The `Periodic Nulls' of Radio Pulsar J1819+1305

We present a single-pulse study of the four-component pulsar J1819+1305, whose ``null'' pulses bunch at periodic intervals of around 57 times the rotation period. The emission bursts between the null bunches exhibit characteristic modulations at two shorter periodicities of approximately 6.2 and 3 times the rotation period, the former found largely in the two outer components, and the latter only in the first component. Many bursts commence with bright emission in second component, exhibit positive six-period drift across the full profile width, and end with 3-period modulation in the leading component. The 57-period cycle can be modelled geometrically as a sparsely filled subbeam carousel with nulls appearing whenever our line of sight intersects a circulating empty region. This interpretation is compatible with other recent evidence for periodic, carousel-related nulling and appears to support the physics of a polar-gap emission model for ``drifting'' subpulses, but the subtle structure of the emission bursts defies an easy explanation.Comment: 8 pages, 9 figure

SAX J1808.4-3657 in Quiescence: A Keystone for Neutron Star Science

The accreting millisecond pulsar SAX J1808.4-3658 may be a transition object between accreting X-ray binaries and millisecond radio pulsars. We have constrained the thermal radiation from its surface through XMM-Newton X-ray observations, providing strong evidence for neutrino cooling processes from the neutron star core. We have also undertaken simultaneous X-ray and optical (Gemini) observations, shedding light on whether the strong heating of the companion star in quiescence may be due to X-ray irradiation, or to a radio pulsar turning on when accretion stops.Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 4 page

Proper motions of ROSAT discovered isolated neutron stars measured with Chandra: First X-ray measurement of the large proper motion of RX J1308.6+2127/RBS 1223

The unprecedented spatial resolution of the Chandra observatory opens the possibility to detect with relatively high accuracy proper motions at X-ray wavelengths. We have conducted an astrometric study of three of the "Magnificent Seven", the thermally emitting radio quiet isolated neutron stars (INSs) discovered by ROSAT. These three INSs (RX J0420.0-5022, RX J0806.4-4123 and RX J1308.6+2127/RBS 1223) either lack an optical counterpart or have one too faint to be used for astrometric purposes. We obtained ACIS observations 3 to 5 years apart to constrain or measure the displacement of the sources on the X-ray sky using as reference the background of extragalactic or remote galactic X-ray sources. Upper limits of 138 mas/yr and 76 mas/yr on the proper motion of RX J0420.0-5022 and RX J0806.4-4123, respectively, have already been presented in Motch et al. (2007). Here we report the very significant measurement (~ 10 sigma) of the proper motion of the third INS of our program, RX J1308.6+2127/RBS1223. Comparing observations obtained in 2002 and 2007 reveals a displacement of 1.1 arcsec implying a yearly proper motion of 223 mas, the second fastest measured for the ROSAT discovered INSs. The source is rapidly moving away from the galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of ~ 740 (d/700pc) km/s might be the largest of the "Magnificent Seven" and among the fastest recorded for neutron stars. RX J1308.6+2127/RBS1223 is thus a young high velocity cooling neutron star. The source may have its origin in the closest part of the Scutum OB2 association about 0.8 Myr ago, an age consistent with that expected from cooling curves, but significantly younger than inferred from pulse timing measurements (1.5 Myr).Comment: 3 pages, 2 figures, proceedings of the conference "40 Years of Pulsars", 12-17 August 2007, Montreal, Canad

Mitigation of Moral Hazard and Adverse Selection in Venture Capital Financing: The Influence of the Country’s Institutional Setting

A venture capitalist (VC) needs to trade off benefits and costs when attempting to mitigate agency problems in their investor-investee relationship. We argue that signals of ventures complement the VC’s capacity to screen and conduct a due diligence during the pre-investment phase, but its attractiveness may diminish in institutional settings supporting greater transparency. Similarly, whereas a VC may opt for contractual covenants to curb potential opportunism by ventures in the post-investment phase, this may only be effective in settings supportive of shareholder rights enforcement. Using an international sample of VC contracts, our study finds broad support for these conjectures. It delineates theoretical and practical implications for how investors can best deploy their capital in different institutional settings whilst nurturing their relationships with entrepreneurs

Hydrostatic Expansion and Spin Changes During Type I X-Ray Bursts

We present calculations of the spin-down of a neutron star atmosphere due to hydrostatic expansion during a Type I X-ray burst. We show that (i) Cumming and Bildsten overestimated the spin-down of rigidly-rotating atmospheres by a factor of two, and (ii) general relativity has a small (5-10%) effect on the angular momentum conservation law. We rescale our results to different neutron star masses, rotation rates and equations of state, and present some detailed rotational profiles. Comparing with recent observations of large frequency shifts in MXB 1658-298 and 4U 1916-053, we find that the spin-down expected if the atmosphere rotates rigidly is a factor of two to three less than the observed values. If differential rotation is allowed to persist, we find that the upper layers of the atmosphere spin down by an amount comparable to the observed values; however, there is no compelling reason to expect the observed spin frequency to be that of only the outermost layers. We conclude that hydrostatic expansion and angular momentum conservation alone cannot account for the largest frequency shifts observed during Type I bursts.Comment: Submitted to the Astrophysical Journal (13 pages, including 4 figures

Polycrystalline Crusts in Accreting Neutron Stars

The crust of accreting neutron stars plays a central role in many different observational phenomena. In these stars, heavy elements produced by H-He burning in the rapid proton capture (rp-) process continually freeze to form new crust. In this paper, we explore the expected composition of the solid phase. We first demonstrate using molecular dynamics that two distinct types of chemical separation occur, depending on the composition of the rp-process ashes. We then calculate phase diagrams for three-component mixtures and use them to determine the allowed crust compositions. We show that, for the large range of atomic numbers produced in the rp-process ($Z\sim 10$--$50$), the solid that forms has only a small number of available compositions. We conclude that accreting neutron star crusts should be polycrystalline, with domains of distinct composition. Our results motivate further work on the size of the compositional domains, and have implications for crust physics and accreting neutron star phenomenology.Comment: 8 pages, 4 figures, Submitted to ApJ, this article supersedes arXiv:1709.0926

An X-ray View of Radio Millisecond Pulsars

In recent years, X-ray observations with Chandra and XMM-Newton have significantly increased our understanding of rotation-powered (radio) millisecond pulsars (MSPs). Deep Chandra studies of several globular clusters have detected X-ray counterparts to a host of MSPs, including 19 in 47 Tuc alone. These surveys have revealed that most MSPs exhibit thermal emission from their heated magnetic polar caps. Realistic models of this thermal X-ray emission have provided important insight into the basic physics of pulsars and neutron stars. In addition, intrabinary shock X-ray radiation observed in ``black-widow'' and peculiar globular cluster ``exchanged'' binary MSPs give interesting insight into MSP winds and relativistic shock. Thus, the X-ray band contains valuable information regarding the basic properties of MSPs that are not accesible by radio timing observations.Comment: 5 pages, 5 figures, To appear in the proceedings of "40 Years of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007, McGill University, Montreal, Canad