454 research outputs found
New Planetary Nebulae in the Galactic Bulge region with l>0deg - I. Discovery method and first results
We present the first results of an [O III] 5007 A interference filter survey
for Planetary Nebulae (PNe) in the Galactic bulge. Covering (at first) the 66
per cent of the survey area, we detected a total of 90 objects, including 25
new PNe, 57 known PNe and 8 known PNe candidates. Deep Halpha+[N II] CCD images
have been obtained as well as low resolution spectra for the newly discovered
PNe. Their spectral signature suggests that the detected emission originates
from a photoionized nebula. In addition, absolute line fluxes have been
measured and the electron densities are given. Accurate optical positions and
optical diameters were also determined.Comment: 16 pages, 5 figures, accepted for publication in MNRA
Are spiral galaxies optically thin or thick?
The opacity of spiral galaxies is examined by modelling the dust and stellar
content of individual galaxies. The model is applied to five late-type spiral
galaxies (NGC 4013, IC 2531, UGC 1082, NGC 5529 and NGC 5907). Having analyzed
a total of seven galaxies thus far, the five galaxies mentioned above plus UGC
2048 and NGC 891 presented in (Xilouris et al. 1997, 1998), we are able to draw
some general conclusions, the most significant of which are: 1) The face-on
central optical depth is less than one in all optical bands indicating that
typical spiral galaxies like the ones that we have modelled would be completely
transparent if they were to be seen face-on. 2) The dust scaleheight is about
half that of the stars, which means that the dust is more concentrated near the
plane of the disk. 3) The dust scalelength is about 1.4 times larger than that
of the stars and the dust is more radially extended than the stars. 4) The dust
mass is found to be about an order of a magnitude more than previously measured
using the IRAS fluxes, indicating the existence of a cold dust component. The
gas-to-dust mass ratio calculated is close to the value derived for our Galaxy.
5) The derived extinction law matches quite well the Galactic extinction law,
indicating a universal dust behaviour.Comment: 13 pages. Accepted for publication in A&
Optical observations of the supernova remnant G 69.4+1.2
We performed deep optical observations of the area of the new supernova
remnant G 69.4+1.2 in the emission lines of [O III], Halpha+[N II] and [S II].
The low ionization images reveal diffuse and filamentary emission in the
central and south, south-west areas of our field. Estimates of the [S
II]/Halpha ratio suggest that the detected emission in these areas originates
from shock heated gas, while the strong extended source in the north must be an
HII region. The medium ionization image of [O III] shows a single filament
close to the field center. Emission from [O III] is not detected elsewhere in
the field but only in the north from LBN 069.96+01.35. Deep long-slit spectra
taken at the position of the [O III] filament suggest shock velocities ~120
km/s, while in other areas velocities around 50 km/s are expected. The sulfur
lines ratio indicates electron densities less than 120 cm^{-3}. The absolute
Halpha flux is ~5 x 10^{-17} erg s^{-1} cm^{-2} arcsec^{-2}. The optical
emission is very well correlated with the radio emission, especially in the
south west. The soft X-ray emission detected in the ROSAT All-Sky survey shows
a satisfactory degree of correlation with the optical data in the south-west
suggesting their association.Comment: 7 pages, 5 figures, accepted for publication in A&
The peculiar supernova remnant CTB 80
Deep CCD exposures of the peculiar supernova remnant CTB 80 in the light of
major optical lines have been obtained. These images reveal significant shock
heated emission in the area of the remnant. The sulfur line image shows
emission in the north along the outer boundary of the IRAS and HI shells. The
comparison between the [OIII] and [OII] line images further suggest the
presence of significant inhomogeneities in the interstellar medium. The flux
calibrated images do not indicate the presence of incomplete recombination
zones, and we estimate that the densities of the preshock clouds should not
exceed a few atoms per cm^3. The area covered by the optical radiation along
with the radio emission at 1410 MHz suggest that CTB 80 occupies a larger
angular extent than was previously known.Comment: 19 pages, 6 png figures. Submitted revised version to A &
Search for AGN counterparts of unidentified Fermi-LAT sources with optical polarimetry: Demonstration of the technique
The third Fermi-LAT catalog (3FGL) presented the data of the first four years
of observations from the Fermi Gamma-ray Space Telescope mission. There are
3034 sources, 1010 of which still remain unidentified. Identifying and
classifying gamma-ray emitters is of high significance with regard to studying
high-energy astrophysics. We demonstrate that optical polarimetry can be an
advantageous and practical tool in the hunt for counterparts of the
unidentified gamma-ray sources (UGSs). Using data from the RoboPol project, we
validated that a significant fraction of active galactic nuclei (AGN)
associated with 3FGL sources can be identified due to their high optical
polarization exceeding that of the field stars. We performed an optical
polarimetric survey within uncertainties of four unidentified 3FGL
sources. We discovered a previously unknown extragalactic object within the
positional uncertainty of 3FGL J0221.2+2518. We obtained its spectrum and
measured a redshift of . Using these measurements and
archival data we demonstrate that this source is a candidate counterpart for
3FGL J0221.2+2518 and most probably is a composite object: a star-forming
galaxy accompanied by AGN. We conclude that polarimetry can be a powerful asset
in the search for AGN candidate counterparts for unidentified Fermi sources.
Future extensive polarimetric surveys at high galactic latitudes (e.g.,
PASIPHAE) will allow the association of a significant fraction of currently
unidentified gamma-ray sources.Comment: accepted to A&
RoboPol: Connection between optical polarization plane rotations and gamma-ray flares in blazars
We use results of our 3 year polarimetric monitoring program to investigate
the previously suggested connection between rotations of the polarization plane
in the optical emission of blazars and their gamma-ray flares in the GeV band.
The homogeneous set of 40 rotation events in 24 sources detected by {\em
RoboPol} is analysed together with the gamma-ray data provided by {\em
Fermi}-LAT. We confirm that polarization plane rotations are indeed related to
the closest gamma-ray flares in blazars and the time lags between these events
are consistent with zero. Amplitudes of the rotations are anticorrelated with
amplitudes of the gamma-ray flares. This is presumably caused by higher
relativistic boosting (higher Doppler factors) in blazars that exhibit smaller
amplitude polarization plane rotations. Moreover, the time scales of rotations
and flares are marginally correlated.Comment: 12 pages, 16 figures, accepted to MNRA
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