Counterparts to X-ray and gamma-ray sources

Most X-ray binary systems are strong IR sources. The XMM-Newton satellite is studying the hard X-ray radiation from these systems, which also penetrates high galactic extinction, so the UKIDSS GPS offers an excellent opportunity for comparative studies which will identify the infrared counterparts. The INTEGRAL satellite will also provide complementary data at hard X-ray and gamma-ray energies.

A clear goal of combining the UKIDSS-GPS/XMM-Newton surveys is to resolve the galactic X-ray Galactic Ridge Emission. This ridge emission may be truly diffuse hot gas, or more likely consists of a large number of currently unresolved point sources, such as Cataclysmic Variables (CVs) and Active Stars. In addition XMM-Newton will locate many new supernova remnants, star formation regions (SFRs) and X-ray binaries (see Technical requirements), all of which will detected by the GPS in statistically useful numbers. The X-ray emission from SFRs will aid in the identification of very young clusters (5-20 Myr) detected by WFCAM but lacking significant extinction after star formation is completed.

In high-mass X-ray binaries (HMXB), IR emission arises from the circumstellar disk which undergoes major expansions in phases of X-ray activity. The near-IR flux and J-K colour provides valuable information on the temperature and density in the circumstellar environment. This information can then be related to the mass accretion rate on to the compact object. In low mass X-ray binaries (LMXB) the emission arises both from an accretion disc around the black hole or neutron star, and from the companion star. Both are heated by X-rays from near the compact object and reradiate this energy at IR to ultraviolet wavelengths.

Transient LMXBs appear to be predominantly black-hole systems which spend most of their time in X-ray quiescence, leaving their optical/near-IR emission dominated by the cool secondary star and with only a small component from the residual disc. Hence accurate IR colours contain valuable information about the luminosity and spectral type of the companion and allow, by comparison of data taken when sources were in quiescence and activity, the unraveling of the physical properties of the system and the details of the outburst events.

The GPS will provide photometry and colours for several hundred known X-ray binaries. Many more will be identified via variability in a multipass survey, which is especially crucial in cases where X-ray coordinates are accurate to only a few arcsec. Thousands of CVs and active stars will also be detected in the regions of the XMM-Newton galactic survey, providing an invaluable database for statistical studies. For X-ray binaries WFCAM will provide the following scientific benefits: