Binary millisecond X-ray pulsars

1. Binary millisecond X-ray pulsars Paul Callanan and Mark Reynolds Department of Physics University College Cork Alexei Filippenko, Department of Astronomy, Berkeley Peter Garnavich, Department of Physics, University of Notre Dame.

2. Galactic X-ray source distribution: LMXBs (open circles) and HMXBs (filled circles) (Grimm et al 2002 from RXTE ASM)

3. More than 300 X-ray binaries known, both persistently bright and quiescent. • Galactic X-ray emission dominated by the Low Mass systems in the bulge (M2<1-2 Mo) • High mass systems in the disk/spiral arms • Globular cluster sources - 13 LMXB, both persistently bright/transient (Pooley et al 2002): fainter sources include quiescent LMXBs, CVs, millisecond pulsars, magnetically active binaries (+ …: Verbunt and Lewin, 2005) • Galactic Centre/diffuse emission (e.g. Muno et al 2003, Hands et al 2004) - faint XRN, HMXBs/magnetic CVs, diffuse emission Grimm et al 2002

4. High Mass X-ray binaries • Vast majority(~96) pulsate (periods • ranging from 69 msec to 1400 s). Recent neutron star mass determinations for HMXBs… Van der Meer et al 2005

5. Low Mass X-ray binaries • More than half of all known Galactic XRB are LMXB • Orbital periods 11 mins to 16.5 days. • Lx (persistently bright) ~1036 -1038 ergs/s. Lower Lx sources often exhibit X-ray bursts.

6. X-ray Novae (aka Soft X-ray Transients) • ~half of all known LMXBs are transient. Typical recurrence times are thought to be ~10-50 years. • Lx(quiescent) as low as 1030 ergs/s: Lx(max) >~ 1039 ergs/s in some systems. • Orbital periods ~4 hrs - 33.5 days • Of the 18 binaries thought to contain a black hole (ie for which Mx>3 Mo from radial velocity studies), 15 are XRN.

7. System parameters for black hole XRN (Orosz)

8. Compact object mass estimates (but beware - e.g. Reynolds et al 2006)

9. Accreting binary millisecond pulsars • 7 discoveries since the first (in 1996), SAX J1808.4-3658 (bursting, pulsating). • Orbital periods from 40 mins ->4.3 hrs: spin periods from 1.67-5.4 msec: Lmax ~1036 ergs/s • Dramatic confirmation of the link between accreting LMXBs and millisecond pulsars. • Evidence for irradiation of secondary by pulsar spin down flux in at least one system. • Spin periods now known for ~20 LMXBs (these + burst oscillations + persistently bright pulsators). • In theory, ideal for constraining neutron star mass…

10. IGR J00291+5934 - discovered by INTEGRAL during a routine Galactic Plane Scan on the 2nd of December 2004. The optical counterpart was subsequently identified two days later and was found to have a magnitude R ≈ 17.4 (Fox et al., 2004). Lx~1036 ergs/s.

11. We obtained a single 300s LRIS spectrum of the proposed optical counterpart, which we display in Figure 6, was obtained 10 days post outburst(Filippenko et al., 2004). We observe broad (FWHM = 1200 km/s) emission lines of Hα 656nm (EW = 0.96 nm), Hβ 486nm (EW = 0.54 nm), and HeI 667.8 nm (EW = 0.1 nm), as well as narrow (FWHM = 300 km/s), very weak (EW = 0.06 nm) HeII 468.6 nm emission.

12. Outburst photometry: variability, but nothing on the orbital period. WIYN (4m)

13. IGR J00291+5934 is the 6th member of the class of accretion powered millisecond X-ray pulsars to be discovered and, with an orbital period of 2.45 hours, the third ‘long’ orbital period system. The other long period sources are SAX J1808.4-3658, (Wijnands et al., 1998; Chakrabarty et al., 1998), XTE J1814-338 (Markwardt et al., 2003b) and HETE J1900.1-2455 (Vanderspek et al., 2005) with periods of 2.01 hr, 4.28 hr and 1.39 hr respectively. The final 3 members of the class are ultra-compact binaries with periods of ∼40 minutes (XTE J1751-305, Markwardt et al., 2002: XTE J0929-314, Galloway et al., 2002 and XTE J1807-294, Markwardt et al., 2003a). Even though IGR J00291+5934 and SAX J1808.4-3658 are geometrically similar, the magnitude of the former in quiescence is at least 2 magnitudes fainter; R ≈ 23 compared to 20.9 ±0.1 (Homer et al., 2001).

14. As the distance to each of these systems is comparable (Jonker et al. 2005; in’t Zand et al. 2001), this implies that IGR J00291+5934 is intrinsically, at least 2 magnitudes fainter than SAX J1808.4-3658 in quiescence. This may suggest that the spindown luminosity of the pulsar in IGR J00291+5934 is considerably less than that of SAX J1808.4-3658, resulting in a fainter quiescent counterpart. However more accurate distance and reddening estimates are required to confirm this.

15. Quiescent observations Can we see the secondary ? … we might expect, by analogy with Cataclysmic Variables…

16. IGR J00291+5934: 30 min Keck integration.

17. Data with NextGen model (T=2845 K).

18. WIYN quiescent photometry + ellipsoidal fit i = 60 degrees, T=4000 K

19. Unfortunately, this is not the whole story….

20. So… Continuum from the disk absent - not only because of short orbital period ? “Naked” secondary - Mv~14.4, V-I~3.5, consistent with the data. Could be at relatively high inclination, but we need to reconcile photometric and spectroscopic fits. No evidence for blue excess (in contrast to SAX J1808.4-3658) - ie for shockfront between wind from secondary and neutron star. Radial velocity measurements/mass estimates tricky…

21. Much remains to be learned by comparing these systems to the short period CVs… Orbital period 81 mins