Tag: observational selection

Peculiar Neutron Stars

There’s a lot we don’t know about neutron stars. Neutron stars are the densest objects we can directly observe, and we have little understanding of how matter behaves under such extreme conditions. Though there are a lot of neutrons in neutron stars, they are not entirely made of neutrons. Whether the interiors of neutron stars contain something other than the known elementary particles is an open question.

The nearest neutron star we know of is RX J1856.5-3754 in Corona Australis, just below Sagittarius. It regales us at a distance between 352 and 437 light years, with the most likely distance being 401 ly. Though most neutron stars we know of are pulsars (a good example of observational selection—we tend to discover what is easiest for us to discover), this one is not.

In addition to its intrinsic properties, how a neutron star looks to us also depends upon its orientation and the environs with which it interacts. These three factors have led to a variety of nomenclature that requires some explanation.

Pulsar – a highly-magnetized, fast-rotating neutron star whose magnetic poles emit beams of electromagnetic radiation. If either of the beams sweeps past the Earth, we observe periodic pulses of electromagnetic radiation coming from the neutron star.

Magnetar – an extremely-highly-magnetized, more-slowly-rotating neutron star that produces bursts of X-rays and gamma rays. Only some magnetars are pulsars. Anomalous X-ray pulsars (AXPs) are now thought to be magnetars.

Rotating Radio Transients (RRATs) – a neutron star that is a pulsar, but with the peculiar property that it emits a single short-lived and extremely bright radio burst quasi-periodically with long lulls in between. The radio bursts last only 2 to 30 milliseconds, with intervals ranging from 4 minutes to 3 hours between pulses.

Soft gamma repeaters (SGRs) – a neutron star—possibly a type of magnetar—that emits large bursts of gamma-rays and X-rays at irregular intervals. If not a magnetar, it may be a neutron star with a disk of material in orbit around it.

Compact Central Objects in Supernova remnants (CCOs in SNRs) – a radio-quiet X-ray-producing neutron star surrounded by a supernova remnant. These have thermal emission spectra, and a weaker magnetic field than most neutron stars.

X-ray Dim Isolated Neutron Stars (XDINS) – an isolated, nearby (otherwise, it would be too faint to see) young neutron star. Only seven of these have been discovered to date (see The Magnificent Seven).

And that’s not all. Clearly, we have a lot more to learn about neutron stars.

There are currently about 3,200 known neutron stars, almost all of them pulsars, and all of them in our Milky Way galaxy and the Magellanic Clouds. About 5% are members of a binary system.

I know of no comprehensive catalog of neutron stars, but here is a catalog of pulsars:

ATNF Pulsar Catalogue
https://www.atnf.csiro.au/research/pulsar/psrcat/

A new and exciting frontier for exploring neutron stars is gravitational wave astronomy. All gravitational-wave observations to date have come from merging binaries consisting of black holes and neutron stars. Events include black hole – black hole mergers, neutron star – neutron star mergers, and neutron star – black hole mergers.

Three Pulsars of Note

The Fastest – PSR J1748-2446ad in the constellation Sagittarius is the fastest-spinning pulsar known, rotating once every 1.40 milliseconds, or 716 times per second (716 Hz). An educated guess at the neutron star’s radius (16 km) tells us that the equatorial surface is spinning at about 24% of the speed of light! PSR J1748-2446ad is located at a distance of about 18,000 ly in the globular cluster Terzan 5. Fortuitously, PSR J1748-2446ad is an eclipsing binary system with a bloated and distorted low-mass main-sequence-star companion.

The Slowest – PSR J0901-4046 in the southern constellation Vela is the slowest-spinning pulsar known*, rotating once every 75.886 seconds. It is located at a distance of approximately 1,300 ly.

The Most Massive – PSR J0952–0607 in the constellation Sextans is the most massive neutron star (2.35±0.17 M☉) known, and the second-fastest-spinning pulsar known (1.41 ms, 707 Hz). PSR J0952–0607 is located in a binary system with a (now) substellar-mass companion that has been largely consumed by the neutron star. The distance to this system is highly uncertain.

* The white dwarf in the red-dwarf – white-dwarf binary system AR Scorpii rotates once every 117 seconds, and is thought to be the only known example of a white-dwarf pulsar.

References

Liz Kruesi (2022, July 2). Slowpoke pulsar stuns scientists. Science News, 202(1), 8.
https://www.sciencenews.org/article/pulsar-radio-waves-neutron-star-astronomy

Govert Schilling (2022, July 28). Black widow pulsar sets mass record.
https://skyandtelescope.org/astronomy-news/black-widow-pulsar-sets-mass-record/

Separating Observer from Observed

One of the most difficult things to do in observational science is to separate the observer from the observed.  For example, in CCD astronomy, we apply bias, dark, and flat-field corrections as well as utilize median combines of shifted images to yield an image that is, ideally, free of any CCD chip defects including differences in pixel sensitivity and zero-point.

We as observers are constrained by other limitations.  For example, when we look at a particular galaxy, we observe it from a single vantage point in space and time, a vantage point we cannot change due to our great distance from the object and our existence within an exceedingly short interval of time.

Yet another limitation is a phenomenon that astronomers often call “observational selection”.  Put simply, we are most likely to see what is easiest to see.  For example, many of the exoplanets we have discovered thus far are “hot Jupiters”.  Is this because massive planets that orbit very close to a star are common?  Not necessarily.  The radial velocity technique we use to detect many exoplanets is biased towards finding massive planets with short-period orbits because such planets cause the biggest radial velocity fluctuations in their parent star over the shortest period of time.  Planets like the Earth with its relatively small mass and long orbital period (1 year) are much more difficult to detect using the radial velocity technique.  The same holds true for the transit method.  Planets orbiting close to a star will transit more often—and are more likely to transit—than comparable planets further out.  Larger planets will exhibit a larger Δm than smaller planets, regardless of their location.  It may be that Earthlike planets are much more prevalent than hot Jupiters, but we can’t really conclude that looking at the data collected so far (though Kepler has helped recently to make a stronger case for abundant terrestrial planets).

Here’s another important observational selection effect to consider in astronomy: the farther away a celestial object is the brighter that object must be for us to even see it.  In other words, many far-away objects cannot be observed because they are too dim.  This means that when we look at a given volume of space, intrinsically bright objects are over-represented.  The average luminosity of objects seems to increase with increasing distance.  This is called the Malmquist bias, named after the Swedish astronomer Gunnar Malmquist (1893-1982).