New transient X-ray source detected in galaxy NGC 4945

Using the Suzaku satellite, Japanese astronomers have detected a transient X-ray source in a nearby galaxy known as NGC 4945. The newly discovered source, named Suzaku J1305-4930, appears to be a black hole binary. The finding is detailed in an article published July 8 in the arXiv preprint repository.

At a distance of approximately 12.1 million light years from Earth, NGC 4945 is a spiral galaxy that houses one of the brightest active galactic nuclei (AGN) in the hard X-ray band. Previous observations of this galaxy have identified several bright X-ray sources, including ultralight X-ray sources (ULX).

Now, a team of astronomers led by Shuntaro Ide of the University of Osaka reports the detection of a new transient X-ray source in NGC 4945 that nearly reaches the brightness required to classify it as ULX. The discovery was made using the X-ray Imaging Spectrometer (XIS) aboard the Japanese spacecraft Suzaku.

“Among the seven Suzaku observations from this region, the source was detected in four observations from July 2010 to August 2010,” the astronomers wrote in the document.

Suzaku J1305−4930 was identified about 9,800 light years from the core of NGC 4945. The highest observed X-ray luminosity of 0.3–10 keV from this source reached 890 undecillion erg / s, while the temperature in its radius of internal disk was approximately 1.12 keV. At its last detection in August 2010, the X-ray luminosity 0.3–10 keV of Suzaku J1305−4930 decreased to approximately 220 erg / s and the temperature in the radius of the inner disk was measured to be around 0.62 keV.

The investigation found that Suzaku J1305-4930’s innermost disk radius is approximately three times the Schwarzschild radius and its mass is approximately 10 solar masses. According to astronomers, these results, along with the measured luminosity, suggest that Suzaku J1305−4930 is a stellar-mass black hole binary.

“If Suzaku J1305−4930 is a stellar-mass black hole binary in NGC 4945, it means that by chance we have obtained multiple observations with relatively short intervals between such a source outside of our galaxy,” the researchers added.

Furthermore, the results suggest that during the four observations made in 2010, Suzaku J1305−4930 was not in the standard disk state but in the thin disk state. Astronomers assume that the source has undergone a transition from the standard disk to the thin disk state. Such a transition has been reported by previous studies for other galactic black hole binaries such as XTE J1550-564 and GRO J1655-40.

“Their Ldisk vs Tin diagrams exhibit clear state transitions from the standard disk state to the thin disk state. Our results in Suzaku J1305-4930 can be interpreted alternatively with a similar state transition,” the authors of the article explained.

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