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A Relativistic Probe of Exotic Matter

We’d like to know a lot more about neutron stars. They’re doubtless the home of exotic matter of the sort we’re unable to create in any laboratory, and their extraordinary density leads to conditions in the space around them that are, shall we say, extreme. Gases whipping around three neutron stars at forty percent of the speed of light have now been used to take measurements of their diameter and mass.

A neutron star binary

Figure out the properties of such gases and you’ve nailed down a maximum size for the diameter of the neutron star Serpens X-1, for example, a figure that turns out to be between 18 and 20.5 miles across. A team led by Edward Cackett (University of Michigan) looked at the spectral lines from hot iron atoms around Serpens X-1 and two other neutron star binaries, GX 349+2, and 4U 1820-30. Independent work by Sudip Bhattacharyya and team (NASA GSFC) bolsters Cackett’s results and demonstrates the efficacy of the method.

Image: Many neutron stars are accompanied by a companion star, as portrayed in this illustration. The powerful gravity of the neutron star siphons off gas from the companion, which then settles into a slowly in-spiraling disk around the neutron star. Credit: NASA.

The factor at work here is the extreme velocity of the gas, which smears and distorts the iron line because of the Doppler effect and beaming effects that are in accordance with Einstein’s Special Theory of Relativity. Moreover, the powerful gravity of the neutron star warps spacetime (now we’re in the realm of the General Theory) and shifts the object’s iron lines to longer and longer wavelengths. These skewed iron lines tell the tale and the GSFC work confirms it. Says Cackett’s colleague Jon Miller:

“Now that we’ve seen this relativistic iron line around three neutron stars, we have established a new technique. It’s very difficult to measure the mass and diameter of a neutron star, so we need several techniques to work together to achieve that goal.”

It’s a goal that goes beyond the study of exotic particles or states of matter, although that’s a major plus. Because the behavior of these gases in the extreme environment near a neutron star can be deduced from the General Theory, observations like these offer a useful tool for examining and testing that theory’s parameters. In such ways do space-based platforms like the European Space Agency’s XMM-Newton x-ray observatory and the Japanese/NASA Suzaku x-ray observatory, both used in this work, help us mine observational data from a cosmic laboratory where no condition is too extreme to to be modeled.

The paper by Cackett and Miller is “Relativistic iron emission lines in neutron star low-mass x-ray binaries as probes of neutron star radii,” submitted to Astrophysical Journal Letters (abstract). The GSFC work by Sudip Bhattacharyya and Tod Strohmayer is “Evidence of a Broad Relativistic Iron Line from the Neutron Star Low-Mass X-ray Binary Serpens X-1,” in Astrophysical Journal Letters Volume 664, Number 2, Part 2 (August 1, 2007), pp. L103-L106 (abstract).

Comments on this entry are closed.

  • Ron S August 29, 2007, 0:40

    “It’s a goal that goes beyond the study of exotic particles or states of matter, although that’s a major plus. Because the behavior of these gases in the extreme environment near a neutron star can be deduced from the General Theory, observations like these offer a useful tool for examining and testing that theory’s parameters.”

    I wonder how this can be true. The authors assume GTR to be true and select a specific metric (Schwarzchild), then use the new data to constrain the size of the neutron star. I would think that to test GTR they would have to essentially do the opposite. That is, assume a model of a neutron star, then use the new data to constrain GTR. I don’t think you can do both.

    I do like the idea of testing equations of state for the neutron star since that would be very interesting. Unfortunately, as they say, their results don’t yet reach that far.

  • ljk January 17, 2008, 13:54

    Neutron stars can be more massive, while black holes are more rare, Arecibo Observatory finds


    Jan. 17, 2008

    Neutron stars and black holes aren’t all they’ve been thought to be.

    In fact, neutron stars can be considerably more massive than previously believed, and it is more difficult to form black holes, according to new research developed by using the Arecibo Observatory in Arecibo, Puerto Rico. Paulo Freire, an astronomer from the observatory, presented his research at the American Astronomical Society national meeting in Austin, Texas, Jan. 11.

    In the cosmic continuum of dead, remnant stars, the Arecibo astronomers have increased the mass limit for when neutron stars turn into black holes.

    “The matter at the center of a neutron star is highly incompressible. Our new measurements of the mass of neutron stars will help nuclear physicists understand the properties of superdense matter,” said Freire. “It also means that to form a black hole, more mass is needed than previously thought. Thus, in our universe, black holes might be more rare and neutron stars slightly more abundant.”

    When the cores of massive stars run out of nuclear fuel, their enormous gravitation causes them to collapse. The core, typically with a mass 1.4 times larger than that of the sun, is compressed into a neutron star. These extreme objects have a radius about 10 to 16 kilometers and a density on the order of a billion tons per cubic centimeter.

    Astronomers had thought the neutron stars needed a maximum mass between 1.6 and 2.5 the mass of our sun in order to collapse and become black holes. However, this new research shows that neutron stars remain neutron stars between the mass of 1.9 and up to possibly 2.7 suns.

    “The matter at the center of the neutron stars is the densest in the universe. It is one to two orders of magnitude denser than matter in the atomic nucleus. It is so dense we don’t know what it is made out of,” said Freire. “For that reason, we have at present no idea of how large or how massive neutron stars can be.”

    From June 2001 to March 2007, Freire used Arecibo’s “L-wide” receiver (sensitive to radio frequencies from 1100 to 1700 MHz) and the Wide-Band Arecibo Pulsar Processors — a very fast spectrometer on the Arecibo telescope — to examine a binary pulsar called M5 B, in the globular cluster M5, which is located in the constellation Serpens. Like a lighthouse emits light, a pulsar is a strongly magnetized neutron star that emits large amounts of electromagnetic radiation, usually from its magnetic pole. As in the case of a lighthouse, distant observers perceive a sequence of pulsations, which are caused by the rotation of the pulsar. In the case of M5 B, these radio pulsations arrive at the Earth every 7.95 milliseconds.

    These radio pulsations were scanned by the wide-band spectrometers once every 64 microseconds for 256 spectral channels and then recorded to a computer disk with accurate timing information. The precise arrival time of the pulses were then used by the astronomers to accurately measure the orbital motion of M5 B about its companion. This allowed the astronomers to estimate the mass (1.9 solar masses) of the pulsar.

    Astronomers also working on this research are: Maureen van den Berg, Northwestern University, Evanston, Ill.; Jason W.T. Hessels, Astronomical Institute “Anton Pannekoek” of the University of Amsterdam in the Netherlands; and Alex Wolszczan, Pennsylvania State University, State College, Pa.

    The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, a national research center operated by Cornell for the National Science Foundation.

  • ljk February 25, 2008, 11:07

    Three-dimensional stability of magnetically confined mountains on accreting neutron stars

    Authors: M. Vigelius, A. Melatos

    (Submitted on 22 Feb 2008)

    Abstract: We examine the hydromagnetic stability of magnetically confined mountains, which arise when material accumulates at the magnetic poles of an accreting neutron star. We extend a previous axisymmetric stability analysis by performing three-dimensional simulations using the ideal-magnetohydrodynamic (ideal-MHD) code \textsc{zeus-mp}, investigating the role played by boundary conditions, accreted mass, stellar curvature, and (briefly) toroidal magnetic field strength. We find that axisymmetric equilibria are susceptible to the undular sub-mode of the Parker instability but are not disrupted. The line-tying boundary condition at the stellar surface is crucial in stabilizing the mountain. The nonlinear three-dimensional saturation state of the instability is characterized by a small degree of nonaxisymmetry ($\la 0.1$ per cent) and a mass ellipticity of $\epsilon \sim 10^{-5}$ for an accreted mass of $M_a = 10^{-5} M_\odot$.

    Hence there is a good prospect of detecting gravitational waves from accreting millisecond pulsars with long-baseline interferometers such as Advanced LIGO. We also investigate the ideal-MHD spectrum of the system, finding that long-wavelength poloidal modes are suppressed in favour of toroidal modes in the nonaxisymmetric saturation state.

    Comments: accepted by MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.3238v1 [astro-ph]

    Submission history

    From: Matthias Vigelius [view email]

    [v1] Fri, 22 Feb 2008 00:00:39 GMT (1149kb)