Previously, Nova Puppis 1991 (V351 Pup) was measured to host one of the most massive ejections claimed in the literature. Multi-frequency radio detections from one epoch were published for this nova ...in the 1990's; and yet, the remaining data collected by the Karl G. Jansky Very Large Array (VLA) have remained unpublished. In this paper, we analyze the remaining unpublished data sets for V351 Pup at frequencies of 4.9, 8.4, 14.9, and 22.5 GHz. We fit the resulting light curve to a model of expanding thermal ejecta, under the assumption that the radio emission is dominated by free-free radiation and accounting for high levels of clumping in the ejecta. Images of V351 Pup in both the radio (from the VLA) and H +N ii (from Hubble Space Telescope) exhibit no aspherical structure, strengthening our assumption of spherical symmetry. From expansion parallax methods, we estimate the distance to V351 Pup to be 5.0 1.5 kpc. Our light-curve fit yields a value of for the ejecta mass, implying that V351 Pup is on the low end of expectations for ejecta mass from classical novae. A comparison between our derived ejecta mass and theoretical models gives evidence for a very massive (1.25 ) white dwarf, which is consistent with spectroscopic evidence for an oxygen-neon white dwarf.
ABSTRACT It has recently been discovered that some, if not all, classical novae emit GeV gamma-rays during outburst. Despite using an unreliable method to determine its distance, previous work showed ...that nova V1324 Sco was the most gamma-ray luminous of all gamma-ray-detected novae. We present here a different, more robust, method to determine the reddening and distance to V1324 Sco using high-resolution optical spectroscopy. Using two independent methods, we derived a reddening of and a distance limit of . This distance is greater than the value used in the gamma-ray analysis, meaning that V1324 Sco has an even higher gamma-ray luminosity than previously calculated. We also use periodic modulations in the brightness, interpreted as the orbital period, in conjunction with pre-outburst photometric limits to show that a main-sequence companion is strongly favored.
It has recently been discovered that some, if not all, classical novae emit GeV gamma-rays during outburst, but the mechanisms involved in the production ofgamma-rays are still not well understood. ...We present here a comprehensive multiwavelength data set-from radio to X-rays-for the most gamma-ray-luminous classical nova to date, V1324 Sco. Using this data set, we show that V1324 Sco is a canonical dusty Fe ii-type nova, with a maximum ejecta velocity of 2600 km s−1 and an ejecta mass of a few . There is also evidence for complex shock interactions, including a double-peaked radio light curve which shows high brightness temperatures at early times. To explore why V1324 Sco was so gamma-ray luminous, we present a model of the nova ejecta featuring strong internal shocks and find that higher gamma-ray luminosities result from higher ejecta velocities and/or mass-loss rates. Comparison of V1324 Sco with other gamma-ray-detected novae does not show clear signatures of either, and we conclude that a larger sample of similarly well-observed novae is needed to understand the origin and variation of gamma-rays in novae.
The distribution of single-event upsets (SEUs) in commercial off-the-shelf (COTS) static random access memory (SRAM) has generally been thought to be uniform in a device. However, process-induced ...variation within a device gives rise to variation within the cell-level electrical characteristic known as the data retention voltage (<inline-formula> <tex-math notation="LaTeX">V_{\mathrm{ DR}} </tex-math></inline-formula>). Furthermore, <inline-formula> <tex-math notation="LaTeX">V_{\mathrm{ DR}} </tex-math></inline-formula> has been shown to directly influence the critical charge (<inline-formula> <tex-math notation="LaTeX">Q_{\mathrm{ C}} </tex-math></inline-formula>) necessary for stored data to upset. Low-energy proton irradiation of COTS SRAMs exhibits a preference toward upsetting cells with lower <inline-formula> <tex-math notation="LaTeX">Q_{\mathrm{ C}} </tex-math></inline-formula>. An electrical procedure is presented to map relative cell critical charge values without knowledge of the underlying circuitry, allowing customized device usage. Given cell-level <inline-formula> <tex-math notation="LaTeX">Q_{\mathrm{ C}} </tex-math></inline-formula> knowledge, a device can have its cells "screened" such that those with low <inline-formula> <tex-math notation="LaTeX">Q_{\mathrm{ C}} </tex-math></inline-formula> are not used to store data. This work presents the results such a screening process has on the SEU per bit cross section.
Disparities in student outcomes, including gendered performance differences, are widespread in introductory physics and other STEM courses. STEM education researchers have investigated many course ...and student factors that could contribute to these inequities, including class size, test formats, assignment weightings, and students’ sense of belonging. These inequities are often largest in the timed, multiple-choice, high-stakes exams that characterize so many traditional introductory STEM courses. Time pressure is widely believed to influence student outcomes on these exams, reducing overall performance and perhaps exaggerating widespread group performance disparities. Reducing time pressure for students by providing more test-taking time is a small, structural change that could have large impacts on student performance and could differentially affect students. To explore this possibility, we offered all 596 students in our introductory physics course a 50% extension in test-taking time and collected data on exam performance, student demographics, and the time each student took to complete each exam. We made careful comparisons of student performance to historical data, across demographic groups, and across time usage on the exams using both raw exam scores and a “Better Than Expected” measure that compares student performance in the course under study to their own performance in other courses. While students overall scored slightly higher with extended exam time, we found that extended time did not reduce the well-established disparities in student outcomes categorized by sex, race/ethnicity, or college generation status present in our introductory physics course. These findings both indicate that extending exam time is not a simple fix for disparities in student outcomes and reinforce that systemic changes towards more authentic assessments of STEM knowledge and capabilities are imperative.
X-ray observations of shocked gas in novae can provide a useful probe of the dynamics of the ejecta. Here we report on X-ray observations of the nova V959 Mon, which was also detected in GeV ...gamma-rays with the Fermi satellite. We find that the X-ray spectra are consistent with a two-temperature plasma model with non-solar abundances. We interpret the X-rays as due to shock interaction between the slow equatorial torus and the fast polar outflow that were inferred from radio observations ofV959 Mon. We further propose that the hotter component, responsible for most of the flux, is from the reverse shock driven into the fast outflow. We find a systematic drop in the column density of the absorber between days 60 and 140, consistent with the expectations for such a picture. We present intriguing evidence for a delay of around 40 d in the expulsion of the ejecta from the central binary. Moreover, we infer a relatively small (a few times 10−6Msun) ejecta mass ahead of the shock, considerably lower than the mass of 104K gas inferred from radio observations. Finally, we infer that the dominant X-ray shock was likely notradiative at the time of our observations, and that the shock power was considerably higher than the observed X-ray luminosity. It is unclear why high X-ray luminosity, closer to the inferred shock power, is never seen in novae at early times, when the shock is expected to have high enough density to be radiative.
ABSTRACT We present multiwavelength observations of the persistent Fermi-Large Area Telescope unidentified γ-ray source 1FGL J1417.7-4407, showing it is likely to be associated with a newly ...discovered X-ray binary containing a massive neutron star (nearly ) and a giant secondary with a 5.4 day period. SOAR optical spectroscopy at a range of orbital phases reveals variable double-peaked H emission, consistent with the presence of an accretion disk. The lack of radio emission and evidence for a disk suggests the γ-ray emission is unlikely to originate in a pulsar magnetosphere, but could instead be associated with a pulsar wind, relativistic jet, or could be due to synchrotron self-Compton at the disk-magnetosphere boundary. Assuming a wind or jet, the high ratio of γ-ray to X-ray luminosity (∼20) suggests efficient production of γ-rays, perhaps due to the giant companion. The system appears to be a low-mass X-ray binary that has not yet completed the pulsar recycling process. This system is a good candidate to monitor for a future transition between accretion-powered and rotational-powered states, but in the context of a giant secondary.
We present an investigation of potential signatures of the formation of multiple stellar populations in recently formed extragalactic star clusters. All of the Galactic globular clusters for which ...good samples of individual stellar abundances are available show evidence for multiple populations. This appears to require that multiple episodes of star formation and light element enrichment are the norm in the history of a globular cluster. We show that there are detectable observational signatures of multiple formation events in the unresolved spectra of massive, young extragalactic star clusters. We present the results of a pilot program to search for one of the cleanest signatures that we identify-the combined presence of emission lines from a very recently formed population and absorption lines from a somewhat older population. A possible example of such a system is identified in the Antennae galaxies. This source's spectrum shows evidence of two stellar populations with ages of 8 Myr and 80 Myr. Further investigation shows that these populations are in fact physically separated, but only by a projected distance of 59 pc. We show that the clusters are consistent with being bound and discuss the possibility that their coalescence could result in a single globular cluster hosting multiple stellar populations. While not the prototypical system proposed by most theories of the formation of multiple populations in clusters, the detection of this system in a small sample is both encouraging and interesting. Our investigation suggests that expanded surveys of massive young star clusters should detect more clusters with such signatures.
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