15 research outputs found
Multiband photometric detection of a huge flare on the M9 dwarf 2MASSW J1707183+643933
We present simultaneous UV-G-R-I monitoring of 19 M dwarfs that revealed a
huge flare on the M9 dwarf 2MASSW J1707183+643933 with an amplitude in the UV
of at least 6 magnitudes. This is one of the strongest detections ever of an
optical flare on an M star and one of the first in an ultracool dwarf (UCD,
spectral types later than about M7). Four intermediate strength flares (Delta
m_UV < 4 mag) were found in this and three other targets. For the whole sample
we deduce a flare probability of 0.013 (rate of 0.018/hr), and 0.049 (0.090/hr)
for 2M1707+64 alone. Deviations of the flare emission from a blackbody is
consistent with strong Halpha line emission. We also confirm the previously
found rotation period for 2M1707+64 (Rockenfeller, Bailer-Jones & Mundt (2006),
http://arxiv.org/abs/astro-ph/0511614/) and determine it more precisely to be
3.619 +/- 0.015 hr.Comment: Accepted to MNRAS. Five page
Fine Structure in the Circumstellar Environment of a Young, Solar-like Star: the Unique Eclipses of KH 15D
Results of an international campaign to photometrically monitor the unique
pre-main sequence eclipsing object KH 15D are reported. An updated ephemeris
for the eclipse is derived that incorporates a slightly revised period of 48.36
d. There is some evidence that the orbital period is actually twice that value,
with two eclipses occurring per cycle. The extraordinary depth (~3.5 mag) and
duration (~18 days) of the eclipse indicate that it is caused by circumstellar
matter, presumably the inner portion of a disk. The eclipse has continued to
lengthen with time and the central brightness reversals are not as extreme as
they once were. V-R and V-I colors indicate that the system is slightly bluer
near minimum light. Ingress and egress are remarkably well modeled by the
passage of a knife-edge across a limb-darkened star. Possible models for the
system are briefly discussed.Comment: 19 pages, 5 figure
The Disappearing Act of KH 15D: Photometric Results from 1995 to 2004
We present results from the most recent (2002-2004) observing campaigns of
the eclipsing system KH 15D, in addition to re-reduced data obtained at Van
Vleck Observatory (VVO) between 1995 and 2000. Phasing nine years of
photometric data shows substantial evolution in the width and depth of the
eclipses. The most recent data indicate that the eclipses are now approximately
24 days in length, or half the orbital period. These results are interpreted
and discussed in the context of the recent models for this system put forward
by Winn et al. and Chiang & Murray-Clay. A periodogram of the entire data set
yields a highly significant peak at 48.37 +/- 0.01 days, which is in accord
with the spectroscopic period of 48.38 +/- 0.01 days determined by Johnson et
al. Another significant peak, at 9.6 days, was found in the periodogram of the
out-of-eclipse data at two different epochs. We interpret this as the rotation
period of the visible star and argue that it may be tidally locked in
pseudosynchronism with its orbital motion. If so, application of Hut's theory
implies that the eccentricity of the orbit is e = 0.65 +/- 0.01. Analysis of
the UVES/VLT spectra obtained by Hamilton et al. shows that the v sin(i) of the
visible star in this system is 6.9 +/- 0.3 km/sec. Using this value of v sin(i)
and the measured rotation period of the star, we calculate the lower limit on
the radius to be R = (1.3 +/- 0.1), R_Sun, which concurs with the value
obtained by Hamilton et al. from its luminosity and effective temperature. Here
we assume that i = 90 degrees since it is likely that the spin and orbital
angular momenta vectors are nearly aligned.Comment: 55 pages, 18 figures, 1 color figure, to appear the September issue
of the Astronomical Journa
Reflected Light from Sand Grains in the Terrestrial Zone of a Protoplanetary Disk
We show that grains have grown to ~mm size (sand sized) or larger in the
terrestrial zone (within ~3 AU) of the protoplanetary disk surrounding the 3
Myr old binary star KH 15D. We also argue that the reflected light in the
system reaches us by back scattering off the far side of the same ring whose
near side causes the obscuration.Comment: 22 pages, 5 figures. To be published in Nature, March 13, 2008.
Contains a Supplemen
Disk-Jet Connection in the Radio Galaxy 3C 120
We present the results of extensive multi-frequency monitoring of the radio
galaxy 3C 120 between 2002 and 2007 at X-ray, optical, and radio wave bands, as
well as imaging with the Very Long Baseline Array (VLBA). Over the 5 yr of
observation, significant dips in the X-ray light curve are followed by
ejections of bright superluminal knots in the VLBA images. Consistent with
this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the
radio variations. This implies that, in this radio galaxy, the radiative state
of accretion disk plus corona system, where the X-rays are produced, has a
direct effect on the events in the jet, where the radio emission originates.
The X-ray power spectral density of 3C 120 shows a break, with steeper slope at
shorter timescale and the break timescale is commensurate with the mass of the
central black hole based on observations of Seyfert galaxies and black hole
X-ray binaries. These findings provide support for the paradigm that black hole
X-ray binaries and active galactic nuclei are fundamentally similar systems,
with characteristic time and size scales linearly proportional to the mass of
the central black hole. The X-ray and optical variations are strongly
correlated in 3C 120, which implies that the optical emission in this object
arises from the same general region as the X-rays, i.e., in the accretion
disk-corona system. We numerically model multi-wavelength light curves of 3C
120 from such a system with the optical-UV emission produced in the disk and
the X-rays generated by scattering of thermal photons by hot electrons in the
corona. From the comparison of the temporal properties of the model light
curves to that of the observed variability, we constrain the physical size of
the corona and the distances of the emitting regions from the central BH.Comment: Accepted for publication in the Astrophysical Journal. 28 pages, 21
figures, 2 table
GRB 050117: Simultaneous Gamma-ray and X-ray Observations with the Swift Satellite
The Swift Gamma-Ray Burst Explorer performed its first autonomous, X-ray
follow-up to a newly detected GRB on 2005 January 17, within 193 seconds of the
burst trigger by the Swift Burst Alert Telescope. While the burst was still in
progress, the X-ray Telescope obtained a position and an image for an
un-catalogued X-ray source; simultaneous with the gamma-ray observation. The
XRT observed flux during the prompt emission was 1.1 x 10^{-8} ergs cm^{-2}
s^{-1} in the 0.5-10 keV energy band. The emission in the X-ray band decreased
by three orders of magnitude within 700 seconds, following the prompt emission.
This is found to be consistent with the gamma-ray decay when extrapolated into
the XRT energy band. During the following 6.3 hours, the XRT observed the
afterglow in an automated sequence for an additional 947 seconds, until the
burst became fully obscured by the Earth limb. A faint, extremely slowly
decaying afterglow, alpha=-0.21. The X-ray position
triggered many follow-up observations: no optical afterglow could be confirmed,
although a candidate was identified 3 arcsecs from the XRT position.Comment: 27 pages, 6 figures. Accepted for publication in Ap
Astroclimatic Conditions at the Hoa Lac and Nha Trang Astronomical Observatories
The paper presents the first results of astroclimatic studies at the sites of the Hoa Lac and Nha Trang astronomical observatories. Our study employs Era-5 data covering a 10-yr time period (2011–2020). An analysis of the main astroclimatic characteristic, namely, the wind speed in the upper layers of the atmosphere, was performed. We calculated space distributions of the wind speed averaged in the height bin from 100 to 200 hPa. Using hourly data on pressure levels we analyzed probability distributions of the wind speed at high-level maxima at the sites of the observatories. At the Nha Trang observatory the period with a potentially high astroclimatic conditions falls on the spring when high recurrence of weak winds is observed. At the Hoa Lac observatory the best conditions are observed in the summer and the autumn. In this period, the median wind speeds are low. Additionally, we calculated spectra of the air temperature using the Fast Fourier Transform. We analyzed the deformations of the spectra with heights in a wide range of scales. At the site of the Nha Trang Astronomical Observatory, the amplitude of daily air temperature variations in the surface layer is approximately 1.5–2.5 times smaller compared to the Hoa Lac Observatory. We showed that the low-frequency maximum in the spectra is pronounced only in the lower layers of the atmosphere
Energy Spectra of Atmospheric Turbulence for Calculating Cn2 Parameter. I. Maidanak and Suffa Observatories in Uzbekistan
Knowledge of the turbulence spectra is of interest for describing atmospheric conditions as applied to astronomical observations. This article discusses the deformations of the turbulence spectra with heights in a wide range of scales at the sites of the Maidanak and Suffa observatories. It is shown that the energy of baroclinic instability is high at the sites of these observatories and should be taken into account in the calculations of the refractive index structure constant Cn2