Research on stellar-mass black holes

BHs are among the Universe most enigmatic, powerful and exotic objects. One of the most challenging questions of BH astrophysics is the nature of their emission. There is a general consensus that radio emission is associated with synchrotron radiation from a jet and the X-rays are produced in the vicinity of the compact object in a hot flow by Comptonization of some seed soft photons whose nature is debated. The origin of the IR/optical/UV emission from the BH in LMXBs is less certain. The contribution from the companion star in such systems is usually too faint and the IR/optical spectra are either connected to the jet, or the accretion process onto the compact object. The main focus of our work is the comprehensive analysis of all available sources of information and a construction of physical models consistent with the data.

Our recent studies concentrated on modeling of the broad-band spectra and temporal properties, in particular the optical-X-ray connection. We have developed the first self-consistent model for spectral formation in BH binaries where photon and electron distributions are computed simultaneously (Vurm & Poutanen 2009). We have shown that non-thermal electrons play an important role in these objects. Interestingly, the synchrotron self-Compton mechanism operating in the hot accretion flow around BHs is capable of producing their broad-band spectra in the hard state from IR to gamma-rays (Vurm & Poutanen 2008; Poutanen & Vurm 2009; Veledina et al. 2013; see Fig. 1).

We also investigated the nature of variability in stellar-mass BHs: rapid flickering and complex correlation between X-ray and optical (UV, IR) radiation on sub-second time-scales, quasi-periodic signals in these energy bands and optical/IR flares during the X-ray state transitions.

We have also measured parameters of the BH binary SWIFT J1753.5−0127 (Neustroev et al. 2014) and showed that it hosts a BH of at most 4 solar masses. This result calls into question the recent claims that there is a gap in the mass distribution of the compact objects between 2 to 5 Msun.



Figure 1: Broad-band spectrum of the BH GX 339−4. The model spectrum includes synchrotron self-Compton emission from various zones of the hot accretion disc (dashed lines) and the irradiated accretion disc and Compton reflection (dotted). From Poutanen & Veledina (2014).

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.