SAX J1808-3658 : Witnessing the Banquet of a Hidden Black Widow? Luciano Burderi

1. SAX J1808-3658 : Witnessing the Banquet of a Hidden Black Widow? Luciano Burderi (Dipartimento di Fisica, Universita’ di Cagliari) Tiziana Di Salvo (Dipartimento di Fisica, Universita’ di Palermo) Collaborators: A. Riggio (Universita’ di Cagliari) A. Papitto (Oss. Astr. Roma) M.T. Menna (Oss. Astr. Roma) Cool Discs, Hot Flows Funasdalen (Sweden) 2008, March 25-30

2. SAX J1808: the outburst of 2002 (Burderi et al. 2006, ApJ Letters; see also similar results for all the outbursts in Hartman et al. 2007, but with a different interpretation) Phase Delays of The First Harmonic Spin up: dotn0 = 4.4 10-13 Hz/s corresponding to a mass accretion rate of dotM = 1.8 10-9 Msun/yr Spin-down: dotn0 = -7.6 10-14 Hz/s corresponding to a NS magnetic field: B = (3.5 +/- 0.5) 108 Gauss Porb = 2 h n = 401 Hz Spin-up: dotn= 4.4 10-13 Hz/s Spin-down at the end of the outburst: dotn= -7.6 10-14 Hz/s

3. New results from timing of SAXJ1808.4-3658:variations of the time of ascending node passage between different outburst(Di Salvo et al. 2007, Hartman et al. 2007) Orbital period increases: dot Porb=(3.40+-0.12) 10-12 s/s (Di Salvo et al. 2007)

4. Orbital Period Derivative dot J / Jorb < 0 and dot Porb / Porb > 0: a lower limit on the positive quantity –dot M2 / M2 can be derived assuming dot J / Jorb = 0 From the definition of the orbital angular momentum, Jorb, and the third Kepler's law, after differentiation, we obtain:

5. Fully Conservative case The mass function gives q >= 4 10-2~ 0 (for M1 = 1.4 Msun). b = 1, g (1, q, a) = 1 – q ~ 1 Excluded! From the observed luminosity in quiescence and in outburst, we derive the average luminosity from the source: Lx = 3.9 1034 ergs/s, and 3 (-dot M2 / M2) = 6.6 10-18 s-1. From experimental data: dot Porb / Porb = 4.7 10-16 s-1. Therefore measured dot Porb / Porb about 70 times higher than predicted from the conservative mass transfer scenario

6. Totally non-conservative case The mass function gives q >= 4 10-2 ~ 0 (for M1 = 1.4 Msun). b = 0, g (0, q, a) = (1 – a + 2/3 q) / (1 + q) ~ 1 – a dot Porb / Porb <= 3 (1 – a) (-dot M2 / M2) Since dot Porb / Porb > 0, a < 1 For matter leaving the system with the specific angular momentum of the primary, a = q2~ 0: similar to the conservative case (as expected). For matter leaving the system with the specific angular momentum of the inner Lagrangian point (with q = 4 10-2from the mass function with M1 = 1.4 Msun), a = [1 - 0.462 (1 + q)2/3 q1/3]2~ 0.7: dot Porb / Porb <= (-dot M2 / M2) Assuming dot Porb / Porb = 4.7 10-16 s-1 (from experimental data) we derive 8.3 10-10 Msun/yr <= (-dot M2) = dot Mejected For matter leaving the system with the specific angular momentum of the secondary, a = 1: the orbital period evolution is frozen (as the orbital period of an Earth-orbiting satellite which does not change halving its mass).

7. Secular evolution - non-conservative Solve the angular momentum equation taking into account losses of angular momentum from the system (which drive the system evolution), and impose contact between the secondary and its Roche lobe along the evolution. dot Porb predicted by non-conservative mass transfer driven by GR angular momentum losses is . -18 for q = 0.564 and n = -1/3

8. Fully Non Conservative mass transfer in SAXJ1808.4-3658 (Di Salvo et al. 2007)

9. Secular evolution - non-conservativePredicted mass loss rate Why high dotM and mass ejection?

10. Folded lightcurve Optical counterpart in quiescence(Homer et al. 2001) In quiescence [Aug 1999, Jul 2000] mV ~ 21.5(uncompatible with intrinsic luminosity from a < 0.1 Msun companion, uncompatible with intrinsic luminosity from an accretion disk in quiescence) • Optical modulation at 2h-orbital period, antiphase with X-ray ephemeris (incompatible with ellipsoidal modulation!) • mV semiamplitude ~ 0.06 mag

11. We proposed an alternative scenario! Optical emission in quiescence interpreted as reprocessed spin-down luminosity of a magneto-dipole rotator by a companion and/or remnant disk Burderi et al. 2003, Campana et al. 2004

12. Estimated reprocessed luminosity

13. Rotating magnetic dipole phase • Radio Ejection phase (Burderi et al. 2001) • Rotating magnetic dipole emission • overflowing matter swept away • by radiation pressure • pulsar pressure given by the • Larmor formula: • Prad = 2 /3c4 m2 (2 p / P)4 /(4 p R2) • matter pressure given by the • ram pressure of the infalling gas: • Pram = dotM (G M1/2)1/2 /(2p R5/2)

14. The first MSP in an interacting binary: J1740-5340 in the Globular Cluster NGC 6397 and in a long period system! is observed during the radio-ejection phase? (Burderi et al. 2002)

15. Is SAXJ1808 in quiescence a radio-ejector? Using: R = RRL2 (Roche lobe radius of the secondary) M2 = 10-9 Msun / yr (as derived from the non-conservative secular evolution) m = 3 - 5 1026 Gauss / cm3 (as derived from 2002 timing) we find: Pram = 150 dyne / cm3 Prad = 80 - 230 dyne / cm3 Pram ~ Prad: living at the border between accretors (outburst) and radio-ejectors (quiescence)

16. Conclusions The high orbital period derivative in SAXJ1808.4-3658 is an indirect proof that • A magnetodipole rotator is active in the system • The system harbors a hidden “Black Widow” eating its companion during outbursts and ablating it during quiescence

17. That’s all Folks!