We report the Suzaku observation of 1E 1740.7-2942, a black hole candidate called the "Great Annihilator" (GA). The high-quality spectrum of Suzaku provides the severest constraints on the parameters of the GA. Two clumpy structures are found around the GA in the line images of FeI Kalpha at 6.4 keV and SXV Kalpha at 2.45 keV. One clump named M359.23-0.04 exhibits the 6.4-keV line with an equivalent width of ~ 1.2 keV, and is associated with a molecular cloud in the radio CS(J=1-0) map. Thus the 6.4-keV line from M359.23-0.04 is likely due to X-ray fluorescence irradiated by an external X-ray source. The irradiating X-rays would be either the past flare of Sagittarius A* or the bright nearby source, the GA. The other clump named G359.12-0.05 is associated with the radio supernova remnant candidate G359.07-0.02. We therefore propose that G359.12-0.05 is an X-ray counterpart of G359.07-0.02. G359.12-0.05 has a thin thermal plasma spectrum with a temperature of kT ~ 0.9 keV. The plasma parameters of G359.12-0.05 are consistent with those of a single supernova remnant in the Galactic center region.
Deep Dive into Suzaku Observations of the Great Annihilator and the Surrounding Diffuse Emissions.
We report the Suzaku observation of 1E 1740.7-2942, a black hole candidate called the “Great Annihilator” (GA). The high-quality spectrum of Suzaku provides the severest constraints on the parameters of the GA. Two clumpy structures are found around the GA in the line images of FeI Kalpha at 6.4 keV and SXV Kalpha at 2.45 keV. One clump named M359.23-0.04 exhibits the 6.4-keV line with an equivalent width of ~ 1.2 keV, and is associated with a molecular cloud in the radio CS(J=1-0) map. Thus the 6.4-keV line from M359.23-0.04 is likely due to X-ray fluorescence irradiated by an external X-ray source. The irradiating X-rays would be either the past flare of Sagittarius A* or the bright nearby source, the GA. The other clump named G359.12-0.05 is associated with the radio supernova remnant candidate G359.07-0.02. We therefore propose that G359.12-0.05 is an X-ray counterpart of G359.07-0.02. G359.12-0.05 has a thin thermal plasma spectrum with a temperature of kT ~ 0.9 keV. The plasma pa
The Galactic center (GC) region has many celestial objects such as a supermassive black hole Sagittarius A * (Sgr A * ), star-forming regions, dense molecular clouds (MCs). The GC region is also full of high-energy phenomena. For example, several supernova remnants (SNRs) or candidates have been recently found (e.g., Nobukawa et al. 2008;Mori et al. 2008;Sawada et al. 2009;Tsuru et al. 2009). The most characteristic phenomenon is the diffuse 6.7-keV lines from highly ionized irons (Koyama et al. 1989;Yamauchi et al. 1990). The origin of this emission is probably due to a plasma with kT ∼ 6.5 keV temperature (Koyama et al. 2007b), but is still a debatable issue. Another notable feature in the GC region is the clumpy 6.4-keV lines from neutral irons (e.g., Koyama et al. 1996). The origin is considered as MCs illuminated by X-rays or electrons. Some of the 6.4-keV clumps are well explained by the irradiation of the past X-ray flare from Sgr A * (e.g., Nobukawa et al. 2008;Inui et al. 2009;Nakajima et al. 2009).
1E 1740.7-2942 was discovered by the Einstein observatory (Hertz & Grindlay 1984) in the direction of the GC region, and was found to be the brightest GC source at above 20 keV (Skinner et al. 1987;Sunyaev et al. 1991). The time variation of the flux and the spectrum of 1E 1740.7-2942 are similar to those of Cygnus X-1, the archetypal black hole candidate, and hence 1E 1740.7-2942 has been considered to be a black hole candidate (Cook et al. 1991;Skinner et al. 1991).
GRANAT detected a prominent bump on the spectrum of 1E 1740.7-2942 at 300-600 keV, which was interpreted as an electron-positron annihilation line at 511 keV (Bouchet et al. 1991;Sunyaev et al. 1991;Churazov et al. 1993a;Cordier et al. 1993). Thus 1E 1740.7-2942 is named the “Great Annihilator” (GA). However, no evidence for the annihilation line has been found so far by other satellites (Harris et al. 1994;Jung et al. 1995;Smith et al. 1996;Cheng et al. 1998;Bouchet et al. 2009).
The VLA radio observations discovered a radio counterpart of the GA and non-thermal double jet-like structures emanating from the GA (Mirabel et al. 1992), and hence the GA is a “micro quasar”. Like the other Galactic jet sources, the GA would be a binary system with a stellar-mass black hole. In fact, the possible orbital period of 12.73 days was discovered by Smith et al. (2002). However, no clear companion star was found with the optical, infrared or radio observations, due mainly to the strong interstellar absorption toward the GC region (e.g., Eikenberry et al. 2001).
The X-ray spectrum of the GA below 10 keV is explained by an absorbed power-law model (Sakano et al. 1999;Gallo & Fender 2002). Cui et al. (2001) reported the extended X-ray emission which is perpendicular to the radio-jets (but see Gallo & Fender 2002). On the other hand, the INTEGRAL observations suggested that the [Vol. , spectrum in the 10-100 keV band is explained by a model of either a comptonized-plasma plus a power-law or two comptonized-plasmas (Bouchet et al. 2009).
The radio observations found a giant MC near the GA (Bally & Leventhal 1991;Mirabel et al. 1991). Then the authors proposed the Bondi-Hoyle accretion mechanism (Bondi & Hoyle 1944); the GA is in the MC, and is powered by the gas accretion from the MC. However the ASCA observations found no clear evidence for dense gas around the GA (Churazov et al. 1996;Sakano et al. 1999).
In order to investigate the accretion mechanisms, whether the Bondi-Hoyle process, due to a binary companion or else, we performed the Suzaku observation on the GA, with its high-quality spectrum and low background. The other objective of the observation is to discover local structures, if any, on the GA and in the close vicinity of the GA, and investigate the physical relation to the GA. We assume the distance to the GC to be 8.5 kpc in this paper.
Two pointing observations toward the GA were performed in September 2008 with the X-ray Imaging Spectrometer (XIS: Koyama et al. 2007a) at the focal plane of the X-Ray Telescope (XRT: Serlemitsos et al. 2007) on board the Suzaku satellite (Mitsuda et al. 2007). The observation log is given in table 1.
The XIS system consists of three sets of frontilluminated (FI) CCD cameras (XIS 0, 2 and 3) and one set of a back-illuminated (BI) CCD camera (XIS 1). The performance of the CCD cameras has been gradually degraded due to the radiation damage by cosmic-ray particles, thus the Spaced-row Charge Injection (SCI) technique was introduced to restore the energy resolution since October 2006 (Uchiyama et al. 2009). Thanks to the SCI, XIS energy resolutions (FI/BI) at 5.9 keV were 155/175 eV (FWHM) during the observations. However, XIS 2 is unusable for scientific observations because of the sudden anomaly in 2006, and we use the data from the remaining three cameras.
The XRT consists of closely nested thin-foil reflectors and has large collecting efficiency (450 cm 2 at 1.5 keV and 250 cm 2 at 7 keV per XRT).
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