Object 17: 4U 1957+115

Podcast release date: 23 March 2020

Right ascension: 19:59:24.0


Epoch: ICRS

Constellation: Aquila

Corresponding Earth location: About 57 km northeast of Tobago

4U 1957+115 is yet another astronomical object identified by its coordinates. What's really annoying is that the object name begins with the rather unfortunate abbreviation 4U for the fourth Uhuru catalog of X-ray sources, the astronomical catalog that gives this object its most commonly-used name [1].

The object was originally discovered as an X-ray source in the early 1970s by the Uhuru satellite, the first X-ray observatory in space [2]. By the way, do not confuse the satellite Uhuru with the Star Trek communications officer, Uhura, or the current President of Kenya, Uhuru Kenyatta. However, all three of these people or things get their name from the Swahili word for freedom, so maybe you can confuse them a little if you want to. In any case, Uhuru was groundbreaking in that it detected multiple X-ray sources in space for the first time.

After Uhuru's observations, astronomers knew that an X-ray source was located roughly somewhere in the location of 4U 1957+115, but its counterpart was not found in light in the visible part of the spectrum until 1978, when the X-ray emission was associated with a star in Aquila which had previously been given the designation V1408 Aquilae [3] . Shortly after this, the object was identified as one of a class of new objects called low mass X-ray binaries (LMXBs). These are thought to be binary star systems consisting of a normal star about the size of the Sun or smaller and a compact object that is the dead core of a star that burned up all of its hydrogen a very long time ago. This compact object could be either a neutron star or a black hole. The two stars in any LMXB are orbiting so close to each other that the compact object is able to strip gas away from the outer atmosphere of the other star. While the gas falls onto the compact object, the gas gets gravitationally compressed and heats up, which leads to the production of X-ray emission. Also, very interestingly, the X-rays and other forms of electromagnetic radiation can heat up one side of the companion star. (Yes, this process actually makes an object that we associate with producing heat actually get hotter.)

The latest astronomical observations have demonstrated that the two stars in 4U 1957+115 orbit each other about once every 9.3 hours and that we are looking close to straight down the orbital axis of the system [4,5]. Models of the X-ray and visible light indicate that the "normal" star has about the same mass as the Sun, while the compact object has a mass about 3 times the mass of the Sun [5]. So far, this all seems rather typical for LMXBs, but 4U 1957+115 is a little weird compared to other LMXBs.

First of all, the X-rays from most other LMXBs tend to appear as bursts, but the X-ray emission from 4U 1957+115 has been relatively constant for over 40 years, which actually makes it similar to high-mass X-ray binaries that contain black holes [5]. Also, the X-ray emission is consistent with originating from a region that is relatively hot and small, which implies that a black hole is present [5]. Moreover, the black hole is probably spinning [6]. This sounds weird at first because it's not possible to actually see anything related to surface features on a black hole. However, spinning black holes will also distort or drag space and time around them in such a way that it is possible for infalling gas to get closer to the black hole without disappearing into it, so the gas will appear hotter and produce more X-rays than if the black hole was not rotating. The X-ray emission from 4U 1957+115 seems to indicate that the event horizon of the black hole is not only spinning but that it is spinning at close to the speed of light.

Aside from the fact that the black hole is spinning very fast, the mass of this particular black hole is rather interesting. In terms of the dead cores of stars, astronomers expect to find some type of mass limit where objects below the limit are neutron stars and objects above the limit are black holes [5]. Quite a few objects have been found that look like black holes but that are larger than about 4 times the mass of the Sun, while the most massive objects that have been identified as neutron stars have masses of up to about 2 times the mass of the Sun. 4U 1957+115 would appear to be a very small black hole, and this may indicate that it's too difficult to make a stable neutron star that is three times the mass of the Sun.

As a final note, I found it weird that no one knew the distance to this object. It's too far to use the technique of parallax, which is the slight shift in the location where a star appears relative to objects behind it as seen while the Earth orbits the Sun and views the star from a slightly different angle. Also, people don't know what the total amount of energy is that is radiated from the star system. If they did know this, they could measure its relative brightness as seen from Earth to determine the distance to it. The distance estimates for 4U 1957+115, which come from the models of the black hole, range from about 30000 to 130000 light years [5].


[1] Forman, W. et al., The fourth Uhuru catalog of X-ray sources., 1978, Astrophysical Journal Supplement Series, 38, 357

[2] Giacconi, R. et al., An X-Ray Scan of the Galactic Plane from UHURU, 1971, Astrophysical Journal Letters, 165, L27

[3] Margon, B. et al., The optical counterpart of 3U 1956+11., 1978, Astrophysical Journal, 221, 907

[4] Mason, Paul A. et al., Long-term Optical Observations of Two LMXBS: UW CrB (=MS 1603+260) and V1408 Aql (=4U 1957+115), 2012, Astronomical Journal, 144, 108

[5] Gomez, Sebastian et al., The Case for a Low Mass Black Hole in the Low Mass X-Ray Binary V1408 Aquilae (= 4U 1957+115), 2015, Astrophysical Journal, 809, 9

[6] Nowak, Michael A. et al., Suzaku Observations of 4U 1957+11: Potentially the Most Rapidly Spinning Black Hole in (the Halo of) the Galaxy, 2012, Astrophysical Journal, 744, 107


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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