July science readings

Articles studied this July - some of which might go to Slashdot.

Not really science. A "Wittgenstein-ism"

A very fat neutron star

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Had a holiday.

Done non-internet stuff. Better try to get back to some sort of normality.

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A "Wittgenstein-ism"

(my quotes file)

Wikiquotes (last entry in the section from this book).

I picked up (from Prof Jerry Coyne, "Professor Ceiling Cat", at Why Evolution Is True (a website, not a blog, despite all appearences to the contrary) a nice little snippet from Wittgenstein, and dropped it into my "quotes" file.

German : Wovon man nicht sprechen kann, darüber muss man schweigen.

English : Whereof one cannot speak, thereof one must be silent.

I like it. A polite way of saying, "If you're unwilling to define your terms, STFU." I'll have to remember it next time there's a god-squaddy needing a slap in the arguments.

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PSR J0952-0607: The Fastest and Heaviest Known Galactic Neutron Star

https://arxiv.org/pdf/2207.05124.pdf

The title is ambiguous - is "fastest" in reference to proper moton (across the "plane of the sky") or it's rotation? My first glance of the abstract says "rotation", but as I start making notes I think - "the forces that impart proper motion (an eccentric supernova explosion) might also be correlated with producing a rotational kick ; so could the two properties be correlated. Through an orientation factor to the plane of the sky/ line of sight, of course.

Abstract : We describe Keck-telescope spectrophotometry and imaging of the companion of the “black widow” pulsar PSR J0952−0607, the fastest known spinning neutron star (NS) in the disk of the Milky Way. The companion is very faint at minimum brightness, presenting observational challenges, but we have measured multicolor light curves and obtained radial velocities over the illuminated “day” half of the orbit. The model fits indicate system inclination i = 59.8 ± 1.9◦ and a pulsar mass M_NS = 2.35 ± 0.17 M_sol , the largest well-measured mass found to date. Modeling uncertainties are small, since the heating is not extreme; the companion lies well within its Roche lobe and a simple direct-heating model provides the best fit. If the NS started at a typical pulsar birth mass, nearly 1 M_sol has been accreted; this may be connected with the especially low intrinsic dipole surface field, estimated at 6 × 10⁷ G. Joined with reanalysis of other black widow and redback pulsars, we find that the minimum value for the maximum NS mass is M_max > 2.19 M_sol (2.09 M_sol) at 1σ (3σ) confidence. This is ∼ 0.15 M_sol heavier than the lower limit on M_max implied by the white-dwarf–pulsar binaries measured via radio Shapiro-delay techniques.

No mention there of the "proper motion". Presumably that's too low to be measured (yet, given the arc since discovery), and unremarkable. The first line of the paper itself remarks on the short arc :

Pulsar PSR J0952−0607 (hereafter J0952) was discovered by Bassa et al. (2017) with a spin period of Ps = 1.41 ms, making it the fastest-spinning pulsar in the disk of the Milky Way.

That's an observation span of 5.2 years - not long for measuring a normal proper motion.

Justification : Table 1 gives the observation dates as MJD 58455.50562 to 59641.37420 (1,185.8 days, 3.25 years), and the Bassa (2017) reference notes it's "timing interval" as starting on MJD 57747.1 which extends the observation arc to 5.2 years.

Future observations are very likely (It's a prime candidate for timing variation studies, having both a very fast spin, and a conpanion to interact with and transfer angular momentum to), so we should get a proper motion eventually. Unless, of course, it's headed directly for us. That'll get the Chicken Little's crowing.

Oh, boring : this occurred to the paper's authors too. And they address it :

Since in addition their optical photometry suggests a large (> 5 kpc) distance, and timing data gave a best-fit (albeit low-significance) proper motion of ∼ 10 mas yr⁻¹

What's the miss distance, for a 5 mas yr⁻¹ proper motion at 5kpc? I make it about 1400AU (0.007 pc), which isn't terribly worrying, and that's assuming that the proper motion is half their "low significance" estimate.

I haven't worked out the travel time, but it's going to be in the millions of years. Let our (your) successor species worry about it.

All of which is good background - weighing NS is a fairly exotic occupation. But what's the paper about? Well, essentially, it's good old Kepler : find the orbital velocity of the bodies in orbit (using the "companion" star mentioned above), and their separation and period, and you can pull the masses of the bodies out of the equations. It's one of the main methods of weighing stars - and has been since the invention of spectroscopy which gives access to absorbtion lines in the spectrum and the radial velocity data (velocity in the line of sight). Very weell-established science, and if you "believe" in spectroscopy and Newton's gravitation, the results come out without complex modelling and points for argument.

But the interpretation of those results - where the NS mass interacts with the particle physics - that's more subtle. In theory, as the NS gets bigger, the central pressure geets higher, until the nuclear force is unable to keep the particles from collapsing to some higher energy state. But the presence of spin on the neutron star produces an apparrant centrifugal force (it's really just inertia, but it looks like a force) which acts to reduce the internal pressure in the NS. Which is why fast-spinning NS represent a close probe into how the pressure of NS matter (a proxy for the nuclei of normal matter) varies with the applied load.

From the orbital motions of the two components of this star-NS system, the mass of the NS is 2.35 ± 0.17 M_sol, which is appreciably higher than non-rotating NS are thought to be able to reach.

That raises two questions :

1. what will happen as the NS accretes mass (and angular momentum from the companion ; will the NS implode under it's own gravity or will the increasing rotation continue to support it?
2. or, will the NS (with it's relatively weak magnetic field) transfer (rotational) angular momentum to the companion star, slowing the NS rotation, increasing the internal pressure until the NS reaches the point that the nuclear forces cannot support the core against the pressure and ... something happens.

Whichever way it goes, getting baseline data now, then watching (for 100 years, or 100 million years - that's a third major question) is possibly a quicker way to probe the finer details of particle physics and nuclear forces than building a particle accelerator large enough (which may be somewhere between the size of the Moon's orbit, or Pluto's orbit). A most fascinating system. Is there an emoticon for Nimoy Spock's eyebrow-raise?

So, what are the critical points for a Slashdot article?

• Who - an international group of astronomers (except they're not - all at California institutions ; not going to guess at nationalities)
• What - identified the most massive known NS (to date).
• Where - Data collection using the Keck telescope in Hawai'i, specifically the LRIS - Low Resolution Imaging Spectrograph - a low-light spectrograph that can acquire good quality data for faint objects.
• When - Dec 02 2018 to Mar 03 2022 ; fairly fast on the paper-writing!
• Why - the physics of neutron stars, particularly rotating ones, probes exquisitely into areas of particle energy (specifically, hadronic/ quark-based particles) which we can't approach with terrestrial particle accelerators. The interplay between the static mass of a large neutron star and the (effective, "centrifugal" force) support that the NS material gets from it's rotation allows us to examine the behaviour of the nuclear force in paramter space we can't directly access. Effectively, a spectroscope on a large telescope can perform observational experiments where the whole of CERn cannot reach.
• Should we be worried - no, it's set for at least a 1400 AU miss, and that not for a million years or more. Someone Elses' Problem - if your Peril-o-matic Sunglasses have darkened, get a warranty repair.

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