DISCOVERY OF PHOTON INDEX SATURATION IN THE BLACK HOLE BINARY GRS 1915+105

Journal Article · Tue Dec 01 04:00:00 EST 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/706/2/1463· OSTI ID:21378097

Titarchuk, Lev ^[1]; Seifina, Elena ^[2]

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1. Dipartimento di fisica, Universita di Ferrara, Via Saragat 1, I-44100 Ferrara (Italy)
2. Moscow State University/Sternberg Astronomical Institute, Universitetsky Prospect 13, Moscow, 119992 (Russian Federation)

We present a study of the correlations between spectral, timing properties, and mass accretion rate observed in X-rays from the Galactic black hole (BH) binary GRS 1915+105 during the transition between hard and soft states. We analyze all transition episodes from this source observed with the Rossi X-ray Timing Explorer, coordinated with Ryle Radio Telescope observations. We show that broadband energy spectra of GRS 1915+105 during all these spectral states can be adequately presented by two bulk motion Comptonization (BMC) components: a hard component (BMC1, photon index GAMMA{sub 1} = 1.7-3.0) with turnover at high energies and soft thermal component (BMC2, GAMMA{sub 2} = 2.7-4.2) with characteristic color temperature <=1 keV, and the redskewed iron-line (LAOR) component. We also present observable correlations between the index and the normalization of the disk 'seed' component. The use of 'seed' disk normalization, which is presumably proportional to mass accretion rate in the disk, is crucial to establish the index saturation effect during the transition to the soft state. We discovered the photon index saturation of the soft and hard spectral components at values of approx<4.2 and 3, respectively. We present a physical model which explains the index-seed photon normalization correlations. We argue that the index saturation effect of the hard component (BMC1) is due to the soft photon Comptonization in the converging inflow close to the BH and that of soft component is due to matter accumulation in the transition layer when mass accretion rate increases. Furthermore, we demonstrate a strong correlation between equivalent width of the iron line and radio flux in GRS 1915+105. In addition to our spectral model components we also find a strong feature of 'blackbody (BB)-like' bump whose color temperature is about 4.5 keV in eight observations of the intermediate and soft states. We discuss a possible origin of this 'BB-like' emission.

• OSTI ID: 21378097
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 706; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCRETION DISKS
ANTENNAS
BLACK HOLES
BOSONS
COLOR
CORRELATIONS
COSMIC RADIATION
ELECTRICAL EQUIPMENT
ELECTRONIC EQUIPMENT
ELEMENTARY PARTICLES
ELEMENTS
ENERGY SPECTRA
EQUIPMENT
HARD COMPONENT
IONIZING RADIATIONS
IRON
MASS
MASSLESS PARTICLES
METALS
OPTICAL PROPERTIES
ORGANOLEPTIC PROPERTIES
PHOTONS
PHYSICAL PROPERTIES
RADIATIONS
RADIO EQUIPMENT
RADIO TELESCOPES
SOFT COMPONENT
SPECTRA
STARS
TELESCOPES
TIMING PROPERTIES
TRANSITION ELEMENTS