Gamma-ray emission from Be/X-ray binaries

1. Gamma-ray emission from Be/X-ray binaries Gustavo E. Romero Instituto Argentino de Radioastronomía, (CONICET) romero@irma.iar.unlp.edu.ar Hong Kong June 2, 2004

2. Variable sources in the Galaxy Nolan et al., ApJ 597, 615 (2003) – see also Torres et al. A&A 370, 478 (2001)

3. What are these variable galactic sources? Some possibilities: * Microquasars * Colliding wind binaries * Pulsar wind nebulae * Interacting neutron stars Be/X-ray binaries

4. Sketch of a Be/X-ray binary System 2S1417-624

5. Cheng and Ruderman (1989, 1991) have studied the magnetosphere of an accreting pulsar when the Keplerian disk rotates faster than the star. Inertial effects of electrons ( - ) and ions (+) lead to a complete charge separation around the “null surface” *B = 0. An electrostatic gap with no charge is created around the null surface. In this gap E*B R 0 and a strong potential drop is established.

6. The potential drop in the gap is: where B is the neutron star's surface dipole magnetic field, R is its radius, r0 is the inner accretion disk radius, M is the compact star mass, and L37 is the X-ray luminosity in units of 1037 erg s-1. =2r0/rA is twice the ratio of the inner accretion disk radius to the Alfven radius. The maximum current that can flow through the gap is:

7. The proton flux on the disk results p-p interactions produce pions that decay in the disk (very energetic charged pions can interact before decaying, leading to a supression of the neutrino flux above 1 TeV, see Anchordoqui et al. 2003). The gamma-rays, electrons and positrons injected in the disk trigger electromagnetic cascades. The energy per unit of time generated in the disk by neutral pion decays( p0g2g ) is: where Ep = fp Ep

8. Anchordoqui et al. ApJ 589, 481 (2003)

9. Adopted X-ray luminosity during both major and normal outbursts • A system with T~ 40 days • Major outbursts last for more than 2 orbits • Normal outbursts last less than 1 orbit After 2S1417-624

10. Specific models: *Neutron star: M = 1.4 Msun , R = 10 6 cm and Bsup =10 12 G. *Standard accretion disk (viscosity parameter a = 0.1) in a strong external field (Gosh & Lamb 1979). The inner radius is located at: where and g0is the magnetic pitch angle (~1). Four models were calculated, characterized by two values of the screening factor inside the disk (h= 0.2 andh= 1) and two values of Lpeak, (corresponding to major and normal outbursts).

11. Variation of disk parameters during a major outburst Dashed: h=1,solid: h=0.2

12. Variation of disk parameters during a normal outburst Dashed: h=1,solid: h=0.2

13. Parameters related to proton interactions during a major outburst Dashed: h=1,solid: h=0.2

14. Parameters related to proton interactions during a normal outburst Dashed: h=1,solid: h=0.2

15. Electromagnetic cascade induced in the disk L=10 E37 erg/s at t=0, h=0.2

16. Soft X-ray emission from the disk Hard X-rays from the neutron star polar cap Opacity to gamma-rays in the photosphere The strong X-ray fields produced by the accretion disk and the neutron star can absorb the gamma-rays that emerge from the pion decays and the associted cascades inside the disk. Since the X-ray fields evolve with time, the gamma-ray absorption will evolve too. When the gs are absorbed (tgX>1), they initiate electromagnetic cascades dominated by inverse Compton scattering in the photosphere. The final result is a standard cascade spectrum with a cutoff energy of(see, e.g., Aharonian et al. 1985).

17. Opacity to gamma-ray propagation in the photosphere Solid line: major outburst Dahsed line: normal outburst

18. Time evolution of the opacity to gamma-rays in the photosphere Solid line: h=0.2 Dashed line: h=1.0

19. Conclusions • Be/X-ray binaries can be variable gamma-ray sources during both major and normal outbursts, when a rapidly rotating accretion disk is formed. • Protons can be accelerated by the Cheng-Ruderman mechanism impacting onto the disk surface with energies above 100 TeV . • TeV gamma-rays from pion decays can be partially absorbed in the disk, depending on the grammage. At energies <10 TeV further absorption can occur in the photosphere. • GeV emission resulting from cascades in the disk is also absorbed in the photosphere. The observed GeV emisison should be anti-correlated with the X-ray emission (Romero et al. 2001). • Low MeV gamma-rays emerging from the cascades in the disk and the photosphere should be correlated with the X-rays. • As noticed by Cheng et al. (1990) and Anchordoqui et al. (2003), Be/X-ray binaries can be strong neutrino sources.

20. Additional slides

21. Neutrinos energy distribution generated by pion decay induced by the collision of 400 TeV protons onto an accretion disc of a typical X-ray binary. The error bars indicate the RMS fluctuations for each of the mean values, represented by the height of each box in the histogram. The latter were obtained averaging over 100 showers. The solid line is a fit to this spectrum, whose parameters are shown in the insert. Zoomed is the high energy tail of the distribution, together with the same fit. A monoenergetic beam of protons impacting onto an accretion disc produces a power law spectrum of neutrinos

22. Lateral distribution for one shower. The xy-plane is parallel to the accretion disc. Most neutrinos are produced very close to where the proton impacted in the disc, both in the x and y-directions.

23. Results The shape of the cutoff given in this figure corresponds to that estimated using arguments about pion survival probability. The muon n-background within a 1x1 square of the source is indicated by the two diagonal thin-solid lines, which corresponds to horizontal and vertical. Assuming the flux is fairly uniform with decreasing the angular bin, we scaled down to a 2x2 bin the current upper limits on fluxes (90% CL) as reported by Frejus as well as by AMANDA (assuming a neutrino flux flavor ratio 1:1:1 at the detector). S/N=1.7, in 50 days, after neutrino Oscillations effect is considered