The Soft Gamma‐Ray Spectral Variability of Cygnus X‐1

Abstract

We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV -ray emission between the hard and soft spectral states, using spectra that cover the energy range from 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder -ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft -ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of $4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.

A year-long AGILE observation of Cygnus X-1 in hard spectral state

“…Following the model by Zdziarski & Gierliński (2004), another possible source of gamma rays, especially in the low/hard spectral state, is the high-energy tail of the electron distribution in the corona. Instead in the high/soft state, the energy spectrum does not show an energy cutoff, as confirmed by the COMPTEL detection up to ∼10 MeV (McConnell et al 2002) and a gamma ray emission above this energy is expected.…”

“…Interestingly, Cyg X-1 was also detected by COMPTEL in high/soft state, up to about 10 MeV energy (McConnell et al 2002). At that time the EGRET instrument was switched off and no GeV observation is available.…”

“…EGRET did not detect the source during that observation and the upper limit to the flux is of the order of 10 × 10 −8 ph cm −2 s −1 , posing no need for a high-energy cut-off. In the soft state, the spectrum of Cyg X-1 can be modelled with a power-law that extends with the same photon index (∼2.5-3) beyond 1 MeV and up to about 10 MeV as detected by COMPTEL (McConnell et al 2002). Unfortunately in this case, no EGRET measurement is available.…”

Gamma‐Ray Spectral State Transitions of GRO J1719−24

“…2b, panels 10 and 11. The high-and low-state spectra intersect at $400 keV, compared to those observed for GRO J0422+32 at $600 keV ( Ling & Wheaton 2003a) and Cygnus X-1 at $1 MeV McConnell et al 2002). contemporaneously with OSSE periods A and B showed a slightly better fit with the exponential truncated power law than the power law ( Table 6).…”

“…GRO J1719À24, a low-mass X-ray binary system with a 14.7 hr periodicity, consisting of a $4.9 M compact object and a $1.6 M companion star ( Della Valle et al 1994;Masetti et al 1996), is one of three Galactic black hole systems, along with GRO J0422+32 ( Ling & Wheaton 2003a) and Cygnus X-1 ( Phlips et al 1996;Ling et al 1997;McConnell et al 2000McConnell et al , 2002, that displayed similar gamma-ray spectral characteristics when undergoing transitions between the high and low gammaray intensity states 1. When these sources were in the high gamma-ray intensity state ( 2 for Cygnus X-1; Ling et al 1997), their spectra showed two components: a Comptonized component from $30 to $200 keV followed by a soft power-law tail of photon index >3 above $200 keV that extended to $500 keV for GRO J1719À24, $1 MeV for Cygnus X-1 ( Ling et al 1997;McConnell et al 2000), and $600 keV for GRO J0422+32 ( Ling & Wheaton 2003a).…”