INTEGRAL Science Operations Centre. For contact information, see the About ISOC page.
Almost all of my research activities are related to accreting X-ray binaries, i.e. systems in which a (usually) main sequence star is losing matter and some of this is accreted by a compact object — a black hole, neutron star or white dwarf — emitting high-energy radiation, fueled by the infalling matter.
To a large extent I am concentrating on accreting X-ray pulsars, where the compact object is a neutron star with a high magnetic field (typically B≈1012 Gauss), funneling the accreted matter onto hotspots at the magnetic poles. Using data from various high-energy missions (INTEGRAL, RXTE, XMM-Newton, Suzaku, Swift and others) my colleagues and I study different aspects of these systems at quite different scales.
On the scale of the binary system, ranging roughly from 109 to 1011 m, we are interested in the physics of mass transfer between companion and compact object. For example in the Be star mechanism, fueling the majority of accreting X-ray pulsars, or in the details of wind accretion and all its complexity. One example of the latter is the very well known wind accretor Vela X-1, which sometimes shows very strong flares but also short 'off-states' of a few 10 minutes duration, both possibly explained by varying accretion in a clumpy stellar wind (Kreykenbohm et al. 2008). More details on Be X-ray binary research are given on a separate page. In recent years, we have also turned our interest to the peculiar system GX 1+4, a symbiotic X-ray Binary, where the details of mass transfer are not well understood.
Studying the period evolution of accreting pulsars tells us something about the interactions between the neutron star's magnetic field and the surrounding matter on the scale of the magnetosphere (typically a few 106 m). Following the rising phase of the 2005 outburst of the transient A0535+26, we have observed a behaviour with can be best understood as accretion through magnetospheric instability (Caballero et al. 2008, Postnov et al. 2008). Since then there have been more outbursts of this source, in February/March 2011 we have finally managed to capture a giant outburst with INTEGRAL - the data are currently being analyzed. To understand the pulse period evolution in GX 1+4 (González-Galán et al. 2010), a new model of accretion from a slow wind has been proposed by Postnov et al. (2010).
Finally, we study the spectra and light curves of the X-ray emission created on the surface of the neutron star or close to it in the accretion column, thus regions of a few km in size or less. A rather complex problem, that we have recently started to tackle is the modeling of the pulsed emission. The complication is the light bending around the neutron star through the effects of general relativity. Results for A0535+26 have been presented, e.g., at the 7th Integral Workshop and published in Caballero et al. 2011.
One of my specialties is the study of cyclotron resonant scattering features, or cyclotron lines in brief. These features, appear as broad, absorption-like structures in the spectra of some accreting X-ray pulsars. Their centroid energies are the only direct method to measure the actual magnetic field strengths. The detailed shapes of the lines, resolved with modern missions, yield additional information on the physical conditions in the accretion columns, when compared to detailed models of the photon transfer in the line-forming region (see, e.g., Schönherr et al. 2007).
Research Activities and Collaborations
While project work dominates my time largely, I maintain involved in active research on the topics mentioned above largely due to collaborations, joining forces both for proposals for observing time, as for the subsequent analysis of the data.
First of all, there is the X-ray binary group within the ESAC Faculty, led by Erik Kuulkers , which gives a framework to discuss and develop scientific activities. Together with Erik or Marion Cadolle Bel I have been and am involved in several scientific trainee projects.
Besides this group, I have kept a close connection to the (informally organized) MAGNET group which ties together scientists from Tübingen, Bamberg, Potsdam, San Diego, Goddard, Southampton, Alicante, Saclay and more places, all interested in accreting X-ray pulsars and their magnetic fields. Besides semi-regular phone conferences and email contact, the group has met at the International Space Science Institute, Bern and at the Astronomical Institute Tübingen for intense working meetings. The talks presented at the meeting in Tübingen can be found here.
My publications in ADS
Besides the ESAC Faculty, I am a member of the International Astronomical Union, the European Astronomical Society and the Swiss Society for Astrophysics and Astronomy