Research on Ultra-luminous X-ray sources

Ultra-luminous X-ray sources (ULX) are bright off-nuclear X-ray point sources discovered in the nearby galaxies already with Einstein observatory. However, their nature is still debated. The main hypothesis are intermediate-mass BH or stellar-mass BHs accreting at very high rates. We have pioneered theoretical models of super-Eddington accretion discs around BH that include advection and winds (Poutanen et al. 2007) and showed that BHs in such a state can produce easily observed X-ray luminosities exceeding 1040 erg/s. On the observational side, we have studied spectral variability of 11 ULX using archival XMM-Newton and Chandra observations (Kajava & Poutanen 2009) demonstrating that the soft excess (often interpreted as a signature of a cold accretion disc around an intermediate mass BH) shows completely different trends on luminosity–temperature plane compared to the standard accretion disc. This suggests that ULX are likely stellar-mass BHs.

We have also been interested in the positions of ULX and found a statistically significant displacement between ULX and young (2–5 Myr) stellar clusters in the Antennae galaxies (Poutanen et al. 2013; see Fig. 1). This gives a strong support to the idea that ULXs are massive X-ray binaries that have been ejected in the process of formation of stellar clusters.

Recently, we discussed other alternative models for ULX where the central engine is a neutron star: either a rapidly rotating pulsar (Medvedev & Poutanen 2013) or an accreting magnetar. A recent discovery with NuSTAR of 1.37 s pulsations in ULX M82 X-2 (Bachetti et al. 2014) confirmed our expectations that some fraction of ULX host a NS instead of a BH. Our future projects include detailed studies of the super-Eddington accretion onto magnetized NSs.

Figure 1: An HST image of the Antennae galaxies with bright point sources being mostly stellar clusters. Green (and black) circles are the positions of ULX. The insets show VIMOS/VLT images that were used to study reddening and to determine cluster ages. From Poutanen et al. (2013).

Selected publications:

Working groups

X-ray binaries

Description of the group activity.
Group meetings take place on Wednesdays (first and third of each month), 15.00 in the coffee room.

Stellar explosions

Description of the group activity.
Group meetings take place on Mondays, 15.00 in the coffee room.

AGNs and Very High Energy Astrophysics

Description of the group activity.
Group meetings take place on Wednesday (second and fourth of each month), 15.00 in the coffee room.