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.

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(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 (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

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Van der Meer et al 2005

Recent neutron star massdeterminations 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.

• Lx (persistently bright) ~1036 -1038 ergs/s. Lower Lx 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.

• 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.

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System parameters for black hole XRN

(Orosz)

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

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…

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.

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.

WIYN (4m)

Outburst photometry:variability, but nothing on the orbital period.

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).

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.

Quiescent observations

Can we see the secondary ?

… we might expect, by analogy with Cataclysmic Variables…

IGR J00291+5934: 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 J1808.4-3658) - 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…