Spectral states and accretion geometries in Galactic black hole binaries

Black hole X-ray binaries, binary systems where matter from a companion star is accreted by a stellar mass black hole, thereby releasing enormous amounts of gravitational energy converted into radiation, are seen as strong X-ray sources in the sky. As a black hole can only be detected via its interaction with its surroundings, these binary systems provide important evidence for the existence of black holes. There are now at least twenty cases where the measured mass of the X-ray emitting compact object in a binary exceeds the upper limit for a neutron star, thus inferring the presence of a black hole. These binary systems serve as excellent laboratories not only to study the physics of accretion but also to test predictions of general relativity in strongly curved space time. An understanding of the accretion flow onto these, the most compact objects in our Universe, is therefore of great importance to physics. We are only now slowly beginning to understand the spectra and variability observed in these X-ray sources. During the last decade, a framework has developed that provides an interpretation of the spectral evolution as a function of changes in the physics and geometry of the accretion flow driven by a variable accretion rate. This doctoral thesis presents studies of two black hole binary systems, Cygnus~X-1 and GRS~1915+105, plus the possible black hole candidate Cygnus~X-3, and the results from an attempt to interpret their observed properties within this emerging framework. The main result presented in this thesis is an interpretation of the spectral variability in the enigmatic source Cygnus~X-3, including the nature and accretion geometry of its so-called hard spectral state. The results suggest that the compact object in this source, which has not been uniquely identified as a black hole on the basis of standard mass measurements, is most probably a massive, ~30 Msun, black hole, and thus the most massive black hole observed in a binary in our Galaxy so far. In addition, results concerning a possible observation of limit-cycle variability in the microquasar GRS~1915+105 are presented as well as evidence of `mini-hysteresis' in the extreme hard state of Cygnus X-1.Över hälften av alla stjärnor på natthimlen är egentligen dubbelstjärnesystem. I ett sådant system är två stjärnor gravitationellt bundna till varandra och kretsar kring ett gemensamt centrum. Om de är mycket nära varandra kan de påverka varandras utveckling mycket starkt. Stjärnorna kan deformera varandra och i vissa fall sker massöverföring från den ena stjärnan till den andra. I de fall då en av komponenterna är en utslocknad massiv stjärna som bildat en neutronstjärna eller ett svart hål ger en sådan massöverföring upphov till röntgenstrålning. Sådana system kallas röntgenbinärer. Eftersom svarta hål endast kan påvisas genom sin påverkan på sin omgivning, ger dessa system oss en möjlighet att observationellt påvisa existensen av svarta hål. De extrema miljöer som finns i omgivningen av det svarta hålet -- med höga temperaturer, kraftig strålning och stark gravitation -- gör att denna typ av system erbjuder möjligheter även att studera fysikaliska processer under förutsättningar som inte kan återskapas på jorden. För att förstå de strålningsprocesser som sker i dessa objekt studerar man den spektrala energifördelningen. Denna säger något om geometrin i närheten av det svarta hålet. I denna avhandling presenteras studier av två röntgenbinärer innehållandes svarta hål, Cygnus X-1 och GRS 1915+105. Huvuddelen av avhandlingen ägnas dock åt det gåtfulla systemet Cygnus X-3. Genom studier av dess spektrum och hur det varierar görs en tolkning av systemets geometri. Resultaten pekar på att även detta system innehåller ett svart hål, troligen det tyngsta hittills upptäckt i en röntgenbinär i vår galax

The aperiodic broad-band X-ray variability of Cygnus X-3

We study the soft X-ray variability of Cygnus X-3. By combining data from the All-Sky Monitor and Proportional Counter Array instruments on the RXTE satellite with EXOSAT Medium Energy (ME) detector observations, we are able to analyse the power density spectrum (PDS) of the source from 10^(-9) - 0.1 Hz, thus covering time-scales from seconds to years. As the data on the longer time-scales are unevenly sampled, we combine traditional power spectral techniques with simulations to analyse the variability in this range. The PDS at higher frequencies (>10^-3 Hz) are for the first time compared for all states of this source. We find that it is for all states well-described by a power-law, with index ~-2 in the soft states and a tendency for a less steep power-law in the hard state. At longer time-scales, we study the effect of the state transitions on the PDS, and find that the variability below ~10^(-7) Hz is dominated by the transitions. Furthermore, we find no correlation between the length of a high/soft state episode and the time since the previous high/soft state. On intermediate time-scales we find evidence for a break in the PDS at time-scales of the order of the orbital period. This may be interpreted as evidence for the existence of a tidal resonance in the accretion disc around the compact object, and constraining the mass ratio to M2/M1<0.3.Comment: 8 pages, 5 figures, accepted for publication in MNRA

PoGOLite - A High Sensitivity Balloon-Borne Soft Gamma-ray Polarimeter

We describe a new balloon-borne instrument (PoGOLite) capable of detecting 10% polarisation from 200mCrab point-like sources between 25 and 80keV in one 6 hour flight. Polarisation measurements in the soft gamma-ray band are expected to provide a powerful probe into high-energy emission mechanisms as well as the distribution of magnetic fields, radiation fields and interstellar matter. At present, only exploratory polarisation measurements have been carried out in the soft gamma-ray band. Reduction of the large background produced by cosmic-ray particles has been the biggest challenge. PoGOLite uses Compton scattering and photo-absorption in an array of 217 well-type phoswich detector cells made of plastic and BGO scintillators surrounded by a BGO anticoincidence shield and a thick polyethylene neutron shield. The narrow FOV (1.25msr) obtained with well-type phoswich detector technology and the use of thick background shields enhance the detected S/N ratio. Event selections based on recorded phototube waveforms and Compton kinematics reduce the background to that expected for a 40-100mCrab source between 25 and 50keV. A 6 hour observation on the Crab will differentiate between the Polar Cap/Slot Gap, Outer Gap, and Caustic models with greater than 5 sigma; and also cleanly identify the Compton reflection component in the Cygnus X-1 hard state. The first flight is planned for 2010 and long-duration flights from Sweden to Northern Canada are foreseen thereafter.Comment: 11 pages, 11 figures, 2 table