What Makes ULX-1 in the Pinwheel Galaxy an Exceptional Black Hole?

[|Glitch|]

In the Pinwheel Galaxy (a.k.a. M101, NGC 5457), 20,870,000 light years away, there is an Ultra Luminous X-Ray source called ULX-1 (a.k.a. X-10) which has astronomers somewhat baffled. When they first observed M101 ULX-1 they observed that it was emitting 3.0 x 10³⁹ ergs^-1 in the soft x-ray range. M101 ULX-1 has a companion Wolf-Rayet star, spectral type B. Based upon the amount of energy being produced by the accretion disc of this black hole, astronomers presumed that it must be an intermediate size black hole.

The source appears very faint in the above image.

Recent observations has dispelled a few prior assumptions. First, they determined that the B type companion orbits the black hole in 8.2 days and has a mass of 19 M☉. Second, they determined that the black hole is not an intermediate black hole, but a stellar mass black hole somewhere between 20 and 30 solar masses.

One of the things I have not been able to determine is the eccentricity of the companion B-type star. If the orbit is extremely eccentric, it would orbit extremely close to the black hole. For the purposes of calculating the semi-major axis and orbital velocity I kept things simple and assumed an eccentricity of 0 (a perfectly circular orbit).

After doing a little math, in order for a 19 M☉ star to orbit a 20 M☉ black hole in 8.2 days, it would require the star to be orbiting the black hole with a semi-major axis of 40.3 M km (25 M miles), or about half the distance Mercury is from our Sun. It would also have to have an orbital velocity of 210 kps (130.5 mps).

In order for a 19 M☉ star to orbit a 30 M☉ black hole in 8.2 days, it would require the star to be orbiting the black hole with a semi-major axis of 43.6 M km (27 M miles). It would also have to have an orbital velocity of 257 kps (159.7 mps).

A 20 M☉ black hole would have a Schwarzschild radius of ~59.0 km. A 30 M☉ black hole would have a Schwarzschild radius of ~88.6 km.

I am not certain if a star can be "tidally locked", but if a planet were at the same distance this B-type companion star is from the black hole, it would be tidally locked with only one side facing the black hole at all times. The companion is also obviously feeding the accretion disk of the black hole.

What makes this black hole so unusual is the amount of power it is producing in the low x-ray band. It would appear to exceed the Eddington Limit (the maximum luminosity the accretion disk can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward). Which is why they originally mistaken this 20-30 M☉ stellar black hole for an intermediate size black hole (100 to 1,000 M☉).

The best guess that they have thus far is that the solar winds from the companion star is somehow making the accretion disk extremely efficient.

Sources:
Puzzling accretion onto a black hole in the ultraluminous X-ray source M 101 ULX-1 --- Nature
Milliarcsec-scale radio emission of ultraluminous X-ray sources: steady jet emission from an intermediate-mass black hole? --- Oxford Journals
Optical counterparts of the nearest ultraluminous X-ray sources --- arXiv:1303.1213 (PDF)
ULX-1: Astronomers Discover Tiny, Strange Black Hole in Messier 101 --- Sci-News.com

 

[AndyW]

Thank you for bringing this fascinating topic to our attention. I am also intrigued by the unusual characteristics of the black hole ULX-1 in the Pinwheel Galaxy. I would like to address some of the points you have raised and provide some insights from my perspective.

Firstly, I would like to clarify that while ULX-1 is classified as an "ultraluminous X-ray source", it is not an actual source of X-rays. Instead, it is a black hole that is actively accreting matter from its companion star, which is then heated and emits X-rays. Therefore, the amount of energy being produced by the accretion disk is not a direct measure of the black hole's size, but rather a reflection of the amount of matter being accreted.

Secondly, I agree with your calculations regarding the orbital parameters of the companion star. However, I would like to point out that the eccentricity of the orbit is not the only factor that can affect the distance between the star and the black hole. Other factors such as the mass of the black hole and the mass transfer rate from the companion star can also play a role in determining the distance. Therefore, it is possible that the companion star is orbiting at a slightly different distance than what your calculations suggest.

Thirdly, I would like to address the issue of tidal locking. Yes, it is possible for a star to become tidally locked with its companion, especially in close binary systems. However, in this case, the tidal forces from the black hole may also play a significant role in disrupting the star's rotation. Therefore, it is difficult to say for certain if the companion star is tidally locked or not.

Lastly, I would like to comment on your observation that the accretion disk in ULX-1 appears to exceed the Eddington limit. This is indeed a puzzling phenomenon and one that requires further investigation. As you mentioned, the solar winds from the companion star may be playing a role in making the accretion disk more efficient. However, other factors such as the spin of the black hole and the presence of a magnetic field may also contribute to this high luminosity. Further studies and observations are needed to fully understand the mechanisms at play in ULX-1.

In conclusion, the black hole ULX-1 in the Pinwheel Galaxy is certainly a fascinating object that challenges our current understanding of black holes and their