Object 138: 4U 1850-087

Podcast release date: 25 November 2024

Right ascension: 18:53:04.9

Declination:-08:42:20

Epoch: ICRS

Constellation: Scutum

Corresponding Earth location: A valley in the foothills of the Andes Mountains in Peru.

The object that this episode's coordinates point to is named 4U 1850-087. Please keep in mind that, when I say "4U" throughout this episode, I am not using the preposition "for" and the pronoun "you" but the number 4 and the letter U. I hope that isn't too confusing for... Well, I almost used that preposition and pronoun combination. Anyhow, I hope it isn't too confusing for people listening to this episode.

In any case, 4U 1850-087 was discovered as an X-ray source by the Uhuru satellite, an X-ray satellite that operated in the 1970s [1]. The number 4 and the letter U in this object's name refers to the fact that it appeared in the fourth catalog of sources discovered by the Uhuru satellite [2], and the digits 1850-087 give the right ascension and declination in the 1950 coordinate system. The object was subsequently identified as a low X-ray binary star system located within the globular cluster NGC 6712, which itself is located at a distance of roughly 22300 light years (6.85 kpc) from Earth in the constellation Scutum [3].

A generic low mass X-ray binary star system consists of two stars. One of the stars is either a neutron star or a black hole that is the leftover core from when a very large star died and exploded as a supernova. The other star is very often a Sun-like star or a red giant that formed from a Sun-like star. The two stars are so close that the neutron star or black hole can gravitationally strip gas away from the outer layers of the other star, and when that gas falls onto the neutron star or black hole, it gets very compressed and very hot and produces X-ray emission.

That is a general description of a generic low mass X-ray binary star system, but 4U 1850-087 is a bit weirder than this. One of the stars is indeed a neutron star with an estimated mass of 1.4 times the mass of the Sun [4, 5]. The other star, however, is not a Sun-like star or a red giant but instead a white dwarf [6], which is the inert ball of mostly carbon and oxygen atoms (but sometimes with a layer of helium on top) left over from when a Sun-like star goes through the red giant stage and a few other stages and then expels its outer gas layers as a planetary nebula.

White dwarfs are very small as far as stars go. The one in 4U 1850-087 has a mass of 0.04 times the mass of the Sun and a diameter about 4.4 times the diameter of the Earth [6], so for the neutron star in the system to gravitationally strip away the outer layers of the white dwarf, the two stars have to be really close. The estimated distance between the two objects is around 15.3 times the diameter of the Earth or about half the distance from the Earth to the Moon [5]. Placing two star sized objects this close to each other will indeed produce severe tidal effects. Additionally, placing two massive objects this close together will cause them to orbit each other really rapidly; the two stars complete an orbit once every 20.6 minutes [6].

So, this system is rather unusual in terms of X-ray binary star systems, or, in other words, this is an unusual version of an already unusual astronomical object. This object and a few other very rare objects like it are classified as ultra compact X-ray binaries.

However, we are not done with the weirdness yet. First of all, the X-ray emission is variable by a huge amount [2, 4, 7, 8, 9, 10, 11, 12, 13, 14, 15]. This is because the gas stripped away from the white dwarf and falling into the neutron star undergoes what could be described as thermonuclear explosions. Second, this binary star system may have a third star orbiting it. That star is some sort of stellar object, probably a white dwarf, about 0.55 times the mass of the Sun orbiting the other two stars at a distance of about 4.5 times the distance from the Earth to the Moon [5]. That could cause extra weird gravitational effects in the system.

To conclude, 4U 1850-087 doesn't just have a weird name. It's a weird example of a weird subclass of a weird class of stars. Astronomers who like weird binary star systems have been studying this thing for decades and are going to continue to do so in the future.

References

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

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

[3] Vasiliev, Eugene and Baumgardt, Holger, Gaia EDR3 view on galactic globular clusters, 2021, Monthly Notices of the Royal Astronomical Society, 505, 5978

[4] Swank, J. H. et al., X-Ray Bursts, 1976, International Astronomical Union Circular, 3010, 1

[5] Prodan, Snezana and Murray, Norman, On the Dynamics of Ultra Compact X-Ray Binaries: 4U 1850-087, 4U 0513-40, and M15 X-2, 2015, Astrophysical Journal, 798, 117

[6] Homer, L. et al., Periodic UV modulation of X1850-087: a double degenerate binary in the globular cluster NGC 6712?, 1996, Monthly Notices of the Royal Astronomical Society, 282, L37

[7] Hoffman, J. A. et al., Repeatable, multiple-peaked structure in type I X-ray bursts., 1980, Astrophysical Journal Letters, 240, L27

[8] Warwick, R. S. et al., The Ariel V (3A) catalogue of X-ray sources. I., 1981, Monthly Notices of the Royal Astronomical Society, 197, 865

[9] Priedhorsky, W. C. and Terrell, J., Long-term observations of X-ray sources : the Aquila-Serpens-Scutum., 1984, Astrophysical Journal, 280, 661

[10] Wood, K. S. et al., The HEAO A-1 X-ray source catalog., 1984, Astrophysical Journal Supplement Series, 56, 507

[11] Warwick, R. S. et al., A survey of the galactic plane with EXOSAT., 1988, Monthly Notices of the Royal Astronomical Society, 232, 551

[12] Kitamoto, Shunji et al., X-Ray Spectrum and Time Variation of X1850-087 in the Globular Cluster NGC 6712, 1992, Astrophysical Journal, 391, 220

[13] Christian, D. J. and Swank, J. H., The Survey of Low-Mass X-Ray Binaries with the Einstein Observatory Solid-State Spectrometer and Monitor Proportional Counter, 1997, Astrophysical Journal Supplement Series, 109, 177

[14] Juett, Adrienne M. et al., Ultracompact X-Ray Binaries with Neon-rich Degenerate Donors, 2001, Astrophysical Journal Letters, 560, L59

[15] Lu, Yongqi et al., Discovery of a Long Thermonuclear X-Ray Burst from the Ultracompact Binary 4U 1850\textendash087, 2024, Astrophysical Journal, 969, 15

Credits

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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