Matter under extreme conditions
Accreting Galactic Compact Objects
•How high in energy does the non-thermal component from accreting BHs extend, and what are the detailed requirements for the process of accelerating electrons?
•What is the rate of positron production from accreting BHs with jets, and what fraction of the Galactic bulge 511 keV emission can be attributed to X-ray binaries?
•Does the polarization of γ-rays from accreting compact objects indicate an origin for this emission in the disk or in the jet?
•Using the nuclear 2.2 MeV line, what is the gravitational redshift at the surface of the neutron star, and what is the neutron star equation of state?
Physical processes in systems with accreting black holes operate to produce non-thermal components that extend to >MeV energies. However, these components have not been well characterized due to lack of sensitivity by previous 1-10 MeV missions, and their origin is not well understood. The unprecedented DUAL sensitivity will allow for the first measurements of the non-thermal components from dozens of black hole binaries.
The previous detections of accreting black holes above 1 MeV illustrate the different types of non-thermal components that may be common. At high accretion rate, Cygnus X-1 shows a power-law component that extends to ~10 MeV (McConnell et al. 2002). While similar components have also been seen for several systems up to ~1 MeV (Grove et al. 1998; Tomsick et al. 1999), the high-energy cut-off to this component has never been detected. Measuring the cut-off with DUAL will determine the efficiency of the electron acceleration mechanism, which may be shocks or plasma waves in the accretion disk (Becker et al. 2008). The cut-off energy is also related to the compactness (power per unit volume) of the emitting region as higher electron/positron pair production leads to a lower cut-off (Zdziarski et al. 2009). A second example of non-thermal emission beyond 1 MeV has been seen from Cyg X-1 at lower accretion rates when the system was producing a powerful radio jet (McConnell et al., 2002), and multiwavelength studies that include DUAL would test whether this non-thermal γ-ray component arises from the jet. Finally, both Cyg X-1 and Cyg X-3 have been detected by Fermi and AGILE at ~100 MeV (Tavani et al. 2009; Abdo et al. 2009), and DUAL coverage in the gap between X-ray and Fermi/AGILE energies will provide critical information about how the X-ray/gamma-ray relationship.
There are two other binary systems that have been detected at GeV/TeV energies, LS 5039 and LS I +61◦ 303. Although these were originally thought to be accreting black hole systems, another possibility is that they are non-accreting binaries harboring rotation-powered pulsars (Dubus 2006). DUAL will provide a new window on these sources, and may uncover a larger population of γ-ray binaries.
The DUAL sensitivity to the electron/positron annihilation line will either provide a measurement or very constraining limits on positron production from X-ray binaries. Although the 511 keV Galactic bulge component suggests X-ray binaries as a positron contributor, SPI has not detected any 511 keV point sources, this is partly related to its sensitivity limit and, for systems with jets, it may also be a consequence of positrons escaping the systems before annihilating. However, there are a small number of black hole jet systems for which the jet appears to be misaligned so that it points very close to or perhaps even strikes the companion star. In these cases, one expects to see a true 511 keV point source, and detailed calculations have shown that even an improvement in detection sensitivity by a factor of a few over SPI is likely to lead to the detection of this line (Guessoum et al. 2006).
When charged particles accrete onto a neutron star X-ray binary, the plasma temperature reaches 1010–1012 K, equivalent to ~MeV energies and exceeding the nucleon binding energy. Hence, nuclei heavier than hydrogen will tend to break up to produce a population of free neutrons, which can be captured by hydrogen to produce deuterium and a characteristic 2.2 MeV photon (e.g., Aharonian & Sunyaev 1984). From RHESSI and INTEGRAL-SPI observations of the Be X-ray binary 1A 0535+262, the upper limit on the line flux has been measured to be (2–11)×10−4 cm-2 s−1 (Boggs & Smith 2006; Caliskan et al. 2009). However, detailed predictions for the strength of this line are significantly lower (Bildsten et al. 1993), and by probing lower flux levels, DUAL will have an opportunity to make an important discovery by detecting this or possibly other nuclear de-excitation lines. Detection of red-shifted lines from the neutron star surface will help constrain the nuclear equation of state under these extreme conditions.
Yet another motivation for DUAL observations of X-ray binaries is the measurement of the γ-ray polarization, which is highly discriminating of emission mechanisms and source geometries but which has not been previously available at X-ray or γ-ray energies. In the infrared, X-ray binaries have shown polarization levels of 4–7%, and this has been interpreted as due to synchrotron emission from a jet (Russell & Fender 2008). The Gravity and Extreme Magnetism Small Explorer (GEMS) mission is planned for launch in 2014 and will make X-ray polarization measurements of X-ray binaries. As the X-rays are largely thermal in origin, comparing polarization strengths and angles measured by GEMS and DUAL will provide new information about the relationship between thermal and non-thermal emission and between the accretion disk and the jet.
