Neutron stars, whether isolated or in a binary system, display a varied and complex phenomenology, often accompanied by extreme variability of many time scales, which takes the form of pulsations due to the object rotation, quasi-periodicities associated to accretion of matter, and explosions due to matter accreted on the surface or to starquakes of highly magnetized objects. This book gives an overview of the current observational and theoretical standpoint in the research on the physics under the extreme conditions that neutron stars naturally provide. The six chapters explore three physical regions of a neutron star: the space around it, where accretion and pulsar companions allow testing of general relativity its surface, where millisecond pulsation and X-ray burts provide clues about general relativistic effects and the equation of state of neutron matter its interior, of course, inaccessible to direct observations, can nevertheless, be probed with all observational parameters related to neutron star variability.
The idea for organizing an Advanced Study Institute devoted largely to neutron star timing arose independently in three places, at Istanbul, Garching and Amster dam; when we became aware of each other's ideas we decided to join forces. The choice of a place for the Institute, in Turkey, appealed much to us all, and it was then quickly decided that Qe§me would be an excellent spot. When the preparations for the Institute started, early in 1987, we could not have guessed how timely the subject actually was. Of course, the recently dis covered QPO phenomena in accreting neutron stars and half a dozen binary and millisecond radio pulsars known at the time formed one of the basic motivations for organizing this Institute. But none of us could have guessed that later in 1987 we were to witness the wonderful discovery of the binary and millisecond radio pulsars in globular clusters and, -as if Nature wished to give us a special present for this the discovery in March 1988 of a millisecond pulsar in an eclipsing binary Institu- system, the first eclipsing radio pulsar ever found, and the second fastest in the sky! The discussion of this pulsar, its formation and fate was one of the highlights of this meeting, especially since its discoverers were among the participants of the Institute and could provide us with first-hand information.
This NATO AS! was the third in the series of Advanced Study Institutes on neutron stars, which started with 'Timing Neutron Stars', held in Qe§me near izmir, Turkey (April 1988), followed by 'Neutron Stars, an Interdis ciplinary Subject', held in Agia Pelagia on the island of Crete (September 1990). The first school centered on our main observational access to neu tron stars, i. e. the timing of radio pulsars and accretion powered neutron stars, and on what timing of neutron stars teaches us of their structure and environment. The second school had as its theme the interplay between diverse areas of physics which find interesting, even exotic applications in the extreme conditions of neutron stars and their magnetospheres. As the field has developed, with the number of observed neutron stars rapidly in creasing, and our knowledge of many individual neutron stars getting deeper and more detailed, an evolutionary picture of neutron stars has started to emerge. This led us to choose 'The Lives of the Neutron Stars' as the uni fying theme of this third Advanced Study Institute on neutron stars. Different types of neutron star activity have been proposed to follow one another in stages during the lives of neutron stars in the same basic population; the evolutionary connection between low-mass X-ray binaries and millisecond radio pulsars is perhaps the prime example.
The NATO Advanced Study Institute 'The Many Faces of Neutron Stars' was the fourth in a series focusing on the astrophysics of neutron stars, which started with the ASI 'Timing Neutron Stars' (Qe§me, Turkey, April 1988), and ~as followed by 'Neutron Stars, and Interdisciplinary Field' (Agia Pelagia, Greece, September 1990), and 'The Lives ofthe Neutron Stars' (Kemer, Turkey, Septem ber 1993). The first ASI had as its theme the main types of observations from which we infer the properties of neutron stars, i.e., the timing of radio pulsars and accret ing neutron stars in X-ray binaries. The second ASI centered on the interplay between various areas of physics and their sometimes exotic applications to the extreme conditions encountered in neutron stars. In the third ASI the evolu tionary connections that exist between different types of neutron stars provided the theme of the lectures. During the last several years the number of neutron stars in their various disguises, e.g., millisecond radio pulsars, single neutron star X-ray sources, and soft gamma-ray repeaters has increased substantially, and new phenomena have been discovered in known populations of neutron star systems, e.g., the kHz QPO in low-mass X-ray binaries. For this reason we have selected 'The Many Faces of Neutron Stars' as the theme of this fourth Advanced Study Institute on neutron stars.
One of the primary science goals of the next generation of hard x-ray timing instruments is to determine the equation of state of matter at supranuclear densities inside neutron stars by measuring the radius of neutron stars with different masses to accuracies of a few percent. Three main techniques can be used to achieve this goal. The first involves waveform modeling. The flux observed from a hotspot on the neutron star surface offset from the rotational pole will be modulated by the star s rotation, and this periodic modulation at the spin frequency is called a pulsation. As the photons propagate through the curved spacetime of the star, information about mass and radius is encoded into the shape of the waveform (pulse profile) via special and general-relativistic effects. Using pulsations from known sources (which have hotspots that develop either during thermo- nuclear bursts or due to channeled accretion) it is possible to obtain tight constraints on mass and radius. The second technique involves characterizing the spin distribution of accreting neutron stars. A large collecting area enables highly sensitive searches for weak or intermittent pulsations (which yield spin) from the many accreting neutron stars whose spin rates are not yet known. The most rapidly rotating stars provide a clean constraint, since the limiting spin rate where the equatorial surface velocity is comparable to the local orbital velocity, at which mass shedding occurs, is a function of mass and radius. However, the overall spin distribution also provides a guide to the torque mechanisms in operation and the moment of inertia, both of which can depend sensitively on dense matter physics. The third technique is to search for quasiperiodic oscillations in x-ray flux associated with global seismic vibrations of magnetars (the most highly magnetized neutron stars), triggered by magnetic explosions. The vibrational frequencies depend on stellar parameters including the dense matter equation of state, and large-area x-ray timing instruments would provide much improved detection capability. In addition, an illustration is given of how these complementary x-ray timing techniques can be used to constrain the dense matter equation of state and the results that might be expected from a 10 m2 instrument are discussed. Also discussed are how the results from such a facility would compare to other astronomical investigations of neutron star properties.
IAU Symposium 291 features a rich harvest of recent scientific discoveries and looks forward to the many exciting avenues for future neutron-star research. The volume starts with general, lively, comprehensive introductions to three main themes that successfully communicate the excitement of current pulsar research. The subsequent reviews and contributions on hot topics cover: ongoing searches for pulsars, both radio and gamma-ray; neutron star formation and properties; binary pulsars; pulsar timing and tests of gravitational theories; magnetars; radio transients; radio, X-ray and gamma-ray pulse properties and emission mechanisms; and future facilities. This range of topics clearly illustrates the diverse nature and wide application of neutron-star research. Through a combination of introductory reviews and practically complete coverage of current results from across the electromagnetic spectrum, IAU S291 is the perfect reference for neutron-star researchers and also provides an excellent read for advanced undergraduate and starting graduate students.
X-ray astronomy began with the detection of the persistent source Scorpius X-1. Shortly afterwards, sources were detected that were variable. Centaurus X-2, was determined to be an X-ray transient, having a quiescent state, and a state that was much brighter. As X-ray astronomy progressed, classifications of transient sources developed. One class of sources, believed to be neutron stars, undergo extreme luminosity transitions lasting a few seconds. These outbursts are believed to be thermonuclear explosions occurring on the surface of neutron stars (type I X-ray bursts). Other sources undergo luminosity changes that cannot be explained by thermonuclear burning and last for days to months. These sources are soft X-ray transients (SXTs) and are believed to be the result of instabilities in the accretion of matter onto either a neutron star or black hole. Type I X-ray bursts provide a tool for probing the surfaces of neutron stars. Requiring a surface for the burning has led authors to use the presence of X-ray bursts to rule out the existence of a black hole (where an event horizon exists not a surface) for systems which exhibit type I X-ray bursts. Distinguishing between neutron stars and black holes has been a problem for decades. Narayan and Heyl have developed a theoretical framework to convert suitable upper limits on type I X-ray bursts from accreting black hole candidates (BHCs) into evidence for an event horizon. We survey 2101.2 ks of data from the USA X-ray timing experiment and 5142 ks of data from the Rossi X-ray Timing Explorer (RXTE) experiment to obtain the first formal constraint of this type. 1122 ks of neutron star data yield a population averaged mean burst rate of 1.7 " 0.4 x 10−5 bursts s−1, while 6081 ks of BHC data yield a 95% confidence level upper limit of 4.9 x 10−7 bursts s−1. Applying the framework of Narayan and Heyl we calculate regions of luminosity where the neutron stars are expected to burst and the BHCs would be expected to burst if they had a similar surface. In this luminosity region 464 ks of neutron star data yield an averaged mean burst rate of 4.1 " 0.9 x 10−5 bursts s−1, and 1512 ks of BHC data yield a 95% confidence level upper limit of 2.0 x 10−6 bursts s−1 and a strong limit that BHCs do not burst with a rate similar to the rate of neutron stars in these regions. This gives evidence that BHCs do not have surfaces. In addition to studying type I X-ray bursts, we analyzed the SXT behavior. In particular, 4U 1630-47, was analyzed throughout its 1999 outburst. This source is one of the oldest known SXTs. This source is assumed to be a BHC in a low-mass X-ray binary system. Despite the length of time devoted to studying this source, there is still little known about it. We report the results of timing and spectral analysis on the 1999 outburst, and compare these results to other outbursts of 4U 1630-47. We found this source progressed from a low-hard state to a high-soft state and then rapidly transitioned back into the low-hard state before returning to quiescence. Timing analysis detected a low frequency quasi-periodic oscillation (LFQPO) during the initial rise of the outburst, which disappeared and did not return. The variability in the X-ray flux in the 0.1-2000 Hz frequency range is low during the high state, but increases as the source progresses into the low-hard state. The next generation Gamma Ray Large Area Space Telescope (GLAST), will measure astrophysical phenomena in the 20 MeV--a few TeV energy range. We describe preliminary design and testing of GLAST. The detector is based on a silicon tracker with similar design characteristics of vertex detectors used in high-energy physics experiments at accelerator based facilities. A beam test engineering model was designed, constructed, and tested at SLAC in 1999-2000. We describe this test, and discuss how the results from this test can improve and demonstrate the viability of the GLAST technology.
"Pulsars are the rotating compact remnants of exploded massive stars. The region surrounding the neutron star, known as the magnetosphere, has properties which are determined by the magnetic ield of the star. In this thesis, I present several observational results involving rotation-powered radio pulsars and magnetars which indicate that pulsar magnetospheres have a more complex structure than a simple dipole, and that the magnetosphere can have a strong efect on all the observed properties of neutron stars. One way to probe the pulsar magnetosphere is through the measurement of braking indices. A braking index quantifies the dependence of the torque on the spin frequency. In Chapter 4 I present a long-term timing study of the rotation-powered pulsar PSR J1846−0258, where we show that the change in braking index reported in this source is long-lived. The most plausible explanation for this changed braking index appears to be due to a change in magnetospheric configuration. In Chapter 5, I present the measurement of a new braking index for the rotation-powered pulsar PSR J1640−4631 of n= 3.15 ± 0.03 - the first measured braking index higher than the canonical three of a magnetic dipole. This result demonstratesthat other physical mechanisms, such as mass or magnetic quadrupole moments most likely need to be taken into account to describe pulsar spin-down & energetics.Another way to probe the magnetospheres of pulsars is by studying the extreme variability seen in the magnetar class. In Chapter 6 I present two years of flux and spin evolution monitoring of the magnetar 1E 1048.1−5937 following an outburst. By comparing to previous outbursts from the source, we show that this pattern of behaviour repeats itself with a quasi-period of ∼1800 days. This behaviour, when compared to similar less extreme events seen in rotation-powered pulsars, appears to implicate processes in the stellar magnetosphere. In Chapter 7, I present the results of monitoring the magnetar 4U 0142+61 over two outbursts, including one with a net spin-down timing event, and compare this timing event to previous such events in other pulsars with high magnetic fields anddiscuss net spin-down glitches now seen in several young, high-B pulsars. The observations that these spin-down events occur in only high-B sources strongly implicates the influence of a large magnetic field in spin-down events and, coupled with the radiatively loud nature of the plurality of spin-down events, suggests an origin in the magnetosphere of the star. In Chapter 8 I present observations of a magnetar-like outburst from the high-magnetic-field pulsar PSR J1119−6127, providing an unambiguous connection between the radio pulsar and magnetar populations.Finally, in Chapter 9, I put these new results in context with recent advances in neutron-star astrophysics, and speculate on avenues for future advancement in the field." --
In this thesis, we present our observational and evolutionary studies of neutron stars in X-ray binary systems. A variety of topics are discussed, which are all related by a single scientific theme, namely, helping to set constraints on the mass-radius relation of neutron stars, and hence on their equations of state (EOS). In Chapter 1 we review the current neutron star masses M and radii R measurement techniques utilizing the X-ray observation of neutron stars in binaries. These techniques fall into two categories: timing and spectral analysis. In Chapter 2 we present our spectral and timing analysis of 4U 2129+47. We show that 4U 2129+47 might be in a hierarchical triple system. The source has been dropping into deeper quiescence during the last decade. The absence of the power-law hard tail in its X-ray spectrum make it a good candidate for measuring neutron star radius. In Chapter 3 we present our analysis of EXO 0748-676. We show that the previously reported narrow absorption lines are inconsistent with the detected high amplitude of the 552 Hz burst oscillations. In Chapter 4 we present our semi-numerical method of evaluating the significance of burst oscillations. With this method, we searched 1187 archived RXTE Type-I X-ray bursts for high frequency oscillation modes. In Chapter 5, we present our evolutionary study of the most massive neutron star that has been recently found: PSR J1614-2230. The study has been carried out with the recently developed star evolution code "MESA". We We have computed an extensive grid of binary evolution tracks to represent low- and intermediate-mass X-ray binaries (LMXBs and IMXBs). The general results will be presented in Chapter 6.
"Over the past few decades, advances in X-ray and gamma-ray astronomy have greatly expanded our knowledge of the neutron-star family. One important recent discovery has been that of the "magnetars," isolated neutron stars whose radiation and occasional bursting activity is thought to be powered by their very high magnetic fields (10^14-10^15 G as inferred from timing), unlike ordinary pulsars that are powered by their rotational energy. There do, however, exist rotation-powered pulsars with inferred magnetic fields that approach those of the magnetars (~10^13 G). These two groups might therefore be expected to show some similarities in their properties or behaviour. Careful study of both the high-magnetic-field pulsars and magnetars, then, may help us to understand magnetar physics and determine their relations and connections with the rest of the pulsar population.In Chapter 3, I present the results of two XMM-Newton observations of the high-magnetic-field radio pulsar PSR J1734-3333. We successfully detect the X-ray counterpart of the pulsar. Its spectrum fits well to a blackbody with temperature 300 +/- 60 eV, and its bolometric luminosity is L_bb = 2.0 x 10^32 erg/s, or ~0.4% of its spin-down power, for a distance of 6.1 kpc. We detect no X-ray pulsations from the source, setting a 1 sigma upper limit on the pulsed fraction of 60% in the 0.5-3 keV band. We compare PSR J1734-3333 to other rotation-powered pulsars of similar age and find that it is significantly hotter, supporting the hypothesis that the magnetic field affects the observed thermal properties of pulsars. We also tabulate the properties of this and all other known high-B radio pulsars with measured thermal X-ray luminosities or luminosity upper limits, and speculate on a possible correlation between L_X and B.In Chapter 4, I present an analysis of the extended emission around the magnetar 1E 1547.0-5408. Based on four XMM-Newton observations taken with the source in various stages from outburst to quiescence, we find that the extended emission flux is highly variable and strongly correlated with the flux of the magnetar. From this result, as well as spectral and energetic considerations, we conclude that the extended emission is dominated by a dust-scattering halo and not a pulsar wind nebula (PWN), as has been previously argued. We obtain an upper limit on the 2-10 keV flux of a possible PWN of 4.7 x 10^-14 erg/s/cm^2, three times less than the previously claimed value. We do, however, find strong evidence for X-ray emission from a supernova remnant surrounding the pulsar, as previously reported.Finally, I present a study of the magnetar population as a whole in Chapter 5, with a catalog of the 26 currently known magnetars and magnetar candidates. Tables are provided of astrometric and timing data for all catalog sources, as well as of their observed radiative properties, particularly the spectral parameters of the quiescent X-ray emission. We show histograms of the spatial and timing properties of the magnetars and compare them with the known pulsar population. We measure the scale height of magnetars to be in the range of 20-31 pc, assuming they are exponentially distributed. This range is smaller than that measured for OB stars, providing evidence that magnetars are born from the most massive O stars. From the same fits, we find that the Sun lies ~13-22 pc above the Galactic plane, consistent with previous measurements. We confirm previously identified correlations between quiescent X-ray luminosity, L_X, and magnetic field, B, as well as X-ray spectral power-law indexes, Gamma and B, and show evidence for an excluded region in a plot of L_X versus Gamma. We observe that while there is a clear correlation between the hard and soft X-ray fluxes in magnetars, the radio-detected magnetars all have low, soft X-ray flux, suggesting, if anything, that the two bands are anti-correlated." --
The Large Area Detector (LAD) on the Large Observatory For X-ray Timing (LOFT), with a 8.5 m 2 photon- collecting area in the 2-30 keV bandpass at CCD-class spectral resolving power ([lambda]/[Delta][lambda] = 10 - 100), is designed for optimum performance on bright X-ray sources. Thus, it is well-suited to study thermonuclear X-ray bursts from Galactic neutron stars. These bursts will typically yield 2 x 105 photon detections per second in the LAD, which is at least 15 times more than with any other instrument past, current or anticipated. The Wide Field Monitor (WFM) foreseen for LOFT uniquely combines 2-50 keV imaging with large (30%) prompt sky coverage. This will enable the detection of tens of thousands of thermonuclear X-ray bursts during a 3-yr mission, including tens of superbursts. Both numbers are similar or more than the current database gathered in 50 years of X-ray astronomy.