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- Axelsson, Magnus, et al.
(författare)
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Vanishing hardness-flux correlation in Cygnus X-1 : Signs of the disc moving out
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 490:1, s. 253-258
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We investigate observations of the X-ray binary Cygnus X-1 with unusually high hardness and low flux. In particular, we study the characteristic frequencies seen in the PDS and the hardness-flux correlation within and between these observations.Methods. We analyse observations of Cyg X-1 during periods when the source reaches its highest hardness levels (≳ 1 for the 9-20 keV over 2-4 keV RXTE/PCA count ratios, corresponding to Γ ≲ 1.6). Using the relativistic precession model to interpret the PDS, we estimate a value for the inner radius of the accretion disc. We also study the hardness-flux correlation.Results. In the selected observations, the characteristic frequencies seen in the power spectrum are shifted to the lowest end of their frequency range. Within a single observation, the hardness-flux correlation is very weak, contrary to the negative correlation normally observed in the hard state. We suggest that this could be interpreted as the inner disc boundary being at large radii (≳50Rg), thereby requiring more time to adjust to a changing accretion rate than allowed by a single RXTE observation, and compare our findings to estimates of the viscous time scale responsible for small scale variability in the system.
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| 2. |
- Takahashi, H., et al.
(författare)
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Beam test results of the polarized gamma-ray observer, PoGOLite
- 2008
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Ingår i: 2008 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE (2008 NSS/MIC), VOLS 1-9. - 9781424427147 ; , s. 732-736
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Konferensbidrag (refereegranskat)abstract
- The Polarized Gamma-ray Observer, PoGOLite, is a balloon experiment with the capability of detecting 10% polarization from a 200 mCrab celestial object in the energy range 25 #x2013;80 keV. During a beam test at KEK-PF in February 2008, 20 detector units were assembled, and a 50 keV X-ray beam with a polarization degree of #x223C;90% was irradiated at the center unit. Signals from all 20 units were fed into flightversion electronics consisting of six circuit boards (four waveform digitizer boards, one digital I/O board and one router board) and one microprocessor (SpaceCube), which communicate using a SpaceWire interface. One digitizer board, which can associate up to 8 PDCs, outputs a trigger signal. The digital I/O board handles the trigger and returns a data acquisition request if there is no veto signal (upper or pulse-shape discriminators) from any detector unit. This data acquisition system worked well, and the modulation factor was successfully measured to be #x223C;34%. These results confirmed the capabilities of both detector and data-acquisition system for a pathfinder flight planned in 2010.
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