Discovery of an Accreting Millisecond Pulsar in the Eclipsing Binary System Swift J1749.4–2807

Abstract

We report on the discovery and the timing analysis of the first eclipsing accretion-powered millisecond X-ray pulsar (AMXP): SWIFT J1749.4-2807. The neutron star rotates at a frequency of ∼517.9 Hz and is in a binary system with an orbital period of 8.8 hr and a projected semimajor axis of ∼1.90 lt-s. Assuming a neutron star between 0.8 and 2.2 M and using the mass function of the system and the eclipse half-angle, we constrain the mass of the companion and the inclination of the system to be in the ∼0.46-0.81 M and ∼ 74.• 4-77.• 3 range, respectively. To date, this is the tightest constraint on the orbital inclination of any AMXP. As in other AMXPs, the pulse profile shows harmonic content up to the third overtone. However, this is the first AMXP to show a first overtone with rms amplitudes between ∼6% and ∼23%, which is the strongest ever seen and which can be more than two times stronger than the fundamental. The fact that SWIFT J1749.4-2807 is an eclipsing system that shows uncommonly strong harmonic content suggests that it might be the best source to date to set constraints on neutron star properties including compactness and geometry.

A search for the near-infrared counterpart of the eclipsing millisecond X-ray pulsar Swift J1749.4–2807

“…The VLT observing epochs were chosen so as to have a phase difference of 0.25 P orb (based on the precise ephemeris of Altamirano et al 2011). On Aug. 30.05 UT the source was observed at phase ∼0.75 (ascending node), while on Aug. 31.05 it was at phase ∼0.5 (inferior conjunction, i.e.…”

“…The companion star is therefore a (possibly evolved) main sequence star. From geometrical considerations in an eclipsing system, Markwardt et al (2010) and Altamirano et al (2011) have derived a companion mass in the 0.46-0.81 M range for a neutron star with mass in the 0.8-2.2 M range, and system inclinations in the 74.4 • −77.9 • range. Assuming an unirradiated main sequence companion star, these mass limits imply that the spectral type of the companion star should be in the K0V-M0V range.…”

“…This was ascribed to a dust-scattering halo located along the line of sight to Swift J1749.4-2807 ). X-ray pulsations are not detected during these intervals, testifying to their genuine eclipse nature (Markwardt & Strohmayer 2010;Altamirano et al 2011;Ferrigno et al 2011). The detection of X-ray eclipses enables the system inclination to be constrained in the 74.4 • −77.9 • range (depending on the assumed neutron star mass; Markwardt & Strohmayer 2010;Altamirano et al 2011).…”

“…Since 1998, fourteen such systems have been discovered, with orbital periods in the range of 40 min to 19 h and spin frequencies from 1.7 to 5.4 ms (Wijnands & van der Klis 1998;Chakrabarty & Morgan 1998;Markwardt et al 2002;Galloway et al 2002;Campana et al 2003;Strohmayer et al 2003;Galloway et al 2005;Kaaret et al 2006;Krimm et al 2007;Casella et al 2008;Altamirano et al 2008;Altamirano et al 2010;Papitto et al 2010;Markwardt & Strohmayer 2010;Altamirano et al 2011;Papitto et al 2011). Since they have been spun up by accretion (see, e.g., Falanga et al 2005), their initial mass should have increased by at least 0.1-0.2 M compared to the canonical neutron star mass (e.g., Thorsett & Chakrabarty 1999).…”

“…The AMXP Swift J1749.4-2807 was discovered in June 2006 by the Swift satellite when a type I burst from it was observed, which was emitted during a dim outburst (Wijnands et al 2009;Campana 2009 the Rossi X-ray Timing Explorer (RossiXTE) satellite led to the discovery of a neutron star spin period of 1.9 ms and of an orbital period of 8.82 h (Markwardt & Strohmayer 2010;Altamirano et al 2011). Short eclipses (with a duration of ∼2200 s) were observed in X-ray from observations with RossiXTE.…”