beecee

Magnetar formed when two Neutron Stars merge:

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https://phys.org/news/2019-04-formation-magnetar-billion-years.html

Researchers observe formation of a magnetar 6.5 billion light years away: 

A University of Arkansas researcher is part of a team of astronomers who have identified an outburst of X-ray emission from a galaxy approximately 6.5 billion light years away, which is consistent with the merger of two neutron stars to form a magnetar—a large neutron star with an extremely powerful magnetic field. Based on this observation, the researchers were able to calculate that mergers like this happen roughly 20 times per year in each region of a billion light years cubed.

 

The research team, which includes Bret Lehmer, assistant professor of physics at the University of Arkansas, analyzed data from the Chandra X-ray Observatory, NASA's flagship X-ray telescope.

The Chandra Deep Field-South survey includes more than 100 X-ray observations of a single area of the sky over a period of more than 16 years to collect information about galaxies throughout the universe. Lehmer, who has worked with the observatory for 15 years, collaborated with colleagues in China, Chile and the Netherlands, and at Pennsylvania State University and the University of Nevada. The study was published in Nature.

more at link...................

 

the paper:

https://www.nature.com/articles/s41586-019-1079-5

A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger:

Abstract

Mergers of neutron stars are known to be associated with short γ-ray bursts1,2,3,4. If the neutron-star equation of state is sufficiently stiff (that is, the pressure increases sharply as the density increases), at least some such mergers will leave behind a supramassive or even a stable neutron star that spins rapidly with a strong magnetic field5,6,7,8 (that is, a magnetar). Such a magnetar signature may have been observed in the form of the X-ray plateau that follows up to half of observed short γ-ray bursts9,10. However, it has been expected that some X-ray transients powered by binary neutron-star mergers may not be associated with a short γ-ray burst11,12. A fast X-ray transient (CDF-S XT1) was recently found to be associated with a faint host galaxy, the redshift of which is unknown13. Its X-ray and host-galaxy properties allow several possible explanations including a short γ-ray burst seen off-axis, a low-luminosity γ-ray burst at high redshift, or a tidal disruption event involving an intermediate-mass black hole and a white dwarf13. Here we report a second X-ray transient, CDF-S XT2, that is associated with a galaxy at redshift z = 0.738 (ref. 14). The measured light curve is fully consistent with the X-ray transient being powered by a millisecond magnetar. More intriguingly, CDF-S XT2 lies in the outskirts of its star-forming host galaxy with a moderate offset from the galaxy centre, as short γ-ray bursts often do15,16. The estimated event-rate density of similar X-ray transients, when corrected to the local value, is consistent with the event-rate density of binary neutron-star mergers that is robustly inferred from the detection of the gravitational-wave event GW170817.

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An interesting side story related to the above....

https://phys.org/news/2018-05-gravitational-event-creation-black-hole.html

Gravitational wave event likely signaled creation of a black hole:

The spectacular merger of two neutron stars that generated gravitational waves announced last fall likely did something else: birthed a black hole. This newly spawned black hole would be the lowest mass black hole ever found.

A new study analyzed data from NASA's Chandra X-ray Observatory taken in the days, weeks, and months after the detection of gravitational waves by the Laser Interferometer Gravitational Wave Observatory (LIGO) and gamma rays by NASA's Fermi mission on August 17, 2017.

While nearly every telescope at professional astronomers' disposal observed this source, known officially as GW170817, X-rays from Chandra are critical for understanding what happened after the two neutron stars collided.

From the LIGO data astronomers have a good estimate that the mass of the object resulting from the neutron star merger is about 2.7 times the mass of the Sun. This puts it on a tightrope of identity, implying it is either the most massive neutron star ever found or the lowest mass black hole ever found. The previous record holders for the latter are no less than about four or five times the Sun's mass.

more at link.....

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the paper:

https://iopscience.iop.org/article/10.3847/2041-8213/aac3d6/meta

GW170817 Most Likely Made a Black Hole:

Abstract

There are two outstanding issues regarding the neutron-star merger event GW170817: the nature of the compact remnant and the interstellar shock. The mass of the remnant of GW170817, ~2.7 , implies that the remnant could be either a massive rotating neutron star, or a black hole. We report Chandra Director's Discretionary Time observations made in 2017 December and 2018 January, and we reanalyze earlier observations from 2017 August and 2017 September, in order to address these unresolved issues. We estimate the X-ray flux from a neutron star remnant and compare that to the measured X-ray flux. If we assume that the spin-down luminosity of any putative neutron star is converted to pulsar wind nebula X-ray emission in the 0.5–8 keV band with an efficiency of 10−3, for a dipole magnetic field with 3 × 1011 G < B < 1014 G, a rising X-ray signal would result and would be brighter than that observed by day 107; we therefore conclude that the remnant of GW170817 is most likely a black hole. Independent of any assumptions of X-ray efficiency, however, if the remnant is a rapidly rotating magnetized neutron star, the total energy in the external shock should rise by a factor ~102 (to ~1052 erg) after a few years; therefore, Chandra observations over the next year or two that do not show substantial brightening will rule out such a remnant. The same observations can distinguish between two different models for the relativistic outflow, either an angular or radially varying structure.

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OK, after absorbing the original article, A question has arisen in my mind....If  these events had not have happened, and someone were to ask me what happens when two neutron stars merge/collide, I would have unhesitatingly said that a BH would be formed. So why a Magnetar in the original article? Did they just graze each other, with one spinning faster as a result of meeting? It seems that if two neutron stars were to smash into each other, that any joining of the two would then probably exceed the NDP? Is the answer [as appears likely] dependent on the combined trajectories of their meet up? Probably I would think. Any other thoughts?

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10 hours ago, beecee said:

OK, after absorbing the original article, A question has arisen in my mind....If  these events had not have happened, and someone were to ask me what happens when two neutron stars merge/collide, I would have unhesitatingly said that a BH would be formed. So why a Magnetar in the original article? Did they just graze each other, with one spinning faster as a result of meeting? It seems that if two neutron stars were to smash into each other, that any joining of the two would then probably exceed the NDP? Is the answer [as appears likely] dependent on the combined trajectories of their meet up? Probably I would think. Any other thoughts?

Interesting questions.

Before reading the abstracts, I'd have assumed that e.g. two coalescing neutron stars each of 1.1 solar masses (apparently the minimum neutron star mass) would produce a neutron star of at most 2.2 solar masses and definitely not a BH.

However, your first source

Quote

If the neutron-star equation of state is sufficiently stiff (that is, the pressure increases sharply as the density increases), at least some such mergers will leave behind a supramassive or even a stable neutron star that spins rapidly with a strong magnetic field5,6,7,8 (that is, a magnetar).

hints that if the neutron-star equation of state is not sufficiently stiff transient high pressure may produce a region sufficiently large and dense in the coalescing neutron stars to form a black hole smaller than the usual minimum.

AFAIK all the collisions observed so far have been binaries coalescing where much of the angular momentum ends up in the final neutron star/black hole e.g. one high spin magnetar.

It's possible that collisions between non binary neutron stars etc are common enough for some to be observed eventually. Those seem to me, especially if the collision is nearly head on, to be the best option for producing unusually small black holes.

Any other thoughts?

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