Accreting neutron star spins and the prospects for GW searches

1. Accreting neutron star spins and the prospects for GW searches Duncan Galloway Monash U. Deepto Chakrabarty Ed Morgan Jake Hartman Center for Space Research, MIT Hans Krimm &c Goddard Space Flight Center NS/LSC meeting, Hannover, October 2007 Galloway, “Accreting neutron star spins and the prospects for GW searches”

2. Rapid NS spins in X-ray binaries • Accreting neutron stars in LMXBs (for which the spin is known) cluster preferentially at millisecond periods -> recycling scenario • These measurements have all been made with the Rossi X-ray Timing Explorer, which has excellent sensitivity to pulsations with a few percent amplitudes and freuqencies well beyond 1 kHz Galloway, “Accreting neutron star spins and the prospects for GW searches”

3. Transient pulsars in LMXBs • No accreting neutron star consistently exhibits pulsations in the LIGO band • Most of the ~100 known neutron stars in LMXBs have never exhibited pulsations! Why this is, we don’t know • We have measured the neutron star spin in 22 systems to date, in three distinct ways: • Persistent pulsations which are mainly present during transient outbursts of certain low-accretion rate binaries; • Coherent oscillations which appear only during thermonuclear bursts; and • Intermittent pulsations (new!) Galloway, “Accreting neutron star spins and the prospects for GW searches”

4. Persistent pulsations: Swift J1756.9-2508 • Discovered 2007 June 7 by the Swift Burst Alert Telescope (BAT; ATel #1105) • Subsequent RXTE/PCA observations revealed 182 Hz pulsations and Doppler delays from a 54.7 min binary orbit • Outburst lasted ~13d; recurrence time >10 years • Pulsations were present consistently while the source was bright, like most other AMSPs(Krimm et al. 2007) Galloway, “Accreting neutron star spins and the prospects for GW searches”

5. The accretion-powered MSP population • With good sensitivity established over a few years, we likely have a complete sample with outburst recurrence times < 6 yr • This confirms the remarkable nature of SAX J1808.4-3658 and it’s close sibling IGR J00291+5934, with outbursts every ~2.5 and 3 years, respectively* • Can track pulse phase while the pulsar is active (13-40 d) for a fully coherent search • Discovery rate since 2002 is approximately 1/year; however Swift J1756.9-2508 is the only system discovered since Dec 2004 (2.5 years; falling off with time?) * Both are expected to be active again within the next 12 months or so Galloway, “Accreting neutron star spins and the prospects for GW searches”

6. What about burst oscillations? • Thermonuclear (type-I) bursts are observed from almost all the known LMXBs • Recur every few hours • Burst physics is well understood; accreted H/He builds up on the surface until a critical temperature and density is reached • In (some) bursts from 14 sources we find coherent oscillations around frequencies in the range 45-620 Hz, characteristic of the source Galloway, “Accreting neutron star spins and the prospects for GW searches”

7. Burst oscillation freq = spin freq • Oscillations during bursts in millisecond pulsars indicate that burst oscillations trace the NS spin (Chakrabarty et al. 2003, Nature 424, 42) • A larger sample of neutron star spins has been measured via this phenomenon; discovery rate of ~1.2/year • However, the typical oscillation lasts only a few seconds, may have discrete phase jumps, as well as drifts in frequency of up to 5 Hz • NOT possible to track phase between bursts • Orbital period for several burst oscillation sources is not known Galloway, “Accreting neutron star spins and the prospects for GW searches”

8. A quasi-persistent accretion-powered millisecond pulsar • Seven of the eight accretion-powered millisecond pulsars are transients with active periods of ~weeks separated by >2 years • HETE J1900.1-2455, in contrast, has been active since discovery in June 2005 (RXTE observations continue) • However, pulsations were only observed within the first two months of the outburst • Pulse amplitude also modulated by the occurrence of bursts (Galloway et al. 2007, ApJ 654, 73L) Galloway, “Accreting neutron star spins and the prospects for GW searches”

9. More intermittent pulsars! • Casella et al. 2007 reported on the discovery of a 120-s interval of persistent pulsations, at the burst oscillation frequency, in Aql X-1! • In addition, intermittent 442 Hz pulsations have also been found in SAX J1748.9-2021, in NGC 6440 (Gavriil et al. 2007, Altimarano et al. 2007) • In that source the oscillations appeared to be related to the bursts, but occurred often enough to measure the orbital Doppler shift • Immediate prospect for new detections from the existing LMXB population, as well as from newly-discovered sources Galloway, “Accreting neutron star spins and the prospects for GW searches”

10. Future prospects • RXTE currently not planned to operate beyond 2008… India’s ASTROSAT satellite, to be launched in 2009, may be able to take over http://meghnad.iucaa.ernet.in/~astrosat … the future for X-ray timing of neutron stars is not too optimistic! • The sources which do exhibit pulsations are not the brightest LMXBs… what about inferring the spin rate from other properties, e.g. the separation frequency of kHz QPOs? • Méndez & Belloni (2007) find that the spin frequency may NOT be related to the kHz QPO separation at all, and instead is consistent (in the mean) with <∆>≈310 Hz Galloway, “Accreting neutron star spins and the prospects for GW searches”