Binary Neutron Stars - Scientific American Digital

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c Meanwhile the energy released in the collapse heats the envelope of the star, which for a few weeks emits more light than an entire galaxy. Observations of old supernovae, such as the Crab NebulaÕs, whose light reached the earth in A.D. 1054, reveal a neutron star surrounded by a luminous cloud of gas, still moving out into interstellar space. More than half the stars in the sky belong to binary systems. As a result, it is not surprising that at least a few massive pairs should remain bound together even after one of them undergoes a supernova explosion. The pair then becomes a massive x-ray binary, so named for the emission that the neutron star produces as it strips the outer atmosphere from its companion. Eventually the second star also explodes as a supernova and turns into a neutron star. The envelope ejected by the second supernova contains most of the mass of the binary (since the remaining neutron star contains a mere 1.4 solar masses). The ejection of such a large fraction of the total mass should therefore disrupt the binary and send the two neutron stars (the old one and the one that has just formed) ßying into space with velocities of hundreds of kilometers per second. Hulse and TaylorÕs discovery demonstrated, however, that some binaries survive the second supernova explosion. In retrospect, astronomers realized that the second supernova explosion might be asymmetrical, propelling the newly formed neutron star into a stable orbit ORBITAL PRECESSION, the rotation of the major axis of an elliptical orbit, results from relativistic perturbations of the motion of fast-moving bodies in intense gravitational Þelds. It is usually almost undetectable; MercuryÕs orbit precesses by less than 0.12 of a degree every century, but that of PSR 1913+16 changes by 4.2 degrees a year. AL KAMAJIAN GEORGE RETSECK rather than out into the void. The second supernova also may be less disruptive if the second star loses its envelope gradually during the massive x-ray binary phase. Since then, the discovery of three other neutron star binaries shows that other massive pairs have survived the second supernova. Several years ago Ramesh Narayan of Harvard University, Amotz Shemi of Tel Aviv University and I, along with E. Sterl Phinney of the California Institute of Technology, working independently, estimated that about 1 percent of massive x-ray binaries survive to form neutron star binaries. This Þgure implies that our galaxy contains a population of about 30,000 neutron star binaries. Following a similar line of argument, we also concluded that there should be a comparable number of binaries, yet unobserved, containing a neutron star and a black hole. Such a pair would form when one of the stars in a massive pair formed a supernova remnant containing more than about two solar masses and so collapsed to a singularity instead of a neutron star. Rarer, but still possible in theory, are black hole binaries, which start their lives as a pair of particularly massive stars; they should number about 300 in our galaxy. Testing General Relativity PSR 1913+16 has implications that reach far beyond the revision of theories of binary stellar evolution. Hulse and Taylor immediately realized that their discovery had provided an ideal site for testing EinsteinÕs General Theory of Relativity. Although this theory is accepted today as the only viable description of gravity, it has had only a few direct tests. Albert Einstein himself computed the precession of MercuryÕs orbit (the shift of the orbital axes and the point of MercuryÕs closest approach to the sun) and showed that observations agreed with his theory. Arthur Eddington detected the bending of light rays during a solar eclipse in 1919. In 1960 Robert V. Pound and Glen A. Rebka, Jr., then both at Harvard, Þrst measured the gravitational redshift, the loss of energy by photons as they climb out of a powerful gravitational Þeld. Finally, in 1964, Irwin I. Shapiro, also at Harvard, pointed out that light signals bent by a gravitational Þeld should be delayed in comparison to those that take a straight path. He measured the delay by bouncing radar signals oÝ other planets in the solar system. Although general relativity passed these tests with ßying colors, they were all carried out in the (relativistically) weak gravitational Þeld of the solar system. That fact left open the possibility that general relativity might break down in stronger gravitational Þelds. Because a pulsar is eÝectively a clock orbiting in the strong gravitational Þeld of its companion, relativity makes a range of clear predictions about how the ticks of that clock (the pulses) will d MASSIVE BINARY (a) evolves through a sequence of violent events. The heavier star in the pair burns its fuel faster and undergoes a supernova explosion; if the two stars stay bound together, the result is a massive x-ray binary (b) in which the neutron star remnant of the Þrst star strips gas from its companion and emits x-radiation. Eventually the second star also exhausts its fuel. In roughly one of 100 cases, the resulting explosion leaves a pair of neutron stars orbiting each other (c); in the other 99, the two drift apart (d). There are enough binary star systems that a typical galaxy contains thousands of neutron star binaries. Copyright 1995 Scientific American, Inc. SCIENTIFIC AMERICAN May 1995 55
FIRST-ORDER DOPPLER EFFECT a SECOND-ORDER DOPPLER EFFECT b OBSERVER OBSERVER GEORGE RETSECK BINARY PULSAR SIGNALS are aÝected by relativistic phenomena. (Each illustration above shows one of the eÝects whose combination produces the timing of the pulses that radio astronomers observe.) The Doppler eÝect slows the rate at which pulses reach an observer when the pulsar is moving away from the earth in its orbit and increases the rate when the pulsar is moving toward the earth (a). The second-order Doppler eÝect and the gravitational redshift (b) impose a sim- 58 SCIENTIFIC AMERICAN May 1995 appear from the earth. First, the Doppler eÝect causes a periodic variation in the pulsesÕ arrival time (the pattern that Þrst alerted Taylor and Hulse). A Òsecond-orderÓ Doppler eÝect, resulting from time dilation caused by the pulsarÕs rapid motion, leads to an additional (but much smaller) variation. This second-order eÝect can be distinguished because it depends on the square of velocity, which varies as the pulsar moves along its elliptical orbit. The second-order Doppler shift combines with the gravitational redshift, a slowing of the pulsarÕs clock when it is in the stronger gravitational Þeld closer to its companion. Like Mercury, PSR 1913+16 precesses in its orbit about its companion. The intense gravitational Þelds involved, however, mean that the periastronÑthe nadir of the orbitÑrotates by 4.2 degrees a year, compared with MercuryÕs perihelion shift of a mere 42 arc seconds a century. The measured eÝects match the predictions of relativistic theory precisely. Remarkably, the precession and other orbital information supplied by the timing of the radio pulses make it possible to calculate the masses of the pulsar and its companion: 1.442 and 1.386 solar masses, respectively, with an uncertainty of 0.003 solar mass. This precision is impressive for a pair of objects 15,000 lightyears away. In 1991 Alexander Wolszczan of the Arecibo observatory found another binary pulsar that is almost a twin to PSR 1913+16. Each neutron star weighs between 1.27 and 1.41 solar masses. The Shapiro time delay, which was only marginally measured in PSR 1913+16, stands out clearly in signals from the pulsar that Wolszczan discovered. Measurements of PSR 1913+16 have also revealed a relativistic eÝect never seen before. In 1918, several years after the publication of his General Theory of Relativity, Einstein predicted the existence of gravitational radiation, an analogue to electromagnetic radiation. When electrically charged particles such as electrons and protons accelerate, they emit electromagnetic waves. Analogously, massive particles that move with varying acceleration emit gravitational waves, small ripples in the gravitational Þeld that also propagate at the speed of light. These ripples exert forces on other masses; if two objects are free to move, the distance between them will vary with the frequency of the wave. The size of the oscillation depends on the separation of the two objects and the strength of the waves. In principle, all objects whose acceleration varies emit gravitational radiation. Most objects are so small and move so slowly, however, that their gravitational radiation is utterly insigniÞcant. Binary pulsars are one of the few exceptions. The emission of gravitational waves produces a detectable eÝect on the binary system. In 1941, long before the discovery of the binary pulsar, the Russian physicists Lev D. Landau and Evgenii M. Lifshitz calculated the eÝect of this emission on the motion of a binary. Energy conservation requires that the energy carried away by the waves come from somewhere, in this case the orbital energy of the two stars. As a result, the distance between them must decrease. PSR 1913+16 emits gravitational radiation at a rate of eight quadrillion gigawatts, about a Þfth as much energy as the total radiation output of the sun. This luminosity is impressive as far as gravitational radiation sources are concerned but still too weak to be detected directly on the earth. Nevertheless, it has a noticeable eÝect on the pulsarÕs orbit. The distance between the two neutron stars decreases by a few meters a year, which suÛces to produce a detectable variation in the timing of the radio pulses. By carefully monitoring the pulses from PSR 1913+16 over the years, Taylor and his collaborators have shown that the orbital separation decreases in exact agreement with the predictions of the General Theory of Relativity. The reduction in the distance between the stars can be compared with the other general relativistic eÝects to arrive at a further conÞrmation. Just as measurements of the orbital decay produce a mathematical function relating the mass of the pulsar to the mass of its companion, so do the periastron shift Copyright 1995 Scientific American, Inc.
Page 1 and 2: AL KAMAJIAN Copyright 1995 Scientif
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Page 7 and 8: Eardley realized that the colliding

Binary Neutron Stars - Scientific American Digital

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