501 research outputs found
Observations of the variability of shallow trade wind cumulus cloudiness and mass flux
Two years of ground-based remote sensing observations are used to study the vertical structure of marine cumulus near the island of Barbados, including their cloud fraction and mass flux profile. Daily radar derived cloud fraction profiles peak at different height levels depending on the depth of the cumuli and thus the extent to which they precipitate. Nonprecipitating cumuli have a peak cloud fraction of about 5% near mean cloud base (700m), whereas precipitating cumuli tend to have a peak of only 2% near cloud base. Nineteen percent of the precipitating cumuli are accompanied by large cloud fractions near the detrainment level of cumulus tops (similar to 1700m). Day-to-day variations in cloud fraction near cloud base are modest (similar to 3%). Nonprecipitating cumuli have their largest reflectivities near cloud top and an ascending core surrounded by a subsiding shell. Precipitating cumuli with enhanced elevated cloudiness (stratiform outflow) are deeper and contain larger vertical gradients in reflectivity and Doppler velocity than precipitating cumuli without such outflow. Bulk (3h) statistics reveal that nonprecipitating shallow cumuli are active and organized. They contain on average 79% in-cloud updrafts with 86% of them being organized in large coherent structures contributing to a maximum updraft mass flux of 8-36gm(-2)s(-1) just above cloud base. Alternatively, downdrafts contribute insignificantly to the mass flux and show little vertical and temporal variability (0-7gm(-2)s(-1)). Complementary Raman lidar information suggests that updraft mass flux profile slope is inversely related to environmental relative humidity
An absorption event in the X-ray lightcurve of NGC 3227
We have monitored the Seyfert galaxy NGC 3227 with the Rossi X-ray Timing
Explorer (RXTE) since January 1999. During late 2000 and early 2001 we observed
an unusual hardening of the 2-10 keV X-ray spectrum which lasted several
months. The spectral hardening was not accompanied by any correlated variation
in flux above 8 keV. We therefore interpret the spectral change as transient
absorption by a gas cloud of column density 2.6 10^23 cm^-2 crossing the line
of sight to the X-ray source. A spectrum obtained by XMM-Newton during an early
phase of the hard-spectrum event confirms the obscuration model and shows that
the absorbing cloud is only weakly ionised. The XMM-Newton spectrum also shows
that ~10% of the X-ray flux is not obscured, but this unabsorbed component is
not significantly variable and may be scattered radiation from a large-scale
scattering medium. Applying the spectral constraints on cloud ionisation
parameter and assuming that the cloud follows a Keplerian orbit, we constrain
the location of the cloud to be R~10-100 light-days from the central X-ray
source, and its density to be n_H~10^8cm^-3, implying that we have witnessed
the eclipse of the X-ray source by a broad line region cloud.Comment: 5 pages, 6 figures, accepted for publication in MNRAS letter
The Meinunger "Nicht Rote" Objects
Four high-latitude slow variable stars have been noted by Meinunger (1972) as
"nicht rote" ("not red") objects and thus curious. We have previously reported
(Margon & Deutsch 1997) that one of these objects, CC Boo, is in fact a QSO.
Here we present observations demonstrating that the remaining three are also
highly variable active galactic nuclei. The most interesting object of the four
is perhaps S 10765 (= NGP9 F324-0276706), which proves to be a resolved galaxy
at z=0.063. Despite the rapid and large reported variability amplitude (~1.6
mag), the spectrum is that of a perfectly normal galaxy, with no emission lines
or evident nonthermal continuum. We also present new spectroscopic and
photometric observations for AR CVn, suggested by Meinunger to be an RR Lyrae
star despite its very faint magnitude (=19.4). The object is indeed one of
the most distant RR Lyrae stars known, at a galactocentric distance of ~40 kpc.Comment: Accepted for publication in Publications of the Astronomical Society
of the Pacific, Volume 111, January 1999; 14 pages including 4 figures and 1
tabl
X-ray spectral variability of the Seyfert galaxy NGC 4051
We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051
observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000 day period
between May 1996 and March 1999. The spectra were obtained as part of
monitoring observations and from two long observations using the RXTE
Proportional Counter Array (PCA). During the monitoring period the 2-10 keV
flux of NGC 4051 varied between 10E-12 and 7x 10E11 (cgs). We re-analysed RXTE
PCA observations from a distinct low state in May 1998 using the latest
background and detector response models. The RXTE and BeppoSAX observations of
NGC 4051 during the low state show a very hard spectrum with a strong
unresolved fluorescence line. This emission, probably due to reflection from a
molecular torus, is likely to be constant over long time-scales and is
therefore assumed as an underlying component at all flux states. By subtracting
the torus component we are able to determine the spectral variability of the
primary continuum. In the variable component we observe a strong
anti-correlation of X-ray flux and spectral hardness in the PCA energy band. We
show that the changes in hardness are caused by slope variability of the
primary power law spectrum rather than by changing reflection or variable
photoelectric absorption. The primary spectral index varies between Gamma=1.6
for the faintest states and Gamma=2.3 during the brightest states, at which
level the spectral index approaches an asympotic value. We find that the
response of the flux of the 6.4 keV iron fluorescence line to changes in the
continuum flux depends on the timescale of the observation. The profile of the
line is very broad and indicates an origin in the innermost regions of the
accretion disk.Comment: accepted for publication in MNRA
Catching NGC4051 in the low state with XMM-Newton
The Narrow Line Seyfert 1 galaxy NGC4051 shows unusual low flux states,
lasting several months, when the 2-10 keV X-ray spectrum becomes unusually hard
(photon index<1) while the spectrum at lower X-ray energies is dominated by a
large soft excess. A Chandra TOO of the low state has shown that the soft
excess and hard components are variable and well-correlated. The variability of
the hard component rules out an origin in a distant reflector. Here we present
results from a recent XMM-Newton TOO of NGC4051 in the low state, which allows
a much more detailed examination of the nature of the hard and soft spectral
components in the low state. We demonstrate that the spectral shape in the low
state is consistent with the extrapolation of the spectral pivoting observed at
higher fluxes. The XMM-Newton data also reveals the warm absorbing gas in
emission, as the drop in the primary continuum flux unmasks prominent emission
lines from a range of ion species.Comment: 4 pages, 4 figures. Proc. of the meeting: "The Restless High-Energy
Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't
Zand, and R.A.M.J. Wijers Ed
XMMU J100750.5+125818: A strong lensing cluster at z=1.082
We report on the discovery of the X-ray luminous cluster XMMU
J100750.5+125818 at redshift 1.082 based on 19 spectroscopic members, which
displays several strong lensing features. SED modeling of the lensed arc
features from multicolor imaging with the VLT and the LBT reveals likely
redshifts ~2.7 for the most prominent of the lensed background galaxies. Mass
estimates are derived for different radii from the velocity dispersion of the
cluster members, M_200 ~ 1.8 10^{14} Msun, from the X-ray spectral parameters,
M_500 ~ 1.0 10^{14} Msun, and the largest lensing arc, M_SL ~ 2.3 10^{13} Msun.
The projected spatial distribution of cluster galaxies appears to be elongated,
and the brightest galaxy lies off center with respect to the X-ray emission
indicating a not yet relaxed structure. XMMU J100750.5+125818 offers excellent
diagnostics of the inner mass distribution of a distant cluster with a
combination of strong and weak lensing, optical and X-ray spectroscopy.Comment: A&A, accepted for publicatio
A giant X-ray dust scattering ring around the black hole transient MAXI J1348-630 discovered with SRG/eROSITA
We report the discovery of a giant dust scattering ring around the Black Hole
transient MAXI J1348-630 with SRG/eROSITA during its first X-ray all-sky
survey. During the discovery observation in February 2020 the ring had an outer
diameter of 1.3 deg, growing to 1.6 deg by the time of the second all sky
survey scan in August 2020. This makes the new dust ring the by far largest
X-ray scattering ring observed so far. Dust scattering halos, in particular the
rings found around transient sources, offer the possibility of precise distance
measurements towards the original X-ray sources. We combine data from
SRG/eROSITA, XMM-Newton, MAXI, and Gaia to measure the geometrical distance of
MAXI J1348-630. The Gaia data place the scattering dust at a distance of 2050
pc, from the measured time lags and the geometry of the ring, we find MAXI
J1348-630 at a distance of 3390 pc with a statistical uncertainty of only 1.1%
and a systematic uncertainty of 10% caused mainly by the parallax offset of
Gaia. This result makes MAXI J1348-630 one of the black hole transients with
the best determined distances. The new distance leads to a revised mass
estimate for the black hole of 11+-2 solar masses, the transition to the soft
state during the outburst occurred when the bolometric luminosity of MAXI
J1348-630 had reached 1.7% of its Eddington luminosity.Comment: 10 pages, 10 figures, 3 tables, accepted for publication in A&
VHE Gamma Rays from PKS 2155-304
The close X-ray selected BL Lac PKS 2155-304 has been observed using the
University of Durham Mark 6 very high energy (VHE) gamma ray telescope during
1996 September/October/November and 1997 October/November. VHE gamma rays with
energy > 300 GeV were detected from this object with a time-averaged integral
flux of (4.2 +/- 0.7 (stat) +/- 2.0 (sys)) x 10^(-11) per cm2 per s. There is
evidence for VHE gamma ray emission during our observations in 1996 September
and 1997 October/November, with the strongest emission being detected in 1997
November, when the object was producing the largest flux ever recorded in
high-energy X-rays and was detected in > 100 MeV gamma-rays. The VHE and X-ray
fluxes show evidence of a correlation.Comment: 14 pages, 6 figures, accepted for publication in Ap.
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