ROSAT PSPC and Hri Observations of the Composite Starburst/Seyfert 2 Galaxy NGC 1672

The nearby barred spiral galaxy NGC 1672 is thought to have a weak Seyfert nucleus in addition to its strong starburst activity. Observations with the PSPC and HRI instruments on board the ROSAT X-ray satellite show that three X-ray sources with luminosities (1--2)\times 10^{40} erg/s are clearly identified with NGC 1672. The strongest X-ray source lies at the nucleus, and the other two lie near the ends of the prominent bar, locations that are also bright in H-alpha and near-infrared images. The nuclear source is resolved by the HRI on about the scale of the recently identified nuclear ring, and one of the sources at the ends of the bar is also probably resolved. The X-ray spectrum of the nuclear source is quite soft, having a Raymond--Smith plasma temperature of about 0.7 keV and little evidence for intrinsic absorption. The ROSAT band X-ray flux of the nuclear source appears to be dominated not by X-ray binary emission but rather by diffuse gas emission. While the properties of the nuclear source are generally supportive of a superbubble interpretation, its large density and emission measure stretch the limits that can be comfortably accommodated by such models. We do not detect direct emission from the putative Seyfert nucleus, although an alternative model for the nuclear source is thermal emission from gas that is photoionized by a hidden Seyfert nucleus. The spectra of the other two X-ray sources are harder than that of the nuclear source, and superbubble models for them have the same strengths and weaknesses.Comment: 11 pages, uuencoded compressed postscript, MNRAS in pres

Analysis of IUE observations of CS in Comet Bradfield (1979 l)

The high resolution rotational band profiles were fitted with theoretical band profiles which are derived using a Boltzmann temperature of 70 K. A very rapid variation with heliocentric distance for the CS brightness was found. The implications of these results for models of the coma along with the origin of the CS species are discussed

Polarized radio emission from the magnetar XTE J1810-197

We have used the Parkes radio telescope to study the polarized emission from the anomalous X-ray pulsar XTE J1810-197 at frequencies of 1.4, 3.2, and 8.4 GHz. We find that the pulsed emission is nearly 100% linearly polarized. The position angle of linear polarization varies gently across the observed pulse profiles, varying little with observing frequency or time, even as the pulse profiles have changed dramatically over a period of 7 months. In the context of the standard pulsar "rotating vector model," there are two possible interpretations of the observed position angle swing coupled with the wide profile. In the first, the magnetic and rotation axes are substantially misaligned and the emission originates high in the magnetosphere, as seen for other young radio pulsars, and the beaming fraction is large. In the second interpretation, the magnetic and rotation axes are nearly aligned and the line of sight remains in the emission zone over almost the entire pulse phase. We deprecate this possibility because of the observed large modulation of thermal X-ray flux. We have also measured the Faraday rotation caused by the Galactic magnetic field, RM = +77 rad/m^2, implying an average magnetic field component along the line of sight of 0.5 microG.Comment: Accepted for publication in ApJ Letters. Six pages with 4 figure

Epistemic Logic with Partial Dependency Operator

In this paper, we introduce $\textit{partial}$ dependency modality $\mathcal{D}$ into epistemic logic so as to reason about $\textit{partial}$ dependency relationship in Kripke models. The resulted dependence epistemic logic possesses decent expressivity and beautiful properties. Several interesting examples are provided, which highlight this logic's practical usage. The logic's bisimulation is then discussed, and we give a sound and strongly complete axiomatization for a sub-language of the logic

RankPL: A Qualitative Probabilistic Programming Language

In this paper we introduce RankPL, a modeling language that can be thought of as a qualitative variant of a probabilistic programming language with a semantics based on Spohn's ranking theory. Broadly speaking, RankPL can be used to represent and reason about processes that exhibit uncertainty expressible by distinguishing "normal" from" surprising" events. RankPL allows (iterated) revision of rankings over alternative program states and supports various types of reasoning, including abduction and causal inference. We present the language, its denotational semantics, and a number of practical examples. We also discuss an implementation of RankPL that is available for download

Follow-Up Chandra Observations of Three Candidate Tidal Disruption Events

Large-amplitude, high-luminosity soft X-ray flares were detected by the ROSAT All-Sky Survey in several galaxies with no evidence of Seyfert activity in their ground-based optical spectra. These flares had the properties predicted for a tidal disruption of a star by a central supermassive black hole. We report Chandra observations of three of these galaxies taken a decade after their flares that reveal weak nuclear X-ray sources that are from 240 to 6000 times fainter than their luminosities at peak, supporting the theory that these were special events and not ongoing active galactic nucleus (AGN) variability. The decline of RX J1624.9+7554 by a factor of 6000 is consistent with the (t-t_D)^(-5/3) decay predicted for the fall-back phase of a tidal disruption event, but only if ROSAT was lucky enough to catch the event exactly at its peak in 1990 October. RX J1242.6-1119A has declined by a factor of 240, also consistent with (t-t_D)^(-5/3). In the H II galaxy NGC 5905 we find only resolved, soft X-ray emission that is undoubtedly associated with starburst activity. When accounting for the starburst component, the ROSAT observations of NGC 5905, as well as the Chandra upper limit on its nuclear flux, are consistent with a (t-t_D)^(-5/3) decay by at least a factor of 1000. Although we found weak Seyfert~2 emission lines in Hubble Space Telescope spectra of NGC 5905, indicating that a low-luminosity AGN was present prior to the X-ray flare, we favor a tidal disruption explanation for the flare itself.Comment: 17 pages, 4 figures, to appear in ApJ April 1 200

Radio disappearance of the magnetar XTE J1810-197 and continued X-ray timing

We report on timing, flux density, and polarimetric observations of the transient magnetar and 5.54 s radio pulsar XTE J1810-197 using the GBT, Nancay, and Parkes radio telescopes beginning in early 2006, until its sudden disappearance as a radio source in late 2008. Repeated observations through 2016 have not detected radio pulsations again. The torque on the neutron star, as inferred from its rotation frequency derivative f-dot, decreased in an unsteady manner by a factor of 3 in the first year of radio monitoring. In contrast, during its final year as a detectable radio source, the torque decreased steadily by only 9%. The period-averaged flux density, after decreasing by a factor of 20 during the first 10 months of radio monitoring, remained steady in the next 22 months, at an average of 0.7+/-0.3 mJy at 1.4 GHz, while still showing day-to-day fluctuations by factors of a few. There is evidence that during this last phase of radio activity the magnetar had a steep radio spectrum, in contrast to earlier behavior. There was no secular decrease that presaged its radio demise. During this time the pulse profile continued to display large variations, and polarimetry indicates that the magnetic geometry remained consistent with that of earlier times. We supplement these results with X-ray timing of the pulsar from its outburst in 2003 up to 2014. For the first 4 years, XTE J1810-197 experienced non-monotonic excursions in f-dot by at least a factor of 8. But since 2007, its f-dot has remained relatively stable near its minimum observed value. The only apparent event in the X-ray record that is possibly contemporaneous with the radio shut-down is a decrease of ~20% in the hot-spot flux in 2008-2009, to a stable, minimum value. However, the permanence of the high-amplitude, thermal X-ray pulse, even after the radio demise, implies continuing magnetar activity.Comment: ApJ, accepted, 12 pages, 9 figure

Composition law for polarizers

The polarization process when polarizers act on an optical field is studied. We give examples for two kinds of polarizers. The first kind presents an anisotropic absorption - as in a polaroid film - and the second one is based on total reflection at the interface with a birefringent medium. Using the Stokes vector representation, we determine explicitly the trajectories of the wave light polarization during the polarization process. We find that such trajectories are not always geodesics of the Poincar\'e sphere as it is usually thought. Using the analogy between light polarization and special relativity, we find that the action of successive polarizers on the light wave polarization is equivalent to the action of a single resulting polarizer followed by a rotation achieved for example by a device with optical activity. We find a composition law for polarizers similar to the composition law for noncollinear velocities in special relativity. We define an angle equivalent to the relativistic Wigner angle which can be used to quantify the quality of two composed polarizers.Comment: 23 pages, 9 figures, accepted for publication in Physical Review

A millimeter-wave antireflection coating for cryogenic silicon lenses

We have developed and tested an antireflection (AR) coating method for silicon lenses at cryogenic temperatures and millimeter wavelengths. Our particular application is a measurement of the cosmic microwave background. The coating consists of machined pieces of Cirlex glued to the silicon. The measured reflection from an AR coated flat piece is less than 1.5% at the design wavelength. The coating has been applied to flats and lenses and has survived multiple thermal cycles from 300 to 4 K. We present the manufacturing method, the material properties, the tests performed, and estimates of the loss that can be achieved in practical lenses