605 research outputs found
Detection of a new, low-brightness supernova remnant possibly associated with EGRET sources
We report on the discovery of a shell-type supernova remnant in the southern
sky. It is a large (8*8), low-brightness source with a nonthermal radio
spectrum, which requires background filtering to isolate it from the diffuse
background emission of the Galaxy. Three 3EG gamma-ray sources are spatially
correlated with the radio structure. We have made 21-cm line observations of
the region and found that two of these sources are coincident with HI clouds.
We propose that the gamma-ray emission is the result of hadronic interactions
between high-energy protons locally accelerated at the remnant shock front and
atomic nuclei in the ambient clouds.Comment: 6 pages, 5 figure
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, , 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
Extreme intranight variability in the BL Lacertae object AO 0235+164
We present results of two-colour photometry with high time resolution of the
violently variable BL Lac object AO 0235+164. We have found extreme intranight
variability with amplitudes of ~ 100 % over time scales of 24 hours. Changes of
0.5 magnitudes in both R and V bands were measured within a single night, and
variations up to 1.2 magnitudes occurred from night to night. A complete
outburst with an amplitude ~ 30 % was observed during one of the nights, while
the spectrum remained unchanged. This seems to support an origin based on a
thin relativistic shock propagating in such a way that it changes the viewing
angle, as recently suggested by Kraus et al. (1999) and Qian et al. (2000).Comment: 4 pages, 3 figures, to appear in Astronomy & Astrophysics (Letters
Remanentes de supernova que brillan en rayos-X
Los remanentes de supernova (RSN), restos gaseosos de la explosión de una estrella (fenómeno comúnmente conocido como supernova - SN), se forman por la propagación de una onda de choque a través del medio interestelar circundante a donde se produjo dicha explosión.
A las SN se las puede clasificar en dos clases bien diferentes: las SN del tipo I y las SN del tipo II. Cada clase se caracteriza por su historia evolutiva y la masa inicial de la estrella que muere. Las del tipo I se originan por la explosión termonuclear de estrellas con unas pocas masas solares. En cambio las del tipo II, son el resultado del colapso gravitacional de estrellas masivas de varias masas solares. En estas explosiones, se pueden producir destellos de radiación intensÃsimos a lo largo de todo el espectro electromagnético, que pueden durar desde varias semanas a varios meses. Se caracterizan por un rápido aumento de la intensidad hasta alcanzar un máximo determinado, para luego decrecer en brillo de forma más o menos suave hasta desaparecer completamente
The P/Halley: Spatial distribution and scale lengths for C2, CN, NH2, and H2O
From P/Halley long slit spectroscopic exposures on 12 dates, extending from Oct. 1985 to May 1986, spatial profiles were obtained for emissions by C2, CN, NH2, and OI(1D). Haser model scale lengths were fitted to these data. The extended time coverage allowed the checking for consistency between the various dates. The time varying production rate of P/Halley severely affected the profiles after perihelion, which is shown in two profile sequences on adjacent dates. Because of the time varying production rate, it was not possible to obtain reliable Haser model scale lengths after perihelion. The pre-perihelion analysis yielded Haser model scale lengths of sufficient consistency that they can be used for production rate determinations, whenever it is necessary to extrapolate from observed column densities within finite observing apertures. Results of scale lengths reduced to 1 AU are given and discussed
P/Halley: Spatial distribution and scale lengths for C2, CN, NH2, and H2O
From P/Halley, long slit spectroscopic exposures on 12 dates, extending from Oct. 1985 to May 1986, spatial profiles were obtained for emissions by C2, CN, NH2, and OI ((sup 1)D). Examples of our derived spatial profiles are given. The qualitative trend of the scale lengths for the different species is nicely exemplified in this example. C2 has the longest parent scale length followed by CN and NH2. OI which tracks the parent H2O distribution is quite narrow but slightly wider than the continuum profile which has a center essentially indistinguishable from the stellar seeing disk. Comparison of C2 and CN also shows that C2 is falling off faster in the wings so that the daughter scale length of CN must be larger than that of C2
Extreme photo-polarimetric behaviour of the blazar AO 0235+164
We present optical photo-polarimetric observations with high temporal
resolution of the blazar AO 0235+164. Our data, the first to test the
photo-polarimetric behaviour of this object at very short time-scales, show
significant micro-variability in total flux, colour index, linear polarization
degree, and position angle. Strong inter-night variations are also detected for
these parameters. Although no correlation between colour index and total flux
was found, our data seem to support the general bluer-when-brighter trend
already known for this object. The polarization degree, in turn, shows no
correlation with total flux, but a clear trend in the sense that colour index
is redder (the spectrum is softer) when the measured polarization is higher.Comment: Accepted by MNRAS (Letters), 5 page
New evidence on the origin of the microquasar GRO J1655-40
Aims. Motivated by the new determination of the distance to the microquasar
GRO J1655-40 by Foellmi et al. (2006), we conduct a detailed study of the
distribution of the atomic and molecular gas, and dust around the open cluster
NGC 6242, the possible birth place of the microquasar. The proximity and
relative height of the cluster on the galactic disk provides a unique
opportunity to study SNR evolution and its possible physical link with
microquasar formation. Methods. We search in the interstellar atomic and
molecular gas around NGC 6242 for traces that may have been left from a
supernova explosion associated to the formation of the black hole in GRO
J1655-40. Furthermore, the 60/100 mu IR color is used as a tracer of
shocked-heated dust. Results. At the kinematical distance of the cluster the
observations have revealed the existence of a HI hole of 1.5*1.5 degrees in
diameter and compressed CO material acumulated along the south-eastern internal
border of the HI cavity. In this same area, we found extended infrared emission
with characteristics of shocked-heated dust. Based on the HI, CO and FIR
emissions, we suggest that the cavity in the ISM was produced by a supernova
explosion occured within NGC 6242. The lower limit to the kinematic energy
transferred by the supernova shock to the surrounding interstellar medium is ~
10^{49} erg and the atomic and molecular mass displaced to form the cavity of ~
16.500 solar masses. The lower limit to the time elapsed since the SN explosion
is ~ 2.2*10^{5} yr, which is consistent with the time required by GRO J1655-40
to move from the cluster up to its present position. The observations suggest
that GRO J1655-40 could have been born inside NGC 6242, being one of the
nearest microquasars known so far.Comment: 6 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
On the origin of two unidentified radio/X-ray sources discovered with XMM-Newton
We aim at clarifying the nature of the emission of two spatially related
unidentified X-ray sources detected with XMM-Newton telescope at
intermediate-low Galactic latitude. Observations reveal a point-like source
aligned with elongated diffuse emission. The X-ray spectra are best-fitted by
absorbed power laws with photon indices ~1.7 for the point-like and ~2.0 for
the extended one. Both sources show nonthermal radio-continuum counterparts
that might indicate a physical association. From the available data, we did not
detect variability on the point-like source in several timescales. Two possible
scenarios are analyzed: first, based on HI line absorption, assuming a Galactic
origin, we infer a distance upper bound of <2 kpc, which poses a constraint on
the height over the Galactic plane of <200 pc and on the linear size of the
system of 10^32 erg/s and
>7.5 x 10^32 erg/s, for the point-like and extended sources, respectively;
second, an extra-Galactic nature is discussed, where the point-like source
might be the core of a radio galaxy and the extended source its lobe. In this
case, we compare derived fluxes, spectral indices, and spatial correlation with
those typical from the radio galaxy population, showing the feasibility of this
alternative astrophysical scenario. From the available observational evidence,
we suggest that the most promising scenario to explain the nature of these
sources is a system consisting of a one-sided radio galaxy, where the
point-like source is an active galactic nucleus and the extended source
corresponds to the emission from its lobe. Other possibilities include a
PSR/PWN origin, where the radio/X-ray emission originates from the synchrotron
cooling of relativistic particles in the PSR magnetic field or a casual
alignment between two unrelated sources, such as an AGN core and a Galactic
X-ray blob.Comment: 6 pages, 2 figures. Accepted for publication in Astronomy and
Astrophysics (A&A
Discovery of a deep Seyfert-2 galaxy at z = 0.222 behind NGC 300
We report on the unveiling of the nature of the unidentified X-ray source
3XMM J005450.3-373849 as a Seyfert-2 galaxy located behind the spiral galaxy
NGC 300 using Hubble Space Telescope data, new spectroscopic Gemini
observations and available XMM-Newton and Chandra data. We show that the X-ray
source is positionally coincident with an extended optical source, composed by
a marginally resolved nucleus/bulge, surrounded by an elliptical disc-like
feature and two symmetrical outer rings. The optical spectrum is typical of a
Seyfert-2 galaxy redshifted to z=0.222 +/- 0.001, which confirms that the
source is not physically related to NGC 300. At this redshift the source would
be located at 909+/-4 Mpc (comoving distance in the standard model). The X-ray
spectra of the source are well-fitted by an absorbed power-law model. By tying
between the six available spectra, we found a variable index
running from ~2 in 2000-2001 years, to 1.4-1.6 in the 2005-2014
period. Alternatively, by tying , we found variable absorption columns
of N_H ~ 0.34 x cm in 2000-2001 years, and 0.54-0.75 x
cm in the 2005-2014 period. Although we cannot distinguish
between an spectral or absorption origin, from the derived unabsorbed X-ray
fluxes, we are able to assure the presence of long-term X-ray variability.
Furthermore, the unabsorbed X-ray luminosities of 0.8-2 x 10 erg
s derived in the X-ray band are in agreement with a weakly obscured
Seyfert-2 AGN at .Comment: MNRAS, accepte
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