Probing Supercritical Accretion in Ultraluminous X-ray Source M82 X-1 by means of X-ray Spectral Evolution Analysis

Izatul Hafizah,Kiki Vierdayanti,W.K Ng,A Yang,A.H Chan,C.H Sow,S.C.C Ng

doi:10.1051/epjconf/202024007005

Izatul Hafizah, Kiki Vierdayanti + Show 5 more

Open Access

https://doi.org/10.1051/epjconf/202024007005

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Abstract

We analyze the spectral evolution of ultraluminous X-ray source (ULX) M82 X-1 by means of spectral fitting. We use selectedSwift/XRT data in 2014 and 2015. The flux of M82 X-1 increased by a factor of 2-3 from 2014 to 2015. Most of the data in 2015 show greater dominance of hard component than those of 2014. Due to moderate signal-to-noise ratio, we only fit each spectrum with power-law and disk blackbody model separately. The data in 2014 are better fitted with powerlaw model based on the value of reduced-chi squared. On the other hand, both powerlaw and diskbb models showed comparable re- duced chi-squared value for the data in 2015. We found that the range of spectral index for 2014 data is 1.65 < Γ < 2.08 and for 2015 data is 1.02 < Γ < 1.95 from the powerlaw model, resembling the range for that of black hole binary system at low mass accretion rate. We obtained higher innermost disk temper- ature from the disk blackbody model, 1.20 keV <Tin< 3.63 keV, compared to that of black hole binary system in the thermal state. The calculated innermost radius of the disk,Rin, varies between 0.99 to 4.89RSassuming 10M0black hole which indicates that the spectral state is not in thermal dominant state but rather we suspect that M82 X-1 exhibits greater mass accretion rate than that of the thermal dominant state.

Highlights

Ultraluminous X-ray sources (ULXs) are defined as point-like X-ray sources located outside the center of galaxies with luminosities exceeding 3 × 1039 erg s-1 [3]
If the Eddington limit applies to these objects, ULXs is one of the best candidates for an intermediate-mass black hole (IMBH)
Modelling of the accretion disk emission shows that M82 X-1 can be explained by a ∼ 30 M stellar remnant black hole radiating at several times its Eddington limit [7]

Summary

Introduction

Ultraluminous X-ray sources (ULXs) are defined as point-like X-ray sources located outside the center of galaxies with luminosities exceeding 3 × 1039 erg s-1 [3]. These apparent luminosities, assuming isotropic emission, are above the Eddington limit of a 20 M black hole. If the Eddington limit applies to these objects, ULXs is one of the best candidates for an intermediate-mass black hole (IMBH). The reality is probably more complex, with some ULXs being possibly more massive than stellar-mass black holes (MULX > 100 M ), but still experiencing the super-Eddington accretion rate. Makes it one of the best intermediate-mass black-hole (IMBH) candidates, assuming Eddington-limited accretion.

Data and Methods

Data Reduction and Fitting Methods

Spectral Analysis

Findings

Conclusion