116 research outputs found
The large amplitude outburst of the young star HBC 722 in NGC 7000/IC 5070, a new FU Orionis candidate
We report the discovery of a large amplitude outburst from the young star HBC
722 (LkHA 188 G4) located in the region of NGC 7000/IC 5070. On the basis of
photometric and spectroscopic observations, we argue that this outburst is of
the FU Orionis type. We gathered photometric and spectroscopic observations of
the object both in the pre-outburst state and during a phase of increase in its
brightness. The photometric BVRI data (Johnson-Cousins system) that we present
were collected from April 2009 to September 2010. To facilitate transformation
from instrumental measurements to the standard system, fifteen comparison stars
in the field of HBC 722 were calibrated in the BVRI bands. Optical spectra of
HBC 722 were obtained with the 1.3-m telescope of Skinakas Observatory (Crete,
Greece) and the 0.6-m telescope of Schiaparelli Observatory in Varese (Italy).
The pre-outburst photometric and spectroscopic observations of HBC 722 show
both low amplitude photometric variations and an emission-line spectrum typical
of T Tau stars. The observed outburst started before May 2010 and reached its
maximum brightness in September 2010, with a recorded Delta V~4.7 mag.
amplitude. Simultaneously with the increase in brightness the color indices
changed significantly and the star became appreciably bluer. The light curve of
HBC 722 during the period of rise in brightness is similar to the light curves
of the classical FUors - FU Ori and V1057 Cyg. The spectral observations during
the time of increase in brightness showed significant changes in both the
profiles and intensity of the spectral lines. Only H alpha remained in
emission, while the H beta, Na I 5890/5896, Mg I triplet 5174, and Ba II
5854/6497 lines were in strong absorption.Comment: 4 pages, 6 figures, accepted for publication in A&
Photometric and spectroscopic variability of the FUor star V582 Aurigae
We carried out BVRI CCD photometric observations in the field of V582 Aur
from 2009 August to 2013 February. We acquired high-, medium-, and
low-resolution spectroscopy of V582 Aur during this period. To study the
pre-outburst variability of the target and construct its historical light
curve, we searched for archival observations in photographic plate collections.
Both CCD and photographic observations were analyzed using a sequence of 14
stars in the field of V582 Aur calibrated in BVRI. The pre-outburst
photographic observations of V582 Aur show low-amplitude light variations
typical of T Tauri stars. Archival photographic observations indicate that the
increase in brightness began in late 1984 or early 1985 and the star reached
the maximum level of brightness at 1986 January. The spectral type of V582 Aur
can be defined as G0I with strong P Cyg profiles of H alpha and Na I D lines,
which are typical of FU Orionis objects. Our BVRI photometric observations show
large amplitude variations V~2.8 mag. during the 3.5 year period of
observations. Most of the time, however, the star remains in a state close to
the maximum brightness. The deepest drop in brightness was observed in the
spring of 2012, when the brightness of the star fell to a level close to the
pre-outburst. The multicolor photometric data show a color reversal during the
minimum in brightness, which is typical of UX Ori variables. The corresponding
spectral observations show strong variability in the profiles and intensities
of the spectral lines (especially H alpha), which indicate significant changes
in the accretion rate. On the basis of photometric monitoring performed over
the past three years, the spectral properties of the maximal light, and the
shape of the long-term light curve, we confirm the affiliation of V582 Aur to
the group of FU Orionis objects.Comment: 9 pages, 8 figures, accepted for publication in A&
Multi-band optical variability of three TeV Blazars on Diverse Timescales
We present our optical photometric observations of three TeV blazars, PKS
1510-089, PG 1553+113 and Mrk 501 taken using two telescopes in India, one in
Bulgaria, one in Greece and one in Serbia during 2012 - 2014. These
observations covered a total of 95 nights with a total of 202 B filter frames,
247 images in V band, 817 in R band while 229 images were taken in the I
filter. This work is focused on multi-band flux and colour variability studies
of these blazars on diverse timescales which are useful in understanding the
emission mechanisms. We studied the variability characteristics of above three
blazars and found all to be active over our entire observational campaigns. We
also searched for any correlation between the brightness of the sources and
their colour indices. During the times of variability, no significant evidence
for the sources to display spectral changes correlated with magnitude was found
on timescales of a few months. We briefly discuss the possible physical
mechanisms most likely responsible for the observed flux variability.Comment: 11 pages, 7 figures, 9 tables. Accepted for publication in MNRA
V1647 Orionis: One Year into Quiescence
We present new optical, near-IR, and mid-IR observations of the young
eruptive variable star V1647 Orionis that went into outburst in late 2004 for
approximately two years. Our observations were taken one year after the star
had faded to its pre-outburst optical brightness and show that V1647Ori is
still actively accreting circumstellar material. We compare and contrast these
data with existing observations of the source from both pre-outburst and
outburst phases. From near-IR spectroscopy we identify photospheric absorption
features for the first time that allow us to constrain the classification of
the young star itself. Our best fit spectral type is M0+-2 sub-classes with a
visual extinction of 19+-2 magnitudes and a K-band veiling of rK~1.5+-0.2. We
estimate that V1647Ori has a quiescent bolometric luminosity of ~9.5Lsun and a
mass accretion rate of ~1.10^-6Msun yr^-1. Our derived mass and age, from
comparison with evolutionary models, are 0.8+-0.2 Msun and ~0.5Myrs,
respectively. The presence towards the star of shock excited optical [S II] and
[Fe II] emission as well as near-IR H2 and [Fe II] emission perhaps suggests
that a new Herbig-Haro flow is becoming visible close to the star.Comment: 22 pages, 19 Figures, accepted AJ 13 October 200
Multi-band optical-NIR variability of blazars on diverse timescales
To search for optical variability on a wide range of timescales, we have
carried out photometric monitoring of two flat spectrum radio quasars, 3C 454.3
and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting
remarkably high activity and pronounced variability at all wavelengths. CCD
magnitudes in B, V, R and I pass-bands were determined for 7000 new
optical observations from 114 nights made during 2011 - 2014, with an average
length of 4 h each, at seven optical telescopes: four in Bulgaria, one
in Greece, and two in India. We measured multiband optical flux and colour
variations on diverse timescales. Discrete correlation functions were computed
among B, V, R, and I observations, to search for any time delays. We found weak
correlations in some cases with no significant time lags. The structure
function method was used to estimate any characteristic time-scales of
variability. We also investigated the spectral energy distribution of the three
blazars using B, V, R, I, J and K pass-band data. We found that the sources
almost always follows a bluer-when-brighter trend. We discuss possible physical
causes of the observed spectral variability.Comment: Accepted for publication in MNRAS, 16 pages, 11 figures, 5 tables,
plus supplementary material containing additional figures and tables (please
contact authors for it
Nature of Intra-night Optical Variability of BL Lacertae
We present the results of extensive multi-band intra-night optical monitoring
of BL Lacertae during 2010--2012. BL Lacertae was very active in this period
and showed intense variability in almost all wavelengths. We extensively
observed it for a total for 38 nights; on 26 of them observations were done
quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with
an average sampling interval of around 8 minutes. BL Lacertae showed
significant variations on hour-like timescales in a total of 19 nights in
different optical bands. We did not find any evidence for periodicities or
characteristic variability time-scales in the light curves.
The intranight variability amplitude is generally greater at higher
frequencies and decreases as the source flux increases.
We found spectral variations in BL Lacertae in the sense that the optical
spectrum becomes flatter as the flux increases but in several flaring states
deviates from the linear trend suggesting different jet components contributing
to the emission at different times.Comment: 12 Pages, 5 figures, 3 Tables, Accepted for Publication in MNRA
A peculiar multi-wavelength flare in the Blazar 3C 454.3
The blazar 3C454.3 exhibited a strong flare seen in gamma-rays, X-rays, and
optical/NIR bands during 3--12 December 2009. Emission in the V and J bands
rose more gradually than did the gamma-rays and soft X-rays, though all peaked
at nearly the same time. Optical polarization measurements showed dramatic
changes during the flare, with a strong anti-correlation between optical flux
and degree of polarization (which rose from ~ 3% to ~ 20%) during the declining
phase of the flare. The flare was accompanied by large rapid swings in
polarization angle of ~ 170 degree. This combination of behaviors appear to be
unique. We have cm-band radio data during the same period but they show no
correlation with variations at higher frequencies. Such peculiar behavior may
be explained using jet models incorporating fully relativistic effects with a
dominant source region moving along a helical path or by a shock-in-jet model
incorporating three-dimensional radiation transfer if there is a dominant
helical magnetic field. We find that spectral energy distributions at different
times during the flare can be fit using modified one-zone models where only the
magnetic field strength and particle break frequencies and normalizations need
change. An optical spectrum taken at nearly the same time provides an estimate
for the central black hole mass of ~ 2.3 * 10^9 M_sun. We also consider two
weaker flares seen during the d span over which multi-band data are
available. In one of them, the V and J bands appear to lead the -ray
and X-ray bands by a few days; in the other, all variations are simultaneous.Comment: 11 pages, 4 figures, 2 tables; MNRAS in pres
Optical and Radio Variability of BL Lacertae
We observed the prototype blazar, BL Lacertae, extensively in optical and
radio bands during an active phase in the period 2010--2013 when the source
showed several prominent outbursts. We searched for possible correlations and
time lags between the optical and radio band flux variations using
multifrequency data to learn about the mechanisms producing variability. During
an active phase of BL Lacertae, we searched for possible correlations and time
lags between multifrequency light curves of several optical and radio bands. We
tried to estimate any possible variability timescales and inter-band lags in
these bands. We performed optical observations in B, V, R and I bands from
seven telescopes in Bulgaria, Georgia, Greece and India and obtained radio data
at 36.8, 22.2, 14.5, 8 and 4.8 GHz frequencies from three telescopes in
Ukraine, Finland and USA. Significant cross-correlations between optical and
radio bands are found in our observations with a delay of cm-fluxes with
respect to optical ones of ~250 days. The optical and radio light curves do not
show any significant timescales of variability. BL Lacertae showed many optical
'mini-flares' on short time-scales. Variations on longer term timescales are
mildly chromatic with superposition of many strong optical outbursts. In radio
bands, the amplitude of variability is frequency dependent. Flux variations at
higher radio frequencies lead the lower frequencies by days or weeks.
The optical variations are consistent with being dominated by a geometric
scenario where a region of emitting plasma moves along a helical path in a
relativistic jet. The frequency dependence of the variability amplitude
supports an origin of the observed variations intrinsic to the source.Comment: 10 pages, 9 figures, Accepted for publication in A&
- …