110 research outputs found
CCD Photometry of Standard Stars at Maidanak Astronomical Observatory in Uzbekstan: Transformations and Comparisons
Observation of standard stars is of crucial importance in stellar photometry.
We have studied the standard transformation relations of the UBVRI CCD
photometric system at the Maidanak Astronomical Observatory in Uzbekistan. All
observations were made with the AZT-22 1.5m telescope, SITe 2k CCD or Fairchild
486 CCD, and standard Bessell UBVRI filters from 2003 August to 2007 September.
We observed many standard stars around the celestial equator observed by SAAO
astronomers. The atmospheric extinction coefficients, photometric zero points,
and time variation of photometric zero points of each night were determined.
Secondary extinction coefficients and photometric zero points were very stable,
while primary extinction coefficients showed a distinct seasonal variation. We
also determined the transformation coefficients for each filter. For B, V, R,
and I filters, the transformation to the SAAO standard system could be achieved
with a straight line or a combination of two straight lines. However, in the
case of the U filter and Fairchild 486 CCD combination, a significant
non-linear correction term - related to the size of Balmer jump or the strength
of the Balmer lines - of up to 0.08 mags was required. We found that our data
matched well the SAAO photometry in V, B-V, V-I, and R-I. But in U-B, the
difference in zero point was about 3.6 mmag and the scatter was about 0.02 mag.
We attribute the relatively large scatter in U-B to the larger error in U of
the SAAO photometry. We confirm the mostly small differences between the SAAO
standard UBVRI system and the Landolt standard system. We also attempted to
interpret the seasonal variation of the atmospheric extinction coefficients in
the context of scattering sources in the earth's atmosphere.Comment: 14 pages, 13 figure
Quantum Algorithm for Dynamic Programming Approach for DAGs. Applications for Zhegalkin Polynomial Evaluation and Some Problems on DAGs
In this paper, we present a quantum algorithm for dynamic programming
approach for problems on directed acyclic graphs (DAGs). The running time of
the algorithm is , and the running time of the
best known deterministic algorithm is , where is the number of
vertices, is the number of vertices with at least one outgoing edge;
is the number of edges. We show that we can solve problems that use OR,
AND, NAND, MAX and MIN functions as the main transition steps. The approach is
useful for a couple of problems. One of them is computing a Boolean formula
that is represented by Zhegalkin polynomial, a Boolean circuit with shared
input and non-constant depth evaluating. Another two are the single source
longest paths search for weighted DAGs and the diameter search problem for
unweighted DAGs.Comment: UCNC2019 Conference pape
Multi-wavelength observations of afterglow of GRB 080319B and the modeling constraints
We present observations of the afterglow of GRB 080319B at optical, mm and
radio frequencies from a few hours to 67 days after the burst. Present
observations along with other published multi-wavelength data have been used to
study the light-curves and spectral energy distributions of the burst
afterglow. The nature of this brightest cosmic explosion has been explored
based on the observed properties and it's comparison with the afterglow models.
Our results show that the observed features of the afterglow fits equally good
with the Inter Stellar Matter and the Stellar Wind density profiles of the
circum-burst medium. In case of both density profiles, location of the maximum
synchrotron frequency is below optical and the value of cooling break
frequency is below rays, s after the burst. Also, the
derived value of the Lorentz factor at the time of naked eye brightness is
with the corresponding blast wave size of cm. The
numerical fit to the multi-wavelength afterglow data constraints the values of
physical parameters and the emission mechanism of the burst.Comment: 8 Pages, 3 Figures, Accepted for publication to Astronomy and
Astrophysics on 02/04/200
Accretion dynamics in the classical T Tauri star V2129 Oph
We analyze the photometric and spectroscopic variability of the classical T
Tauri star V2129 Oph over several rotational cycles to test the dynamical
predictions of magnetospheric accretion models. The photometric variability and
the radial velocity variations in the photospheric lines can be explained by
rotational modulation due to cold spots, while the radial velocity variations
of the He I (5876 \AA) line and the veiling variability are due to hot spot
rotational modulation. The hot and cold spots are located at high latitudes and
about the same phase, but the hot spot is expected to sit at the chromospheric
level, while the cold spot is at the photospheric level. Using the
dipole+octupole magnetic-field configuration previously proposed in the
literature for the system, we compute 3D MHD magnetospheric simulations of the
star-disk system. We use the simulation's density, velocity and scaled
temperature structures as input to a radiative transfer code, from which we
calculate theoretical line profiles at all rotational phases. The theoretical
profiles tend to be narrower than the observed ones, but the qualitative
behavior and the observed rotational modulation of the H\alpha and H\beta
emission lines are well reproduced by the theoretical profiles. The
spectroscopic and photometric variability observed in V2129 Oph support the
general predictions of complex magnetospheric accretion models with
non-axisymmetric, multipolar fields.Comment: Accepted by Astronomy and Astrophysic
An unusual very low-mass high-amplitude pre-main sequence periodic variable
We have investigated the nature of the variability of CHS7797, an unusual
periodic variable in the Orion Nebula Cluster. An extensive I-band photometric
data set of CHS7797 was compiled between 2004-2010 using various telescopes.
Further optical data have been collected in R and z' bands. In addition,
simultaneous observations of the ONC region including CHS7797 were performed in
the I, J, Ks and IRAC [3.6] and [4.5] bands over a time interval of about 40d.
CHS7797 shows an unusual large-amplitude variation of about 1.7 mag in the R,
I, and z' bands with a period 17.786. The amplitude of the brightness
modulation decreases only slightly at longer wavelengths. The star is faint
during 2/3 of the period and the shape of the phased light-curves for seven
different observing seasons shows minor changes and small-amplitude variations.
Interestingly, there are no significant colour-flux correlations for
wavelengths smaller than 2microns, while the object becomes redder when fainter
at longer wavelengths. CHS7797 has a spectral type of M6 and an estimated mass
between 0.04-0.1Msun. The analysis of the data suggests that the periodic
variability of CHS7797 is most probably caused by an orbital motion.
Variability as a result of rotational brightness modulation by spots is
excluded by the lack of any color-brightness correlation in the optical. The
latter indicates that CHS7797 is most probably occulted by circumstellar matter
in which grains have grown from typical 0.1 microns to 1-2 micron sizes. We
discuss two possible scenarios in which CHS7797 is periodically eclipsed by
structures in a disc, namely that CHS7797 is a single object with a
circumstellar disc, or that CHS7797 is a binary system, similar to KH15D, in
which an inclined circumbinary disc is responsible of the variability. Possible
reasons for the typical 0.3mag variations in I-band at a given phase are
discussed.Comment: 11 pages, 9 figures, accepted for publication A&
Evidence for energy injection and a fine-tuned central engine at optical wavelengths in GRB 070419A
We present a comprehensive multiwavelength temporal and spectral analysis of
the FRED GRB 070419A. The early-time emission in the -ray and X-ray
bands can be explained by a central engine active for at least 250 s, while at
late times the X-ray light curve displays a simple power-law decay. In
contrast, the observed behaviour in the optical band is complex (from 10 up
to 10 s). We investigate the light curve behaviour in the context of the
standard forward/reverse shock model; associating the peak in the optical light
curve at 450 s with the fireball deceleration time results in a Lorenz
factor at this time. In contrast, the shallow optical
decay between 450 and 1500 s remains problematic, requiring a reverse shock
component whose typical frequency is above the optical band at the optical peak
time for it to be explained within the standard model. This predicts an
increasing flux density for the forward shock component until t 4
10 s, inconsistent with the observed decay of the optical emission
from t 10 s. A highly magnetized fireball is also ruled out due to
unrealistic microphysic parameters and predicted light curve behaviour that is
not observed. We conclude that a long-lived central engine with a finely tuned
energy injection rate and a sudden cessation of the injection is required to
create the observed light curves - consistent with the same conditions that are
invoked to explain the plateau phase of canonical X-ray light curves of GRBs.Comment: 9 pages, 10 figures, accepted for publication in MNRA
Adaptive Optics Integral Field Spectroscopy of the Young Stellar Objects in LkH_alpha 225
Progress in understanding the embedded stars in LkHa225 has been hampered by
their variability, making it hard to compare data taken at different times, and
by the limited resolution of the available data, which cannot probe the small
scales between the two stars. In an attempt to overcome these difficulties, we
present new near-infrared data on this object taken using the ALFA adaptive
optics system with the MPE 3D integral field spectrometer and the near-infrared
camera Omega-Cass. The stars themselves have K-band spectra which are dominated
by warm dust emission, analagous to class I-II for low mass YSOs, suggesting
that the stars are in a phase where they are still accreting matter. On the
other hand, the ridge of continuum emission between them is rather bluer,
suggestive of extincted and/or scattered stellar light rather than direct dust
emission. The compactness of the CO emission seen toward each star argues for
accretion disks (which can also account for much of the K-band veiling) rather
than a neutral wind. In contrast to other YSOs with CO emission, LkHa225 has no
detectable Br_gamma emission. Additionally there is no H_2 detected on the
northern star, although we do confirm that the strongest H_2 emission is on the
southern star, where we find it is excited primarily by thermal mechanisms. A
second knot of H_2 is observed to its northeast, with a velocity shift of
-75kms and a higher fraction of non-thermal emission. This is discussed with
reference to the H2O maser, the molecular outflow, and [S II] emission observed
between the stars.Comment: to appear in ApJ, April 2001. 18 pages, including 6 figure
The WEBT Campaign on the Blazar 3C279 in 2006
The quasar 3C279 was the target of an extensive multiwavelength monitoring
campaign from January through April 2006, including an optical-IR-radio
monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration.
In this paper we focus on the results of the WEBT campaign. The source
exhibited substantial variability of optical flux and spectral shape, with a
characteristic time scale of a few days. The variability patterns throughout
the optical BVRI bands were very closely correlated with each other. In
intriguing contrast to other (in particular, BL Lac type) blazars, we find a
lag of shorter- behind longer-wavelength variability throughout the RVB ranges,
with a time delay increasing with increasing frequency. Spectral hardening
during flares appears delayed with respect to a rising optical flux. This, in
combination with the very steep IR-optical continuum spectral index of ~ 1.5 -
2.0, may indicate a highly oblique magnetic field configuration near the base
of the jet. An alternative explanation through a slow (time scale of several
days) acceleration mechanism would require an unusually low magnetic field of <
0.2 G, about an order of magnitude lower than inferred from previous analyses
of simultaneous SEDs of 3C279 and other FSRQs with similar properties.Comment: Accepted for publication in Ap
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