FNWI --- IMAPP Department of Astrophysics
Radboud University > Faculty of Science > Department of Astrophysics


In a survey we observe a region of the sky containing many stars and other objects, rather than to target one individual star. This can be done once, or if the aim is to find variability, multiple times. Observations can be made in various wavelengths, such as optical, ultraviolet and X-ray, or a combination of these. The large sample of sources allows for doing statistics, improved understanding of the population and a better chance to find rare objects. The surveys we work on mainly look for binary stars and white dwarfs in our galaxy, the Milky Way.

Group members

List of Surveys


The UV-excess Survey of the Northern Galactic Plane (UVEX) is part of the European Galactic Plane Surveys (EGAPS). These surveys image a 10 degree wide band of the Milky Way through 6 different color filters. The complete Galactic Plane is photographed in 15,000 exposures per color. EGAPS will contain digital multi-color information of one billion objects in the Milky Way.

Fig 1:Close-up of the Rosette Nebula Figure 1: Close-up of the Rosette Nebula. Light-years long dust filaments are silhouetted by luminous hydrogen gas in this star-forming region in the Milky Way at 5000 light-years. (Credits: Nick Wright)

These data will be used for several studies, e.g. the selection of different stellar populations, a high quality proper motion study and a 3D absorption dust map of the Milky Way.

The main aim is to find hot, blue, but relatively low-luminosity stars in the Milky Way, such as white dwarfs and compact binary stars. These populations of stellar and binary remnants are intrinsically blue, faint objects. These can be distinguished from regular main-sequence stars like the Sun because of their blue color and the reddening of the background main-sequence stars due to dust absorption (Groot et al. 2009).

Galactic Bulge Survey

Fig 2: Dust absorption based on COBE dust maps

The Galactic Bulge Survey (GBS) observes X-ray binaries in the distant Bulge, where the stellar density is much higher than in the spiral arms. In the Galactic Plane is a lot of dust, which absorbs the optical light of the binaries. Since optical observations at the locations of the X-ray sources are necessary to identify the component stars of the binary, the GBS looks between 1 and 2 degrees outside the Galactic Plane (see Figure 2).

The GBS so far has found 1200 X-ray sources, including many low-mass X-ray binaries, which consist of a very compact neutron star or black hole which steals gas from a low-mass companion star. This gas heats up in an accretion disk and emits X-rays. The number, orbital periods and luminosities of the binaries tell us about their formation and evolution. Neutron star masses and radii give insight into the structure of high-density matter (Jonker et al. 2011).

Figure 2: Dust absorption based on COBE dust maps by Schlegel et al. (1998). The rectangular boxes indicate the GBS area, where dust absorption is relatively low but still many X-ray binaries are located.

Variability Surveys

Variability surveys image part of the sky multiple times per night to pick up optical transients and variable objects, such as binaries, supernovae, flaring stars, active galactic nuclei, transiting exoplanets and rocks in our solar system.

Fig 3:Variable binaries Figure 3: Variable binaries with hot blue-white accretion disks around a hardly visible white dwarf (left, middle) or black hole (right). Credits: Mark. A. Garlick.

Two variability surveys designed to find these short-period variables are the Palomar Transient Factory on the Oschin Telescope at Palomar Observatory in California, and the OmegaWhite survey on the VLT Survey Telescope in Chile.