Detection of a High-Temperature Blackbody Hump in Black Hole Spectra. The strongly redshifted annihilation line

TL;DR

This paper reports the discovery of a high-temperature blackbody component in the 15-40 keV range in black hole spectra, interpreted as a gravitationally redshifted annihilation line, with detailed analysis across multiple sources and states.

Contribution

It is the first to identify and analyze the high-temperature blackbody hump as a gravitational redshifted annihilation line in black hole spectra, expanding understanding of spectral features.

Findings

HBB component detected in intermediate states with kT_HBB 4.5-5.9 keV

HBB feature appears only in the intermediate spectral state

HBB timescale is much shorter than iron line timescale

Abstract

We discovered a so called high-temperature blackbody (HBB) component, found in the 15 -- 40 keV range, in the broad-band X-ray energy spectra of black hole (BH) candidate sources. A detailed study of this spectral feature is presented using data from five of the Galactic BH binaries, Cyg X-1, GX 339-4, GRS 1915+105, SS 433 and V4641~Sgr in the low/hard, intermediate, high/soft and very soft spectral states (LHS, IS, HSS and VSS, respectively) and spectral transitions between them using {\it RXTE}, INTEGRAL and BeppoSAX data. In order to fit the broad-band energy spectra of these sources we used an additive XSPEC model, composed of the Comptonization component and the Gaussian line component. In particular, we reveal that the IS spectra have the HBB component which color temperature, kT_HBB} is in the range of 4.5 -- 5.9 keV. This HBB feature has been detected in some spectra of these five sources only in the IS (for the photon index Gamma>1.9) using different X-ray telescopes. We also demonstrate that a timescale of the HBB-feature is of orders of magnitude shorter than the timescale of the iron line and its edge. That leads us to conclude that these spectral features are formed in geometrically different parts of the source and which are not connected to each other. Laurent & Titarchuk (2018) demonstrated a presence of a gravitational redshifted annihilation line emission in a BH using the Monte-Carlo simulations and therefore the observed HBB hump leads us to suggest this feature is a gravitational redshifted annihilation line observed in these black holes