Neutron star 4U 1820-30 spins at 716 revolutions per second

[Astronuc]

I recently saw a short video describing some characteristics of neutron star, 4U 1820-30, in the Sagittarius constellation, i.e., in that direction.


For some reason, it made the news recently, e.g., NICER X-ray Telescope Discovers Fast-Spinning Neutron Star, probably because some results were reported.
https://www.sci.news/astronomy/nicer-fast-spinning-neutron-star-13382.html

A neutron star in the X-ray binary system 4U 1820-30 spins 716 times per second, making it one of the fastest-spinning objects ever observed, according to an analysis of data gathered by NASA’s Neutron Star Interior Composition Explorer (NICER) between 2017 and 2022.

So, it has been observed for some time.

4U 1820-30 is located about 26,000 light-years away from Earth in the constellation of Sagittarius.

This X-ray binary system is part of a metal-rich globular cluster called NGC 6624.

It consists of two stars — a neutron star and a white dwarf companion. The latter orbits the neutron star once every 11 minutes, making it the system with the shortest known orbital period.

4U 1820-30 typically displays short X-ray bursts, only lasting for 10-15 seconds. This is likely because the ignited helium-rich fuel burns out rapidly on its surface.

“Due to its intense gravity, the neutron star pulls material away from its companion star,” said Dr. Gaurava Jaisawal from DTU Space and colleagues.

There is this paper from 2010, The Mass and Radius of the Neutron Star in 4U 1820-30,
https://arxiv.org/abs/1002.3825

This is not to be confused with SGR 1830, which is 13,000 ly distant from earth. https://svs.gsfc.nasa.gov/14115/, or SGR 1820 (or SGR 1820-06, unless some are actually referring to SGR 1806-20)

On Oct. 10, 2020, NASA’s Neil Gehrels Swift Observatory discovered just such an outburst from a new magnetar, called SGR 1830-0645 (SGR 1830 for short). It’s located in the constellation Scutum, and while its distance is not precisely known, astronomers estimate that the object lies about 13,000 light-years away. Swift turned its X-Ray Telescope to the source, detecting repeated pulses that revealed the object was rotating every 10.4 seconds.

NICER measurements from the same day show that the X-ray emission exhibited three close peaks with every rotation. They were caused when three individual surface regions much hotter than their surroundings spun into and out of our view.

NICER observed SGR 1830 almost daily from its discovery to Nov. 17, after which the Sun was too close to the field of view for safe observation. Over this period, the emission peaks gradually shifted, occurring at slightly different times in the magnetar’s rotation. The results favor a model where the spots form and move as a result of crustal motion, in much the same way as the motion of tectonic plates on Earth drives seismic activity.

From 2022 - Properties of magnetar SGR 1830−0645 inspected with NICER
https://phys.org/news/2022-01-properties-magnetar-sgr-nicer.html

https://www.sci.news/astronomy/magnetar-x-ray-spots-10606.html

SGR is an acronym for Soft Gamma Repeater, in addition to an acronym for Sagittarius (Sgr, or SGR).
https://solomon.as.utexas.edu/magnetar.html

The patches of sky described as Sagittarius and Scutum constellations are adjacent neighbors.
https://en.wikipedia.org/wiki/Sagittarius_(constellation)
https://en.wikipedia.org/wiki/Scutum_(constellation)

I've discovered that some authors use X-ray and soft (low energy) gamma rays interchangably. I use X-rays to refer to photons originating from transitions of bound electrons in atoms, or unbound captured in an atomic orbital). For all other photons arising from nuclear transitions, or subatomic interactions (bremsstrahlung), I use gamma rays.

Not having done a review of the distribution of observed neutron stars and magnetars (or SGRs) in our galaxy, I'm wondering if neutron stars and magnetars are more prevalent as one approaches the center of the galaxy.

From 1992, The Neutron Star Population in the Galaxy
https://link.springer.com/chapter/10.1007/978-94-011-2434-8_49

More recently (2021) - The Galactic neutron star population I - an extragalactic view of the Milky Way and the implications for fast radio bursts
https://arxiv.org/abs/2105.04549

One practical consideration for understanding the presence and distribution on neutron stars, or rather gamma and X-ray emitting objects, is to be able to discern their signals from possible nuclear tests on the earth. There are sensitive instruments around the earth's surface and in orbit, which monitor for nuclear detonations.

https://imagine.gsfc.nasa.gov/observatories/learning/swift/classroom/grb_educator_guide.html

Reference to SGR 1820-06 (which should probably be SGR 1806-20) in paper
https://www.osti.gov/servlets/purl/900650

Ref. 42. Figer, D. F., Najarro, F., Geballe, T. R., Blum, R. D. & Kudritzki, R. P. Massive Stars in the
SGR 1806-20 Cluster. Astrophys. J. 622, L49–L52 (2005).