Suzaku Observation of the Symbiotic X-Ray Binary IGR J16194-2810

We observed IGR J16194-2810 in the low/hard state with the Suzaku X-ray satellite in 2009. The source is a Symbiotic X-ray Binary (SyXB) classified as a category of a Low-Mass X-ray Binary (LMXB), since the system is composed of an M-type giant and probably a neutron star (NS). We detected the 0.8-50 keV signal with the XIS and HXD-PIN. The 2-10 keV luminosity was L ~ 7 x 10^34 erg s^-1 corresponding to ~10^-3 L_Edd, where L_Edd is the Eddington Luminosity of a 1.4 M_o NS and a source distance of 3.7 kpc is assumed. The luminosity is similar to those of past observations. The spectral analysis showed that there are two emission components below and above ~2 keV. The hard emission component is represented by a Comptonized black-body emission model with the seed-photon temperature ~1.0 keV and the emission radius ~700 m. The seed photon is considered to come from a small fraction of the NS surface. The soft component is reproduced by either a raw black-body (~0.4 keV, ~1.7 km) or a Comptonized emission (~0.1 keV, ~75 km). We think the origin is the emission from other part of the NS surface or the accreting stream. The physical parameters of the hard emission component of IGR J16194-2810 are compared with those of an SyXB (4U 1700+24) and LMXBs (Aql X-1 and 4U 0614+091). This comparison reveals that these SyXBs in the low/hard state have a smaller radiation region (< 1 km) on the NS surface with a higher seed-photon temperature (~1 keV) than the compared LMXBs.

RXTE Observations of the Low-Mass X-Ray Binary 4U 1608-522 in Upper-Banana State

To investigate the physics of mass accretion onto weakly-magnetized neutron stars, 95 archival RXTE datasets of an atoll source 4U 1608-522, acquired over 1996-2004 in so-called upper-banana state, were analyzed. The object meantime exhibited 3-30 keV luminosity in the range of <~ 10^35 - 4 x 10^37 erg s^-1, assuming a distance of 3.6 kpc. The 3-30 keV PCA spectra, produced one from each dataset, were represented successfully with a combination of a soft and a hard component, of which the presence was revealed in a model-independent manner by studying spectral variations among the observations. The soft component is expressed by so-called multi-color disk model with a temperature of ~1.8 keV, and is attributed to the emission from an optically-thick standard accretion disk. The hard component is a blackbody emission with a temperature of ~2.7 keV, thought to be emitted from the neutron-star surface. As the total luminosity increases, a continuous decrease was observed in the ratio of the blackbody luminosity to that of the disk component. This property suggests that the matter flowing through the accretion disk gradually becomes difficult to reach the neutron-star surface, presumably forming outflows driven by the increased radiation pressure. On time scales of hours to days, the overall source variability was found to be controlled by two independent variables; the mass accretion rate, and the innermost disk radius which changes both physically and artificially.

Fermi Large Area Telescope Observation of a Gamma-ray Source at the Position of Eta Carinae

The Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope detected a gamma-ray source that is spatially consistent with the location of Eta Carinae. This source has been persistently bright since the beginning of the LAT survey observations (from 2008 August to 2009 July, the time interval considered here). The gamma-ray signal is detected significantly throughout the LAT energy band (i.e., up to ~100 GeV). The 0.1-100 GeV energy spectrum is well represented by a combination of a cutoff power-law model (< 10 GeV) and a hard power-law component (> 10 GeV). The total flux (> 100 MeV) is $3.7^{+0.3}_{-0.1} times 10^{-7}$ photons s$^{-1}$ cm$^{-2}$, with additional systematic uncertainties of 10%, and consistent with the average flux measured by AGILE (Tavani et al. 2009). The light curve obtained by Fermi is consistent with steady emission. Our observations do not confirm the presence of a gamma-ray flare in 2008 October as reported by Tavani et al. (2009), although we cannot exclude that a flare lasting only a few hours escaped detection by the Fermi LAT. We also do not find any evidence for gamma-ray variability that correlates with the large X-ray variability of Eta Carinae observed during 2008 December and 2009 January. We are thus not able to establish an unambiguous identification of the LAT source with Eta Carinae.

Suzaku Results on Cygnus X-1 in the Low/Hard State

The black-hole binary Cygnus X-1 was observed for 17 ks with the Suzaku X-ray observatory in 2005 October, while it was in a low/hard state with a 0.7-300 keV luminosity of 4.6 x 10^37 erg/s. The XIS and HXD spectra, spanning 0.7-400 keV, were reproduced successfully incorporating a cool accretion disk and a hot Comptonizing corona. The corona is characterized by an electron temperature of ~100 keV, and two optical depths of ~0.4 and ~1.5 which account for the harder and softer continua, respectively. The disk has the innermost temperature of ~0.2 keV, and is thought to protrude half way into the corona. The disk not only provides seed photons to the Compton cloud, but also produces a soft spectral excess, a mild reflection hump, and a weakly broadened iron line. A comparison with the Suzaku data on GRO J1655-40 reveals several interesting spectral differences, which can mostly be attributed to inclination effects assuming that the disk has a flat geometry while the corona is grossly spherical. An intensity-sorted spectroscopy indicates that the continuum becomes less Comptonized when the source flares up on times scales of 1-200 s, while the underlying disk remains unchanged.

Low/Hard State Spectra of GRO J1655-40 Observed with Suzaku

The Galactic black-hole binary GRO J1655$-$40 was observed with Suzaku on 2005 September 22--23, for a net exposure of 35 ks with the X-ray Imaging Spectrometer (XIS) and 20 ks with the Hard X-ray Detector (HXD). The source was detected over a broad and continuous energy range of 0.7--300 keV, with an intensity of $sim$50 mCrab at 20 keV. At a distance of 3.2 kpc, the 0.7--300 keV luminosity is $ sim 5.1 times 10^{36}$ erg s$^{-1}$ ($sim 0.7$ % of the Eddington luminosity for a 6 $M_{odot}$ black hole). The source was in a typical low/hard state, exhibiting a power-law shaped continuum with a photon index of $sim 1.6$. During the observation, the source intensity gradually decreased by 25% at energies above $sim 3$ keV, and by 35% below 2 keV. This, together with the soft X-ray spectra taken with the XIS, suggests the presence of an independent soft component that can be represented by emission from a cool ($sim 0.2$ keV) disk. The hard X-ray spectra obtained with the HXD reveal a high-energy spectral cutoff, with an e-folding energy of $sim 200$ keV. Since the spectral photon index above 10 keV is harder by $sim 0.4$ than that observed in the softer energy band, and the e-folding energy is higher than those of typical reflection humps, the entire 0.7--300 keV spectrum cannot be reproduced by a single thermal Comptonization model, even considering reflection effects. Instead, the spectrum (except the soft excess) can be successfully explained by invoking two thermal-Comptonization components with different $y$-parameters. In contrast to the high/soft state spectra of this object in which narrow iron absorption lines are detected with equivalent widths of 60--100 eV, the present XIS spectra bear no such features beyond an upper-limit equivalent width of 25 eV.