Observations of Type I Bursts from Neutron Stars

Observations of Type I X-ray bursts have long been taken as evidence that the sources are neutron stars. Black body models approximate the spectral data and imply a suddenly heated neutron star cooling over characteristic times of seconds to minutes. The phenomena are convincingly explained in terms of nuclear burning of accreted gas on neutron stars with low mass companion stars. Prospects are promising that detailed theory and data from RXTE and future missions will lead to better determinations of important physical parameters (neutron star mass and radius, composition of the accreting gas, distance of the source). Among the variety of bursts observed, there are probably representatives of different kinds of explosive burning. RXTE's discovery of a 2.5 ms persistent coherent period from one Type I burster has now linked bursters indisputably to the epitome of a neutron star, a fast spinning magnetic compact object. Oscillations in some bursts had already been thought to arise from the neutron stars' rotations. Detailed observations of these oscillations are touchstones of how the explosive bursts originate and progress, as well as independent measures of the neutron star parameters.Comment: 10 pages, 4 figures, to appear in "Cosmic Explosions, Proceedings of the 10th October Astrophysics in Maryland, AIP Conf. Proceedings 522, ed. S. S. Holt & W. W. Zhang, (AIP: Woodbury, N. Y.), 200

X-Ray Observations of Low-Mass X-Ray Binaries: Accretion Instabilities on Long and Short Time-Scales

X-rays trace accretion onto compact objects in binaries with low mass companions at rates ranging up to near Eddington. Accretion at high rates onto neutron stars goes through cycles with time-scales of days to months. At lower average rates the sources are recurrent transients; after months to years of quiescence, during a few weeks some part of a disk dumps onto the neutron star. Quasiperiodic oscillations near 1 kHz in the persistent X-ray flux attest to circular motion close to the surface of the neutron star. The neutron stars are probably inside their innermost stable circular orbits and the x-ray oscillations reflect the structure of that region. The long term variations show us the phenomena for a range of accretion rates. For black hole compact objects in the binary, the disk flow tends to be in the transient regime. Again, at high rates of flow from the disk to the black hole there are quasiperiodic oscillations in the frequency range expected for the innermost part of an accretion disk. There are differences between the neutron star and black hole systems, such as two oscillation frequencies versus one. For both types of compact object there are strong oscillations below 100 Hz. Interpretations differ on the role of the nature of the compact object.Comment: 12 pages, 5 Postscript figures, in "Astrophysical Sources of Gravitational Radiation for Ground-Based Detectors", American Institute of Physics, 200

Decision Making and Learning in a Globalizing World

Decision-makers can benefit from the experience of others with solutions to common problems. If a best practice exists, the challenge is to recognize it and to ensure its diffusion. Information about different solutions is often dispersed, and decision-makers may be reluctant to switch for reputational reasons. We study how (i) the assignment of decision rights (who decides on the solutions.implementation?) and (ii) globalization (who knows what about solutions adopted in other places?) in.uence both the quality of the information on locally adopted solutions that decision-makers exchange and the quality of the solutions that are actually being used next.centralization, decentralization, learning, cheap talk, reputational concerns, globalization, health care consensus panels, EU Open Method of Coordination

Producing and Manipulating Information

To reduce the chances of policy failures, policy makers need information about the effects of policies. Sometimes, policy makers can rely on agents who already possess the information. Often, the information has yet to be produced. This raises two problems. First, for a policy maker it is hard to ascertain how much effort an expert has put in acquiring information. Second, when the expert has an interest in the policy outcome, she may manipulate information to bring the policy decision more in line with her preferences. We show that experts who are unbiased toward the policy alternatives put highest effort in collecting information. Eliminating manipulation of information, however, requires that the preferences of the policy maker and the expert are aligned. Hence, when selecting an expert, policy makers face a trade-off. We show that policy makers optimally appoint experts with policy preferences which are less extreme than their own.

Is Transparency to No Avail? Committee Decision-Making, Pre-Meetings, and Credible Deals

Transparent decision-making processes are widely regarded as a prerequisite for the working of a representative democracy. It facilitates accountability, and citizens may suspect that decisions, if taken behind closed doors, do not promote their interests. Why else the secrecy? We provide a model of committee decision-making that explains the public.s demand for transparency, and committee members. aversion to it. In line with case study evidence, we show how pressures to become transparent induce committee members to organize pre-meetings away from the public eye. Outcomes of pre-meetings, deals, are less determined, more anarchic, than those of formal meetings, but within bounds. We characterize deals that are self-enforcing in the formal meeting.Committee decision-making, reputational concerns, transparency, pre-meetings, deliberation, self-enforcing deals, coalitions

The discovery of 50 minute periodic absorption events from 4U1915-05

The steady flux from 4U1916-05 which undergoes periodic absorption dips every 50 minutes was demonstrated. This period represents the underlying orbital period of the system. It is suggested that variations in the depth and duration of these events are caused by a bulge in the edge of the accretion disk, at the point where the gas stream impacts the disk. The mass losing star in this system is probably a low mass white dwarf. The spectrum of the dips indicates that the metallicity of the absorbing material is at least a factor 17 below solar values