Abstract
We present results from the second-generation Energetic X-ray Imaging Telescope (EXITE2) observations of the black hole X-ray binary Cyg X-1 during the experiment's 1997 and 2001 flights. The EXITE2 phoswich [NaI(Tl)/CsI(Na)] detector is designed to image cosmic X-ray sources in the hard X-ray band by using the coded-aperture imaging technique from a high-altitude scientific balloon. The sky image reconstruction methodology used for EXITE2 is also discussed in detail. Background reduction (PSD rejection), subtractive flat-fielding, pixel shuffling, and image functions are introduced. During the observations from the EXITE2 1997 and 2001 flights, Cyg X-1 is easily detected in the 37-237 keV energy range. During the 1997 observations, the spectrum is well fitted by a Comptonization model. The spectral signatures and the observed 100 keV flux, together with the RXTE ASM light curve, indicate that the source was in the typical low state during this observation. Evidence is seen for hard X-ray variability on timescales on the order of 10 minutes. During the 2001 flight the spectrum is best fit by an extended power law with no observable cutoff. This is possible evidence of a transition to the high state, which is indeed seen in the RXTE ASM light curve shortly after our observation.
Highlights
The hard X-ray band (20–600 keV) provides a window into $109 K thermal and nonthermal phenomena, such as highenergy synchrotron radiation, magnetic cyclotron absorption and emission, and inverse Compton scattering of cosmic X-ray sources
We present results from the second-generation Energetic X-ray Imaging Telescope (EXITE2) observations of the black hole X-ray binary Cyg X-1 during the experiment’s 1997 and 2001 flights
A photomultiplier tube (PMT; a list of abbreviations is given in Table 1) array and processing electronics are applied to process the signals from the combined NaI/CsI
Summary
The hard X-ray band (20–600 keV) provides a window into $109 K thermal and nonthermal phenomena, such as highenergy synchrotron radiation, magnetic cyclotron absorption and emission, and inverse Compton scattering of cosmic X-ray sources. The coded aperture imaging telescope technique (Caroli et al 1987, and references therein) has been widely applied to hard X-ray and soft gamma-ray imaging telescopes (e.g., the French SIGMA telescope, 30 keV–1.3 MeV; see Paul et al 1991). This technique requires large area detectors with good spatial and spectral resolution. The high-energy spectrum of Cyg X-1, like that of most black hole X-ray binaries, is widely accepted to originate from the combination of a geometrically thin, optically thick accretion disk, radiating thermally in the soft X-ray band, and a geometrically thick, optically thin hot corona, which upscatters the soft photons from the disk into the observed Comptonized hard X-ray spectrum. Results for the Crab Nebula and 3C 273/GRS 1227+025 are given by Bloser et al (2002) and Grindlay et al (2005), respectively
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