A Neutron Star in the Neighborhood

by Paul Gilster on August 21, 2007

A long-time old movie buff, I am delighted with the choice of name for a recently discovered neutron star that may be the closest such object to Earth. It’s being called Calvera, after the bandit played so brilliantly by Eli Wallach in the 1960 western The Magnificent Seven. In astronomical terms, the ‘Magnificent Seven’ are the seven isolated neutron stars — neutron stars with no associated binary companion, supernova remnant or radio pulsations — known until the discovery of Calvera, which brings the number to eight.

Isolated neutron star Calvera

Co-discoverer Derek Fox (Penn State) calls the choice of names “…a bit of an inside joke on our part.” And indeed, Calvera started life with a far less interesting name. The German-American ROSAT satellite (named after x-ray discoverer Wilhelm Röntgen) had compiled 18,000 x-ray sources. Comparing these with catalogs of visible, infrared and radio objects, Robert Rutledge (McGill University) discovered that the source known as 1RXS J141256.0+792204 had no known counterpart at any other wavelength. Subsequent observations with NASA’s Swift satellite, the Gemini North instrument in Hawaii and the Chandra X-Ray Observatory were able to confirm the neutron star explanation.

Image: Artist’s illustration of an “isolated neutron star” — a neutron star that does not have an associated supernova remnant, binary companion, or radio pulsations. Credit: Casey Reed, courtesy of Penn State.

Calvera looks to be a promising object indeed. The current working assumption is that it is within 1000 light years and perhaps as close as 250, making it possibly the closest known neutron star. The discovery team believe Calvera to be a radio pulsar, in which case it could be useful. In the passage below, the team discusses Calvera’s classification, relating it to millisecond pulsars (MSPs) with a rotational period between one and ten milliseconds (internal references deleted for brevity):

The only class of objects consistent with the demonstrated properties of Calvera are the radio pulsars; in particular, pulsars analogous to the MSPs observed in the globular cluster 47 Tuc. We therefore conclude that Calvera is most likely a radio pulsar. This could be confirmed by detection of radio pulsations from this source.

If Calvera is a new (perhaps fast) radio pulsar, it would be an observationally useful object. It exhibits an X-ray flux equal to the X-ray-brightest and second closest millisecond radio pulsar, PSR J0437−4715. There are only 5 known radio pulsars within d <260 pc, and only one in the northern hemisphere. If Calvera indeed turns out to be an MSP, it would be the third closest MSP in the sky (after PSR J0437-4715 at d = 160 pc and PSR J2124-3358 and d = 250 pc) and potentially the closest MSP at d = 80 pc. It would be the closest MSP in the northern hemisphere (followed by PSR J0030+0451, d=300 pc) making it a potentially useful target both for a pulsar timing array and for targeted search with the Laser Interferometric Gravitational Wave Observatory (LIGO).

Many more isolated neutron stars may be found to exist as we continue to identify which ROSAT sources are missing optical counterparts. Longer observations with Chandra, planned by the discovery team, should show whether the source pulsates in X-rays (radio pulsations have yet to be identified). The team’s long-term ROSAT survey is ongoing, so we expect to hear more about Calvera-like objects. The paper is Rutledge, “Discovery of an Isolated Compact Object at High Galactic Latitude,” to be published in The Astrophysical Journal (preprint available).

{ 12 comments }

ljk August 21, 2007 at 11:23

Apparently the USAF detected pulsars going back to 1964,
before their “official” discovery in 1967.

See this blog for the details:

http://scienceblogs.com/catdynamics/2007/08/schisler.php

Ron S August 21, 2007 at 11:36

I quickly skimmed the preprint to understand why this object could be a LIGO target. However there’s no more on this than is hinted at in the abstract.

Can anyone say why this possibly close neutron star could have gravitational anomalies of sufficient magnitude to be detectable by LIGO? Would something like rapid internal mass-shifting events (starquakes) be detectable at 250 ly?

Administrator August 21, 2007 at 13:20

Ron, let me see if I can get something on this from the research team.

Robert Spero August 21, 2007 at 13:28

“Targeted search” probably means a search for gravitational waves at twice the repetition rate of the radio signal, if/when that’s found. A tiny deviation from spherical shape would generate waves at twice the neutron star’s rotation frequency. Starquakes could also give pulsed gravitational waves, though that’s more speculative.

philw August 21, 2007 at 13:55

Outstandingly interesting stuff about the Air Force pulsar detection. I remember the LGM article on pulsars in Newsweek in the 60s.

Administrator August 21, 2007 at 14:07

Ron S, I have two responses from the research team. The first is from Bob Rutledge:

“LIGO is involved in detecting gravitational waves, and one possible source of gravitational waves is a fast spinning neutron star which is not perfectly spherical. The closer such an object is to us, the stronger the gravitational wave signal which could be detected with LIGO. As a source which may be the closest known neutron star, Calvera may therefore be used for LIGO search for gravitational waves. To perform such a search — which are computationally expensive, requiring 1-2 months on a fast super computer — they can simply make use of the direction of Calvera.”

The second is from Derek Fox, also on the Calvera research team:

“Your reader is absolutely correct, in that we have no evidence of gravitational anomalies that would make Calvera detectable by LIGO. In order to be detected, some substantial mass quadrupole moment, along with a fast spin rate of at least 20 Hz or so, would be required. In addition, the source would have to be at the near end of our current distance estimates.

“That is a lot of “ifs”. However, we do not actually have to know any of those things in advance in order to “target” Calvera with LIGO. The observatory has been on almost continuously for 1 1/2 years now, collecting data (I refer to the current science run, S5, which has much-improved sensitivity by comparison to earlier runs). All of these data are retained in an archive and can be subject to analysis at any time. Since LIGO has not detected any source of gravitational radiation to-date, it is these searches (via computer analysis) that occupy their time.

“Thus our statement that Calvera is a “promising target” for LIGO is founded solely on our observation that it is likely near to Earth, and may be spinning fast – no more than that.”

Ron S August 21, 2007 at 16:02

Paul, thank you for this. It was also kind of Bob Rutledge and Derek Fox to respond quickly to my curiosity-inspired question. I am very interested in seeing LIGO successfully see something out there so my curiosity was piqued.

It could take quite the mass distribution anomaly that is rotationally asymmetric that can also withstand the gravitational compression of a neutron star, even if it rotates in 50 ms, to be detectable by LIGO. Probably the LIGO team has already looked at this and knows how big an anomaly and how close it’d have to be to be detectable. Perhaps as Derek (and also Robert Spero above) says that could be a targeted search in their ongoing data analysis.

Administrator August 21, 2007 at 16:48

Glad to help, Ron, and I too thank Bob Rutledge and Derek Fox for their help, especially the quick turnaround.

ljk August 27, 2007 at 12:36

More stories of pre-1967 Bell pulsar discoveries here:

http://www.nature.com/news/2007/070820/full/448974a.html

To quote:

Schisler was not the only one to “pre-discover” a pulsar, though, according to Bell Burnell. “There are actually a lot of stories,” she says. In the 1950s, a woman visiting the observatory at the University of Chicago, Illinois, pointed out that there was a regularly pulsating source of visible light in the Crab Nebula. Elliot Moore, an astronomer at the university, dismissed the woman’s claim, telling her that all stars seem to flicker. Another radio astronomer she knows of will, after a drink or two, confess to having dismissed observations of a pulsating source as the result of faulty equipment. “He’s a bit embarrassed now,” says Bell Burnell.

ljk August 28, 2007 at 10:32

Explosion on a neutron star

http://www.spaceref.com/news/viewpr.nl.html?pid=23413

NASA: Astronomers Pioneer New Method for Probing Matter

http://www.spaceref.com/news/viewpr.nl.html?pid=23412

Neutron stars warp space-time, U-M astronomers observe

http://www.spaceref.com/news/viewpr.nl.html?pid=23411

“Einstein’s predicted distortion of space-time occurs around
neutron stars, University of Michigan astronomers and others
have observed.

Using European and Japanese/NASA X-ray observatory satellites,
teams of researchers have pioneered a groundbreaking technique
for determining the properties of these ultradense objects.”

ljk January 23, 2008 at 23:59

What Millisecond Pulsars Can Tell Us About Matter In The Galaxy

Authors: E. R. Siegel

(Submitted on 22 Jan 2008)

Abstract: I demonstrate that precision timing of millisecond pulsars possess the capabilities of detecting the gravitational effects of intervening galactic substructure. This analysis is applicable to all types of collapsed baryons including stars, planets, and MACHOs, as well as many types of dark matter, including primordial black holes, scalar miniclusters, and sufficiently dense clumps of cold dark matter. The physical signal is quantified and decomposed into observable and unobservable components; templates for the observable signals are also presented. Additionally, I calculate the expected changes in the observed period and period derivatives that will result from intervening matter.

I find that pulsar timing is potentially a very useful tool for probing the nature of dark matter and to learn more about the substructure present within our galaxy.

Comments: 6 pages, 6 figures, submitted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.3458v1 [astro-ph]

Submission history

From: Ethan Siegel [view email]

[v1] Tue, 22 Jan 2008 21:00:00 GMT (246kb)

http://arxiv.org/abs/0801.3458

ljk May 28, 2008 at 14:06

Direct Detection of Gravity Waves from Neutron Stars

Authors: Redouane Al Fakir, William G. Unruh

(Submitted on 24 May 2008)

Abstract: In light of the discovery of the first-ever double pulsar system, PSR J0737-3039, we re-examine an earlier proposal to directly detect gravity waves from neutron stars, which was predicated on a hypothetical system almost identical to the later discovered double pulsar.

We re-derive the effect in more detail, and confirm the initial estimate–sometimes doubted in the literature–that it includes a 1/b dependence, where b is the impact parameter of a pulsar with respect to its foreground, gravity-wave emitting, neutron star companion. A coherent modulation in pulsar time-of-arrival measurements of 10 nano-sec/sec is possible.

A one-year intermittent experiment on an instrument comparable to the SKA could thus detect the exceedingly faint gravity waves from individual neutron stars.

Comments: 19 pages, 2 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.3750v1 [astro-ph]

Submission history

From: Redouane Fakir [view email]

[v1] Sat, 24 May 2008 09:12:10 GMT (149kb)

http://arxiv.org/abs/0805.3750

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