689 research outputs found
Multi-wavelength Observations of the Giant X-ray Flare Galaxy NGC 5905: signatures of tidal disruption
NGC 5905 is one of the few galaxies with no prior evidence for an AGN in
which an X-ray flare, due to the tidal disruption of a star by the massive
black hole in the center of the galaxy, was detected by the RASS in 1990-91.
Here we present analysis of late-time follow-up observations of NGC 5905 using
Chandra, Spitzer VLA 3 GHz and 8 GHz archival data and GMRT 1.28 GHz radio
observations. The X-ray image shows no compact source that could be associated
with an AGN. Instead, the emission is extended -- likely due to nuclear star
formation and the total measured X-ray luminosity is comparable to the X-ray
luminosity determined from the 2002 Chandra observations. Diffuse X-ray
emission was detected close to the circum-nuclear star forming ring. The
Spitzer 2006 mid-infrared spectrum also shows strong evidence of nuclear star
formation but no clear AGN signatures. The semi-analytical models of Tommasin
et. al. 2010 together with the measured [OIV]/[NeII] line ratio suggest that at
most only 5.6% of the total IR Flux at 19 m is being contributed by the
AGN. The GMRT 1.28 GHz observations reveal a nuclear source. In the much higher
resolution VLA 3 GHz map, the emission has a double lobed structure of size
2.7'' due to the circumnuclear star forming ring. The GMRT 1.28 GHz peak
emission coincides with the center of the circumnuclear ring. We did not detect
any emission in the VLA 8 GHz (1996) archival data. The 3 upper limits
for the radio afterglow of the TDE at 1.28 GHz, 3 GHz and 8 GHz are 0.17 mJy,
0.09 mJy and 0.09 mJy, respectively. Our studies thus show that (i) NGC 5905
has a declining X-ray flux consistent with a TDE, (ii) the IR flux is dominated
by nuclear star formation, (iii) the nuclear radio emission observed from the
galaxy is due to circumnuclear star formation, (iv) no compact radio emission
associated with a radio afterglow from the TDE is detected.Comment: 12 pages, 8 figures, accepted to be published in Astrophysics and
Space Scienc
Silicon saw-tooth refractive lens for high-energy X-rays made using a diamond saw
A Si saw-tooth refractive lens, fabricated by a dicing process, is demonstrated to focus a 115 keV X-ray beam
Low-frequency radio observations of Seyfert galaxies: A test to the unification scheme
Aims. We present low-frequency radio imaging and spectral properties of a
well defined sample of Seyfert galaxies using GMRT 240/610 MHz dual frequency
observations. Radio spectra of Seyfert galaxies over 240 MHz to 5.0 GHz are
investigated using 240 MHz, 610 MHz flux densities derived from GMRT, and 1.4
GHz and 5.0 GHz flux densities mainly from published VLA data. We test the
predictions of Seyfert unification scheme by comparing the radio properties of
Seyfert type 1s and type 2s. Methods. We choose a sample such that the two
Seyfert subtypes have matched distributions in parameters that are independent
to the orientation of AGN, obscuring torus and the host galaxy. Our sample
selection criteria allow us to assume that the two Seyfert subtypes are
intrinsically similar within the framework of the unification scheme. Results.
The new observations at 240/610 MHz, together with archival observations at 1.4
GHz, 5.0 GHz show that type 1s and type 2s have statistically similar radio
luminosity distributions at 240 MHz, 610 MHz, 1.4 GHz and 5.0 GHz. The spectral
indices at selected frequency intervals as well as index measured over 240 MHz
to 5.0 GHz for the two Seyfert subtypes have similar distributions with median
spectral index -0.7, consistent with the synchrotron emission from
optically thin plasma. In our snap-shot 240/610 MHz GMRT observations, most of
the Seyfert galaxies show primarily an unresolved central radio component,
except a few sources in which faint kpc-scale extended emission is apparent at
610 MHz. Our results on the statistical comparison of the multifrequency radio
properties of our sample Seyfert galaxies are in agreement with the predictions
of the Seyfert unification scheme.Comment: 21 pages, 5 figures, Accepted for publication in Astronomy &
Astrophysic
Communication With The SAS 58 Auditor’s Standard Report: An Exploratory Study
In a field experiment, the authors examine whether three alternative versions of the auditor’s standard report communicate effectively the cognitive dimensions of understand-ability, engagement risk, and needed accommodation to a user group (investment and banking professionals) and an expert group (audit partners and managers). The study focuses on the mandated SAS 58 (AICPA 1988) three-paragraph auditor’s standard report (SR), the previously mandated two-paragraph auditor’s standard report (OSR) and a modified auditor’s standard report (MSR) more in harmony with the stated auditor’s responsibility for detecting fraud, as mandated by SAS 53 (AICPA 1988b) which was subsequently superceded by SAS 82 and SAS 99. The results indicate that both auditors and users are consistent in their belief that the SR represents an enhancement in understandability of the audit message over OSR, and that a format along the lines of MSR would not have represented an improvement over the SR format given the inconsistencies in ratings between auditors and users of the MSR (which contains explicit language relative to fraud). Specifically, auditors’ perception of engagement risk associated with the MSR is much higher than users’ perception and the demand for needed accommodation (additional information) is also greater for auditors than users. Overall, the results suggest that the ASB was effective in responding to the user needs with respect to the message communicated in the auditor’s report, a critical link in the financial information reporting process. This investigation has the potential to inform policy-making bodies concerned with adopting a report standard that fairly communicates the risks borne by both auditor and user groups
Time dependence of Bragg forward scattering and self-seeding of hard x-ray free-electron lasers
Free-electron lasers (FELs) can now generate temporally short, high power
x-ray pulses of unprecedented brightness, even though their longitudinal
coherence is relatively poor. The longitudinal coherence can be potentially
improved by employing narrow bandwidth x-ray crystal optics, in which case one
must also understand how the crystal affects the field profile in time and
space. We frame the dynamical theory of x-ray diffraction as a set of coupled
waves in order to derive analytic expressions for the spatiotemporal response
of Bragg scattering from temporally short incident pulses. We compute the
profiles of both the reflected and forward scattered x-ray pulses, showing that
the time delay of the wave is linked to its transverse spatial shift
through the simple relationship , where
is the grazing angle of incidence to the diffracting planes. Finally,
we apply our findings to obtain an analytic description of Bragg forward
scattering relevant to monochromatically seed hard x-ray FELs.Comment: 11 pages, 6 figure
Quantum error-correcting codes and 4-dimensional arithmetic hyperbolic manifolds
Using 4-dimensional arithmetic hyperbolic manifolds, we construct some new
homological quantum error correcting codes. They are LDPC codes with linear
rate and distance . Their rate is evaluated via Euler
characteristic arguments and their distance using -systolic
geometry. This construction answers a queston of Z\'emor, who asked whether
homological codes with such parameters could exist at all.Comment: 21 page
Evaluation of a low-cost inkjet printed slot antenna for energy harvesting applications
A circularly polarized patch antenna fabricated using commercially available, low-cost, printers is described. Two additive processes are combined for the fabrication: stereolithography (SLA) and inkjet printing of silver inks. A widely available SLA 3D printer is employed to fabricate the substrate of the antenna. Inkjet printing is used to deposit the metallic layers of the radiating element on the substrate. The two machines employed are very low-cost in comparison to those used in previously reported work. Good adhesion of the metallic patterns to the substrates is observed. Furthermore, the resistance of the added metallic patch is relatively low. The aim is to demonstrate the use of alternative, inexpensive machines, for the prototyping and manufacturing of antennas on 3D printed substrates. In this work, the antenna operates at the 1.575GHz GPS frequency band. Finite-difference-time-domain simulations compare well with the practical experiments
The WiFeS S7 AGN survey: Current status and recent results on NGC 6300
The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) is a
targeted survey probing the narrow-line regions (NLRs) of a representative
sample of ~140 nearby (z<0.02) Seyfert galaxies by means of optical integral
field spectroscopy. The survey is based on a homogeneous data set observed
using the Wide Field Spectrograph WiFeS. The data provide a 25x38 arcsec
field-of-view around the galaxy centre at typically ~1.5 arcsec spatial
resolution and cover a wavelength range between ~3400 - 7100 at spectral
resolutions of ~100 km s and ~50 km s in the blue and red parts,
respectively. The survey is primarily designed to study gas excitation and star
formation around AGN, with a special focus on the shape of the AGN ionising
continuum, the interaction between radio jets and the NLR gas, and the nature
of nuclear LINER emission. We provide an overview of the current status of
S7-based results and present new results for NGC 6300.Comment: 5 pages, 1 figure, Refereed Proceeding of the "The Universe of
Digital Sky Surveys" conference held at the INAF - Observatory of
Capodimonte, Naples, on 25th-28th november 2014, to be published on
Astrophysics and Space Science Proceedings, edited by Longo, Napolitano,
Marconi, Paolillo, Iodic
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