Monte-Carlo simulations of thermal/nonthermal radiation from a neutron-star magnetospheric accretion shell

We discuss the space-and-time-dependent Monte Carlo code we have developed to simulate the relativistic radiation output from compact astrophysical objects, coupled to a Fokker-Planck code to determine the self-consistent lepton populations. We have applied this code to model the emission from a magnetized neutron star accretion shell near the Alfven radius, reprocessing the radiation from the neutron sar surface. We explore the parameter space defined by the accretion rate, stellar surface field and the level of wave turbulence in the shell. Our results are relevant to the emission from atoll sources, soft-X-ray transient X-ray binaries containing weakly magnetized neutron stars, and to recently suggested models of accretion-powered emission from anomalous X-ray pulsars.Comment: 24 pages, including 7 figures; uses epsf.sty. final version, accepted for publication in ApJ. Extended introduction and discussio

The Supernova Remnant W44: confirmations and challenges for cosmic-ray acceleration

The middle-aged supernova remnant (SNR) W44 has recently attracted attention because of its relevance regarding the origin of Galactic cosmic-rays. The gamma-ray missions AGILE and Fermi have established, for the first time for a SNR, the spectral continuum below 200 MeV which can be attributed to neutral pion emission. Confirming the hadronic origin of the gamma-ray emission near 100 MeV is then of the greatest importance. Our paper is focused on a global re-assessment of all available data and models of particle acceleration in W44, with the goal of determining on a firm ground the hadronic and leptonic contributions to the overall spectrum. We also present new gamma-ray and CO NANTEN2 data on W44, and compare them with recently published AGILE and Fermi data. Our analysis strengthens previous studies and observations of the W44 complex environment and provides new information for a more detailed modeling. In particular, we determine that the average gas density of the regions emitting 100 MeV - 10 GeV gamma-rays is relatively high (n= 250 - 300 cm^-3). The hadronic interpretation of the gamma-ray spectrum of W44 is viable, and supported by strong evidence. It implies a relatively large value for the average magnetic field (B > 10^2 microG) in the SNR surroundings, sign of field amplification by shock-driven turbulence. Our new analysis establishes that the spectral index of the proton energy distribution function is p1 = 2.2 +/- 0.1 at low energies and p2 = 3.2 +/- 0.1 at high energies. We critically discuss hadronic versus leptonic-only models of emission taking into account simultaneously radio and gamma-ray data. We find that the leptonic models are disfavored by the combination of radio and gamma-ray data. Having determined the hadronic nature of the gamma-ray emission on firm ground, a number of theoretical challenges remains to be addressed.Comment: 13 pages, 11 figures, accepted by A&

A variability analysis of low-latitude unidentified gamma-ray sources

We present a study of 40 low-latitude unidentified 3EG gamma-ray sources which were found to be not positionally coincident with any known class of potential gamma-ray emitters in the Galaxy (Romero, Benaglia & Torres, 1999). We have performed a variability analysis which reveals that many of these 40 sources are variable. These sources have, in addition, a steep mean value of the gamma-ray spectral index, $= 2.41 \pm 0.2$, which, combined with the high level of variability seems to rule out a pulsar origin. The positional coincidences with uncatalogued candidates to supernova remnants were also studied. Only 7 sources in the sample are spatially coincident with these candidates, a result that is shown to be consistent with the expected level of pure chance association. A complementary search for weak radio counterparts was also conducted and the results are presented as an extensive table containing all significant point-like radio sources within the 40 EGRET fields. We argue that in order to produce the high variability, steep gamma-ray spectra, and absence of strong radio counterparts observed in some of the gamma-ray sources of our sample a new class of objects should be postulated, and we analyze a viable candidate.Comment: Paper updated to match the accepted version to appear in Astronomy and Astrophysics, 2001. Tables 5,6,7 and 8 are in ascii format and need to be printed separately. they can also be obtained from http://www.iar.unlp.edu.ar/garra Table 5 is 62 pages long. Download the source to obtain the table

Revisión del proceso de osmosis inversa aplicado a la descontaminación de un líquido residual de curtido

En este trabajo se estudiaron aspectos relevantes del proceso de ósmosis inversa cuando es utilizado para reducir el contenido de sustancias nocivas disueltas en un líquido residual de curtido. La muestra provino de una curtiembre que trabaja con sulfato de cromo(III) 33% básico y el enmascarante ácido fórmico. Inicialmente se separaron materiales proteínicos y grasas por medio de tamizado seguido de ultrafiltración. La composición química después de reducirse la mencionada carga orgánica fue: 0,99 g L"1 de cromo(III); 12,51 g L"1 de sodio; 11,53 g L"1 de sulfato; 11,20 g L"1 de cloruro; 0,26 g L"1 de materiales proteínicos; 0,18 g L"1 de grasas y pH 3,63. Volúmenes de 24 L del efluente pre"tratado fueron descontaminados mediante ósmosis inversa, finalizándose cada ciclo luego de producir 18 L de permeado. La capacidad del equipo empleado fue determinada en términos de los porcentajes factor de conversión y paso aparente de sales. Se comprobó que bajo nuestras condiciones experimentales el ensuciamiento de la membrana semipermeable ocurría muy rápido y que la remoción del cromo(III) fue cuasi cuantitativa. Los otros componentes inorgánicos mayoritarios también fueron separados, pero con menor eficiencia: sulfato > sodio > cloruro. Finalmente, se estableció que el tiempo es una variable operativa capaz de potenciar el transporte por difusión de aquellas especies cuyo gradiente de concentración más ha aumentado, principalmente cromo(III) y en menor proporción sulfato.Publicado en Terceras Jornadas de Investigación, Transferencia y Extensión. La Plata : Universidad Nacional de La Plata, 2015.Facultad de Ingenierí

Revisión del proceso de osmosis inversa aplicado a la descontaminación de un líquido residual de curtido

En este trabajo se estudiaron aspectos relevantes del proceso de ósmosis inversa cuando es utilizado para reducir el contenido de sustancias nocivas disueltas en un líquido residual de curtido. La muestra provino de una curtiembre que trabaja con sulfato de cromo(III) 33% básico y el enmascarante ácido fórmico. Inicialmente se separaron materiales proteínicos y grasas por medio de tamizado seguido de ultrafiltración. La composición química después de reducirse la mencionada carga orgánica fue: 0,99 g L"1 de cromo(III); 12,51 g L"1 de sodio; 11,53 g L"1 de sulfato; 11,20 g L"1 de cloruro; 0,26 g L"1 de materiales proteínicos; 0,18 g L"1 de grasas y pH 3,63. Volúmenes de 24 L del efluente pre"tratado fueron descontaminados mediante ósmosis inversa, finalizándose cada ciclo luego de producir 18 L de permeado. La capacidad del equipo empleado fue determinada en términos de los porcentajes factor de conversión y paso aparente de sales. Se comprobó que bajo nuestras condiciones experimentales el ensuciamiento de la membrana semipermeable ocurría muy rápido y que la remoción del cromo(III) fue cuasi cuantitativa. Los otros componentes inorgánicos mayoritarios también fueron separados, pero con menor eficiencia: sulfato > sodio > cloruro. Finalmente, se estableció que el tiempo es una variable operativa capaz de potenciar el transporte por difusión de aquellas especies cuyo gradiente de concentración más ha aumentado, principalmente cromo(III) y en menor proporción sulfato.Publicado en Terceras Jornadas de Investigación, Transferencia y Extensión. La Plata : Universidad Nacional de La Plata, 2015.Facultad de Ingenierí

Revisión del proceso de osmosis inversa aplicado a la descontaminación de un líquido residual de curtido

En este trabajo se estudiaron aspectos relevantes del proceso de ósmosis inversa cuando es utilizado para reducir el contenido de sustancias nocivas disueltas en un líquido residual de curtido. La muestra provino de una curtiembre que trabaja con sulfato de cromo(III) 33% básico y el enmascarante ácido fórmico. Inicialmente se separaron materiales proteínicos y grasas por medio de tamizado seguido de ultrafiltración. La composición química después de reducirse la mencionada carga orgánica fue: 0,99 g L"1 de cromo(III); 12,51 g L"1 de sodio; 11,53 g L"1 de sulfato; 11,20 g L"1 de cloruro; 0,26 g L"1 de materiales proteínicos; 0,18 g L"1 de grasas y pH 3,63. Volúmenes de 24 L del efluente pre"tratado fueron descontaminados mediante ósmosis inversa, finalizándose cada ciclo luego de producir 18 L de permeado. La capacidad del equipo empleado fue determinada en términos de los porcentajes factor de conversión y paso aparente de sales. Se comprobó que bajo nuestras condiciones experimentales el ensuciamiento de la membrana semipermeable ocurría muy rápido y que la remoción del cromo(III) fue cuasi cuantitativa. Los otros componentes inorgánicos mayoritarios también fueron separados, pero con menor eficiencia: sulfato > sodio > cloruro. Finalmente, se estableció que el tiempo es una variable operativa capaz de potenciar el transporte por difusión de aquellas especies cuyo gradiente de concentración más ha aumentado, principalmente cromo(III) y en menor proporción sulfato.Publicado en Terceras Jornadas de Investigación, Transferencia y Extensión. La Plata : Universidad Nacional de La Plata, 2015.Facultad de Ingenierí

Gamma-rays from binary system with energetic pulsar and Be star with aspherical wind: PSR B1259-63/SS2883

At least one massive binary system containing an energetic pulsar, PSR B1259-63/SS2883, has been recently detected in the TeV gamma-rays by the HESS telescopes. These gamma-rays are likely produced by particles accelerated in the vicinity of the pulsar and/or at the pulsar wind shock, in comptonization of soft radiation from the massive star. However, the process of gamma-ray production in such systems can be quite complicated due to the anisotropy of the radiation field, complex structure of the pulsar wind termination shock and possible absorption of produced gamma-rays which might initiate leptonic cascades. In this paper we consider in detail all these effects. We calculate the gamma-ray light curves and spectra for different geometries of the binary system PSR B1259-63/SS2883 and compare them with the TeV gamma-ray observations. We conclude that the leptonic IC model, which takes into account the complex structure of the pulsar wind shock due to the aspherical wind of the massive star, can explain the details of the observed gamma-ray light curve.Comment: 12 pages, 11 figures, accepted for publication in MNRA

HST Observations of the Host Galaxy of GRB970508

We report on observations of the field of GRB~970508 made in early August 1998, 454 days after outburst, with the STIS CCD camera onboard the Hubble Space Telescope. The images, taken in open filter (50CCD) mode, clearly reveal the presence of a galaxy which was obscured in earlier (June 1997) HST images by emission from the optical transient (OT). The galaxy is regular in shape: after correcting for the HST/STIS PSF, it is well-fitted by an exponential disk with a scale length of 0."046 +/- 0."006 and an ellipticity of 0.70 +/- 0.07. All observations are marginally consistent with a continuous decline in OT emission as t^{-1.3} beginning two days after outburst; however, we find no direct evidence in the image for emission from the OT, and the surface brightness profile of the galaxy is most regular if we assume that the OT emission is negligible, suggesting that the OT may have faded more rapidly at late times than is predicted by the power-law decay. Due to the wide bandwidth of the STIS clear mode, the estimated magnitude of the galaxy is dependent on the galaxy spectrum that is assumed. Using colors obtained from late-time ground-based observations to constrain the spectrum, we find V = 25.4 +/- 0.15, a few tenths of a magnitude brighter than earlier ground-based estimates that were obtained by observing the total light of the galaxy and the OT and then subtracting the estimated OT brightness assuming it fades as a single power-law. This again suggests that the OT may have faded faster at late time than the power-law predicts. The position of the OT agrees with that of the isophotal center of the galaxy to 0."01. This remarkable agreement raises the possibility that the GRB may have been associated with either an active galactic nucleus or a nuclear starburst.Comment: Submitted to the Astrophysical Journal (Letters). Thirteen pages, three encapsulated figures. Abstract slightly abridge

Identification of Ambient Molecular Clouds Associated with Galactic Supernova Remnant IC443

The Galactic supernova remnant (SNR) IC443 is one of the most studied core-collapse SNRs for its interaction with molecular clouds. However, the ambient molecular clouds with which IC443 is interacting have not been thoroughly studied and remain poorly understood. Using Five College Radio Astronomy Observatory 14m telescope, we obtained fully sampled maps of ~ 1{\deg} \times 1{\deg} region toward IC443 in the 12CO J=1-0 and HCO+ J=1-0 lines. In addition to the previously known molecular clouds in the velocity range v_lsr = -6 to -1 km/s (-3 km/s clouds), our observations reveal two new ambient molecular cloud components: small (~ 1') bright clouds in v_lsr = -8 to -3 km/s (SCs), and diffuse clouds in v_lsr = +3 to +10 km/s (+5 km/s clouds). Our data also reveal the detailed kinematics of the shocked molecular gas in IC443, however the focus of this paper is the physical relationship between the shocked clumps and the ambient cloud components. We find strong evidence that the SCs are associated with the shocked clumps. This is supported by the positional coincidence of the SCs with shocked clumps and other tracers of shocks. Furthermore, the kinematic features of some shocked clumps suggest that these are the ablated material from the SCs upon the impact of the SNR shock. The SCs are interpreted as dense cores of parental molecular clouds that survived the destruction by the pre-supernova evolution of the progenitor star or its nearby stars. We propose that the expanding SNR shock is now impacting some of the remaining cores and the gas is being ablated and accelerated producing the shocked molecular gas. The morphology of the +5 km/s clouds suggests an association with IC443. On the other hand, the -3 km/s clouds show no evidence for interaction.Comment: Accepted for publication in ApJ. 15 pages (with emulateapj.cls), 17 figures, and 2 table