Unveiling the high energy emission of accreting compact objects with SPI/INTEGRAL

Dr. Robert Droulans
(CESR, Université de Toulouse, France)
Mittwoch/Wednesday, 2011-4-6 14:00, EW809/10
Eugene-Paul-Wigner Gebäude (Physik-Neubau), Hardenbergstr. 36, 10623 Berlin

Abstract

The study of the high energy emission (>20 keV) is essential for understanding the radiative processes inherent to accretion flows onto compact objects (black holes and neutron stars). The X/gamma-ray continuum of these systems is successfully interpreted in terms of two components. The first component corresponds to blackbody emission from a geometrically thin optically thick accretion disk while the second component is generally associated to Compton scattering of the thermal disk flux off hot electrons. Despite considerable advances throughout the years, the heating mechanisms as well as the structure of the Comptonizing medium remain poorly understood. In order to shed light on the physical processes that govern the innermost regions of the accretion flow, we take advantage of the data archive accumulated by the SPI instrument, a high energy spectrometer (20 keV – 8 MeV) developed at the CESR (now IRAP, Toulouse, France) for the INTEGRAL mission. Above 150 keV, SPI combines a unique spectral resolution with unequalled sensitivity, being thus an ideal tool to study the high energy emission of accreting compact objects.

At this seminar, I will present the results of timing and spectral studies of three particular systems. First, I adress the high energy emission of the enigmatic microquasar GRS 1915+105, a source characterized by colossal luminosity and strong chaotic variability in X-rays. On a timescale of about one day, the photon index of the 20 – 200 keV spectrum varies between 2.7 and 3.5; at higher energies (>150 keV), SPI unveils the systematic presence of an additional emission component, extending without folding energy up to ~500 keV. Second, I address the high energy emission of GX 339–4, a source whose spectral properties are representative of black hole transients. The spectrum of the luminous hard state of this source shows a variable high energy tail (>150 keV), with significant flux changes on a short timescale (several hours). I explain the observed spectral variability in the framework of a new Comptonization model which self-consistently accounts for the presence of a magnetic field and introduce a purely non-thermal scenario as an alternative interpretation of the luminous hard state of accreting black hole binaries. Finally, I present a long term study of the high energy emission of the X-ray burster GS 1826–24. The accretion flow being extraordinarily stable, I integrated over 8 Msec of data allowing to measure the average source spectrum up to 500 keV. Once again, I find strong evidence for a hard spectral tail above 150 keV, establishing that this feature is not exclusively associated to black hole systems. I compare the results obtained for the three sources and discuss the possible physical origins of the high energy emission of stellar-size compact objects, showing that all observed spectral shapes can be explained by a non-thermal magnetized corona model.