Carbon Flashes in the Heavy Element Ocean on Accreting Neutron Stars

We show that burning of a small mass fraction of carbon in a neutron star ocean is thermally unstable at low accumulated masses when the ocean contains heavy ashes from the hydrogen burning rapid proton (rp) process. The key to early unstable ignition is the low thermal conductivity of a heavy element ocean. The instability requires accretion rates in excess of one-tenth the Eddington limit when the carbon mass fraction is 0.1 or less. The unstable flashes release 10^{42} to 10^{43} ergs over hours to days, and are likely the cause of the recently discovered large Type I X-ray bursts (so-called ``superbursts'') from six Galactic low mass X-ray binaries. In addition to explaining the energetics, recurrence times, and durations of the superbursts, these mixed carbon/heavy element flashes have an accretion rate dependence of unstable burning similar to that observed. Though the instability is present at accretion rates near Eddington, there is less contrast with the accretion luminosity there, explaining why most detections are made at accretion rates between 0.1 and 0.3 Eddington. Future comparisons of time dependent calculations with observations will provide new insights into the rp process.Comment: Submitted to Astrophysical Journal Letters (6 pages, 3 figures

Radio Monitoring of the January 11, 1997 Gamma-Ray Burst

We report on a comprehensive radio monitoring program of the bright gamma-ray burster GRB970111. These VLA observations were made at a frequency of 1.4 GHz and span a range of post-burst timescales between 28 hours and one month. Despite extensive sampling at sub-milliJansky sensitivities, no radio source was detected above 0.5 mJy in the current best error box (~14 arcmin^2) for GRB970111. A highly unusual radio source, VLA J1528.7+1945, was seen to drop in flux density by a factor of two in our monitoring period but it lies outside the error box and thus it is unlikely to be related to GRB970111. Cosmological fireball models of gamma-ray bursts make predictions of late-time emission occurring at longer wavelengths. The absence of a flaring or fading radio counterpart to GRB970111 provides strong constraints on these models.Comment: ApJ Let (accepted

Is transactional sex exploitative? A social norms perspective, with implications for interventions with adolescent girls and young women in Tanzania

Although transactional sex is common in many sexual relationships, there has been little research into the degree to which the practice is considered exploitative in the settings in which it is practiced. We describe the social norms that influence transactional sex in two sites in Mwanza, Tanzania, and explore local understandings of whether and under what conditions it is considered exploitative. We then compare these "emic" understandings of exploitation to international definitions and norms around sexual exploitation. This study employed a qualitative research design involving 18 focus group discussions and 43 in-depth interviews with young people aged 14-24 years and parents with children aged 14-24 years in a rural area and an urban center within Mwanza, Tanzania. Thematic analysis was conducted with the aid of NVivo 10. The social norms influencing the practice of transactional sex included: reciprocity as a core cultural value that permeates the way exchange in sexual relationships is judged; gendered expectations that men should provide for women's material needs in sexual relationships and that women should reciprocate by means of sex; and peer pressure to be perceived as "fashionable". Adolescent girls and young women (AGYW) are under strong peer pressure to conform to a "modern lifestyle" as reflected in stylish clothing and other items of modernity such as cellphones. The emic conceptualization of exploitation is defined by circumstances surrounding the relationship or a sexual encounter. Important factors that characterize local notions of when transactional relationships are considered exploitative include: when the encounter or relationship involves an imbalance of power (based on age, male economic power and social status); when a man fails to reciprocate; and when sex is coerced. According to community perspectives, young women's behavior should be considered exploitative of men when they take gifts or money yet refuse sex or when they demand large sums of money. Interventions aimed at reducing AGYW's exploitation through transactional sex need to be cognizant of the variations in the understanding of what constitutes sexual exploitation as well as the social and gender norms influencing the practice of transactional sex. Interventions need to involve communities and families in critical thinking that helps them identify positive alternatives to current gendered social norms that shape the involvement of AGYW and men in transactional sex

A rotating cavity for high-field angle-dependent microwave spectroscopy of low-dimensional conductors and magnets

The cavity perturbation technique is an extremely powerful method for measuring the electrodynamic response of a material in the millimeter- and sub-millimeter spectral range (10 GHz to 1 THz), particularly in the case of high-field/frequency magnetic resonance spectroscopy. However, the application of such techniques within the limited space of a high-field magnet presents significant technical challenges. We describe a 7.62 mm x 7.62 mm (diameter x length) rotating cylindrical cavity which overcomes these problems.Comment: 11 pages including 8 figure

Wide band observations of the new X-ray burster SAX J1747.0-2853 during the March 1998 outburst

We report on our discovery and follow-up observations of the X-ray source SAX J1747.0-2853 detected in outburst on 1998, March 10 with the BeppoSAX Wide Field Cameras in the energy range 2-28 keV. The source is located about half degree off the Galactic Nucleus. A total of 14 type-I X-ray bursts were detected in Spring 1998, thus identifying the object as a likely low-mass X-ray binary harboring a weakly magnetized neutron star. Evidence for photospheric radius expansion is present in at least one of the observed bursts, leading to an estimate of the source distance of about 9 kpc. We performed a follow-up target of opportunity observation with the BeppoSAX Narrow Field Instruments on March 23 for a total elapsed time of 72 ks. The source persistent luminosity was 2.6x10^36 erg/s in the 2-10 keV energy range. The wide band spectral data (1-200 keV) are consistent with a remarkable hard X-ray spectrum detected up to 150 keV, highly absorbed at low energies (Nh of the order of 10^23 cm^-2) and with clear evidence for an absorption edge near 7 keV. A soft thermal component is also observed, which can be described by single temperature blackbody emission at about 0.6 keV.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter