62 research outputs found
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&
Multi-band optical micro-variability observations of the BL Lac object S4 0954+658
We have observed S4 0954+658 in the BVRI bands in 2001, and in the BI bands
in 2002. The observations resulted in almost evenly sampled light curves, 3-8
hours long, with an average sampling interval of ~5-15 min. Because of the
dense sampling and the availability of light curves in more than one optical
bands we are able to study the intra-night flux and spectral variability of the
source in detail. Significant observations were observed in all but one cases.
On average, the flux variability amplitude, on time scales of minutes/hours,
increases from ~2-3% in the I, to ~3.5-5.5% in the B band light curves. We do
not detect any flares within the individual light curves. However, there is a
possibility that the April 2001 and late May 2002 observations sample two
flares which lasted longer than ~1-3 days. The evidence is only suggestive
though, due to the limited length of the present light curves with regard to
the duration of the assumed flares. No spectral variations are detected during
the April 2001 observations. The source flux rises and decays with the same
rate, in all bands. This variability behaviour is typical of S4 0954+658, and
is attributed to geometrical effects. However, significant spectral variations
are observed in May 2002. We find that the spectrum hardens/softens as the flux
increases/decreases, respectively. Furthermore, the "hardening" rate of the
energy spectrum is faster than the rate with which the spectrum becomes
"redder" as the flux decays. We also find evidence (although of low statistical
significance) that the I band variations are delayed with respect to the B band
variations. If the May 2002 observations sample a flaring event, these results
suggest that the variations are caused by energetic processes which are
associated with the particle cooling and the source light travel time scales.Comment: 7 pages, 7 figures, accepted for publication in 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
RoboPol: First season rotations of optical polarization plane in blazars
We present first results on polarization swings in optical emission of
blazars obtained by RoboPol, a monitoring program of an unbiased sample of
gamma-ray bright blazars specially designed for effective detection of such
events. A possible connection of polarization swing events with periods of high
activity in gamma rays is investigated using the dataset obtained during the
first season of operation. It was found that the brightest gamma-ray flares
tend to be located closer in time to rotation events, which may be an
indication of two separate mechanisms responsible for the rotations. Blazars
with detected rotations have significantly larger amplitude and faster
variations of polarization angle in optical than blazars without rotations. Our
simulations show that the full set of observed rotations is not a likely
outcome (probability ) of a random walk of the
polarization vector simulated by a multicell model. Furthermore, it is highly
unlikely () that none of our rotations is physically
connected with an increase in gamma-ray activity.Comment: 16 pages, 9 figure
B and I-band optical micro-variability observations of the BL Lac objects S5 2007+777 and 3C371
We have observed S5 2007+777 and 3C371 in the B and I bands for 13 and 8
nights, respectively, during various observing runs in 2001, 2002 and 2004. The
observations resulted in almost evenly sampled light curves, 6-9 hours long. We
do not detect any flares within the observed light curves, but we do observe
small amplitude, significant variations, in both bands, on time scales of hours
and days. The average variability amplitude on time scales of minutes/hours is
2.5% and 1-1.5% in the case of S5 2007+777 and 3C371, respectively. The average
amplitudes increase to 5-12% and 4-6%, respectively, on time scales of days. We
find that the B and I band variations are highly correlated, on both short and
long time scales. During the 2004 observations, which resulted in the longest
light curves, we observe two well defined flux-decay and rising trends in the
light curves of both objects. When the flux decays, we observe significant
delays, with the B band flux decaying faster than the flux in the I band. As a
result, we also observe significant, flux related spectral variations as well.
The flux-spectral relation is rather complicated, with loop-like structures
forming during the flux evolution. The presence of spectral variations imply
that the observed variability is not caused by geometric effects. On the other
hand, our results are fully consistent with the hypothesis that the observed
variations are caused by perturbations which affect different regions in the
jet of the sources.Comment: Accepted for publication in Astronomy and Astrophysic
Multi-band optical micro-variability observations of BL Lacertae
We have observed BL Lacertae in the B, R and I bands for 2 nights in July,
1999, and 3 nights in July, 2001. The observations resulted in almost evenly
sampled light curves, with an average sampling interval of ~5 min. The source
is significantly variable in all bands. On average, the variability amplitude
increases from ~5% in the I band, to ~5.5% in the R and ~6.5% in the B band
light curves. The rising and decaying time scales are comparable within each
band, but they increase from the B, to R and I band light curves. The optical
power spectrum shows a red noise component with a slope of ~ -2.
Cross-correlation analysis shows that in most cases the delay between the
variations in the B and I band light curves is less than ~ 0.4 hrs. The
cross-correlation functions are asymmetric, implying complex delays of the I
band variations with respect to the B band variations. Furthermore, in one case
we find that the I band variations are significantly delayed (by ~0.2 hrs) with
respect to the B band variations. We also detect significant spectral
variations: the spectrum becomes steeper as the flux increases, and the
flattest spectral index corresponds to the maximum B band flux. Our results
imply that the fast, intra-night variations of the source correspond to
perturbations of different regions in the jet which cause localized injections
of relativistic particles on time scales much sorter that the average sampling
interval of the light curves. The variations are controlled by the cooling and
light crossing time scales, which are probably comparable.Comment: Accepted for publication in A&
M31N 2005-09c: a fast FeII nova in the disk of M31
Classical novae are quite frequent in M~31. However, very few spectra of M31
novae have been studied to date, especially during the early decline phase. Our
aim is to study the photometric and spectral evolution of a M31 nova event
close to outburst. We present photometric and spectroscopic observations of
M31N 2005-09c, a classical nova in the disk of M31, using the 1.3m telescope of
the Skinakas Observatory in Crete (Greece), starting on the 28th September,
i.e. about 5 days after outburst, and ending on the 5th October 2005, i.e.
about 12 days after outburst. We also have supplementary photometric
observations from the La Sagra Observatory in Northern Andalucia, Spain, on
September 29 and 30, October 3, 6 and 9 and November 1, 2005. The wavelength
range covered by the spectra is from 3565 A to 8365 A. The spectra are of high
S/N allowing the study of the evolution of the equivalent widths of the Balmer
lines, as well as the identification of non-Balmer lines. The nova displays a
typical early decline spectrum that is characterized by many weak FeII
multiplet emissions. It is classified as a P nova. From the nova light
curve, we have also derived its speed class, t=14+-2.5 days. As the nova
evolved the Balmer lines became stronger and narrower. The early decline of the
expansion velocity of the nova follows a power law in time with an exponent of
\~-0.2
Discovery of a Be/X-ray Binary Consistent with the Position of GRO J2058+42
GRO J2058+42 is a 195 s transient X-ray pulsar discovered in 1995 with BATSE.
In 1996, RXTE located GRO J2058+42 to a 90% confidence error circle with a 4'
radius. On 20 February 2004, the region including the error circle was observed
with Chandra ACIS-I. No X-ray sources were detected within the error circle,
however, 2 faint sources were detected in the ACIS-I field-of-view. We obtained
optical observations of the brightest object, CXOU J205847.5+414637, that had
about 64 X-ray counts and was just 0.3' outside the error circle. The optical
spectrum contained a strong H alpha line and corresponds to an infrared object
in the 2MASS catalog, indicating a Be/X-ray binary system. Pulsations were not
detected in the Chandra observations, but similar flux variations and distance
estimates suggest that CXOU J205847.5+414637 and GRO J2058+42 are the same
object. We present results from the Chandra observation, optical observations,
new and previously unreported RXTE observations, and a reanalysis of a ROSAT
observation.Comment: 18 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
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