass function of f(M_1)>0.10M_sun. Applying the maximum K-correction gives 228<K_2<322 km/s 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 warranted to confirm this. If the derived K_em is correct, the largest uncertainty in the determination of the mass of the neutron star in SAX J1808.4-3658 using dynamical studies lies with the poorly known inclination.
In this paper we present a coherent timing analysis of the 401 Hz pulsations of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst. Using observations collected with the Neutron Star Interior Composition Explorer (NICER)
An outburst of the accreting X-ray millisecond pulsar SAX J1808.4-3658 in October-November 2002 was followed by the Rossi X-ray Timing Explorer for more than a month. We demonstrate how the area covered by the hotspot at the neutron star surface is d
We report the detection of a possible gamma-ray counterpart of the accreting millisecond pulsar SAX J1808.4-3658. The analysis of ~6 years of data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (Fermi-LAT) within a region
We report the discovery of phase shifts between X-ray pulses at different energies in the newly discovered millisecond (ms) X-ray pulsar SAX J1808.4-3658. The results show that low-energy pulses lag high-energy pulses by as much as $sim$0.2 ms (or $s
We have observed the millisecond X-ray pulsar SAX J1808.4-3658 on three occasions during its 2000 outburst with the BeppoSAX satellite. The source was highly variable and erratic during this outburst, and by coincidence we obtained data only during t