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

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

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 (M 2 <1-2 M o ) 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

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

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

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. L x (quiescent) as low as ergs/s: L x (max) >~ ergs/s in some systems. Orbital periods ~4 hrs days Of the 18 binaries thought to contain a black hole (ie for which M x >3 M o from radial velocity studies), 15 are XRN.

System parameters for black hole XRN (Orosz)

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

Accreting binary millisecond pulsars 7 discoveries since the first (in 1996), SAX J (bursting, pulsating). Orbital periods from 40 mins ->4.3 hrs: spin periods from msec: Lmax ~10 36 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…

IGR J discovered by INTEGRAL during a routine Galactic Plane Scan on the 2nd of December The optical counterpart was subsequently identified two days later and was found to have a magnitude R ≈ 17.4 (Fox et al., 2004). Lx~10 36 ergs/s.

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 H α 656nm (EW = 0.96 nm), H β 486nm (EW = 0.54 nm), and HeI nm (EW = 0.1 nm), as well as narrow (FWHM = 300 km/s), very weak (EW = 0.06 nm) HeII nm emission.

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

IGR J 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 J , (Wijnands et al., 1998; Chakrabarty et al., 1998), XTE J (Markwardt et al., 2003b) and HETE J (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 J , Markwardt et al., 2002: XTE J , Galloway et al., 2002 and XTE J , Markwardt et al., 2003a). Even though IGR J and SAX J 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).

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

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

IGR J : 30 min Keck integration.

Data with NextGen model (T=2845 K).

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

Unfortunately, this is not the whole story….

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 J ) - ie for shockfront between wind from secondary and neutron star. Radial velocity measurements/mass estimates tricky…

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