# The Black Hole Candidate LSI+61303

## TL;DR

This study analyzes Swift X-ray observations of LSI+61303 to determine if it behaves like a black hole binary, finding its spectral properties align with black hole systems rather than pulsars, suggesting it is a black hole candidate.

## Contribution

The paper provides evidence that LSI+61303 exhibits spectral trends consistent with black hole X-ray binaries, supporting its classification as a black hole candidate based on X-ray spectral analysis.

## Key findings

- LSI+61303 shows an anti-correlation between photon index and luminosity, similar to black hole binaries.
- The spectral trend of LSI+61303 overlaps with known black hole systems in the luminosity-Eddington ratio plane.
- PSR B1259-63 exhibits a positive correlation, contrasting with black hole systems.

## Abstract

In recent years, fundamental relationships for the black hole X-ray binaries have been established between their X-ray luminosity $L_X$ and the photon index $\Gamma$ of their X-ray spectrum. For the moderate-luminosity regime, an anti-correlation between $\Gamma$ and $L_X$ has been observed. In this article, aimed to verify if the moderate luminous X-ray binary system LSI +61303 is a black hole, we analyse $Swift$ observations of LSI +61303. We compare the derived $L_X$ vs $\Gamma$ distribution, first with the statistical trend for black hole X-ray binaries, then with the trend of the pulsar PSR B1259-63, and finally with the individual trends of the black hole X-ray binaries Swift J1357.2-0933 and V404 Cygni. We find that the system PSR B1259-63 shows a positive correlation between $\Gamma$ and $L_X$, whereas in contrast LSI +61303 shows the same anti-correlation as for black hole X-ray binaries. Moreover, the trend of LSI +61303 in the $L_X$/$L_{Eddington} - \Gamma$ plane overlaps with that of the two black holes Swift J1357.2-0933 and V404 Cygni. All three systems, Swift J1357.2-0933, V404 Cygni and LSI +61303 well trace the last part of the evolution of accreting black holes at moderate-luminosity until their drop to quiescence.