Linear Polarization in a Black Hole, 4U~1957+115 and its X-ray Spectral Analysis

2025/08/13 by Titarchuk, Lev, Seifina, Elena, Soffiitta, Paolo +1
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE) #High Energy Physics - Phenomenology (hep-ph)

paper · doi:10.48550/arxiv.2508.09692

Abstract

We present a new spectro-polarimetric method for estimation of the inclination of X-ray binaries using the linear polarization (LP) value in black hole (BH) sources, based on the early paper by Sunyaev & Titarchuk (1985, ST85). The X-ray LP arises from multiple scattering of initially soft photons in a hot, optically thick Compton cloud (CC) using a flat geometry. ST85 showed that the LP degree P depends on the binary inclination i and remains independent of the photon energy. Moreover, the P follows a characteristic angular distribution based on the CC optical depth τ₀, in particular for τ₀ > 10 the LP follows the Chandrasekhar (1950) distribution. In light of the vast number of recent IXPE spectro-polarimetric observations of BHs, testing this approach in combination with spectral analysis of these sources by other missions is desirable. In particular, the BH source, 4U 1957+115 was observed with IXPE in the high/soft state for which P∼ 2% was found (Marra et al. 2024). To apply the new method, in addition to knowing P for a given source, it is necessary to estimate the CC optical depth τ₀ and then to calculate of the best-fit photon index Γ and the plasma temperature kT_e. We present the X-ray spectral analysis of 4U~1957+11 using data from IXPE, NuSTAR, NICER, RXTE, Swift, Suzaku and ASCA. We show that the X-ray source spectra are well described by the Comptonization model with Γ varying from 1.5 to 3. We find a monotonic increase in Γ with the mass accretion rate, M, and a final saturation of the index at Γ∼3 for high M. We determine a BH mass by the scaling method: M₁₉₅₇=4.8 ± 1.8~M_\odot, assuming a source distance of 22 kpc, using H 1743-322, 4U 1630-47, and GRS 1915+105 as reference sources.

Citations

• A Comparison of the X-ray Polarimetric Properties of Stellar and Supermassive Black Holes
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• The first X-ray polarimetric observation of the black hole binary LMC X-1
• The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images
• Methods and results of an automatic analysis of a complete sample ofSwift-XRT observations of GRBs

Related

• X-ray linear polarization prediction in Black-Hole binaries and Active Galactic Nuclei and its measurements by IXPE
• The First X-ray Polarization Observation of the Black Hole X-ray Binary 4U 1630-47 in the Steep Power Law State
• Spectro-polarimetry of 4U 1957+115 with IXPE: Effects of spin and returning radiation on polarised emission of black hole in thermal state
• Detection of X-ray Polarization in the Hard State of IGR J17091-3624: Spectro-Polarimetric Study with IXPE and NuSTAR Data
• Evaluation of several explanations of the strong X-ray polarization of the black hole X-ray binary 4U 1630-47