439 research outputs found
Realization of the farad from the dc quantum Hall effect with digitally-assisted impedance bridges
A new traceability chain for the derivation of the farad from dc quantum Hall
effect has been implemented at INRIM. Main components of the chain are two new
coaxial transformer bridges: a resistance ratio bridge, and a quadrature
bridge, both operating at 1541 Hz. The bridges are energized and controlled
with a polyphase direct-digital-synthesizer, which permits to achieve both main
and auxiliary equilibria in an automated way; the bridges and do not include
any variable inductive divider or variable impedance box. The relative
uncertainty in the realization of the farad, at the level of 1000 pF, is
estimated to be 64E-9. A first verification of the realization is given by a
comparison with the maintained national capacitance standard, where an
agreement between measurements within their relative combined uncertainty of
420E-9 is obtained.Comment: 15 pages, 11 figures, 3 table
Discovery of the Onset of Rapid Accretion by a Dormant Massive Black Hole
Massive black holes are believed to reside at the centres of most galaxies.
They can be- come detectable by accretion of matter, either continuously from a
large gas reservoir or impulsively from the tidal disruption of a passing star,
and conversion of the gravitational energy of the infalling matter to light.
Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be
variable but have never been observed to turn on or off. Tidal disruption of
stars by dormant massive black holes has been inferred indirectly but the on-
set of a tidal disruption event has never been observed. Here we report the
first discovery of the onset of a relativistic accretion-powered jet in the new
extragalactic transient, Swift J164449.3+573451. The behaviour of this new
source differs from both theoretical models of tidal disruption events and
observations of the jet-dominated AGN known as blazars. These differences may
stem from transient effects associated with the onset of a powerful jet. Such
an event in the massive black hole at the centre of our Milky Way galaxy could
strongly ionize the upper atmosphere of the Earth, if beamed towards us.Comment: Submitted to Nature. 4 pages, 3 figures (main paper). 26 pages, 13
figures (supplementary information
VLT/X-shooter spectroscopy of the afterglow of the Swift GRB 130606A: Chemical abundances and reionisation at
The reionisation of the Universe is thought to have ended around z~6, as
inferred from spectroscopy of distant bright background sources, such as
quasars (QSO) and gamma-ray burst (GRB) afterglows. Furthermore, spectroscopy
of a GRB afterglow provides insight in its host galaxy, which is often too dim
and distant to study otherwise. We present the high S/N VLT/X-shooter spectrum
of GRB130606A at z=5.913. We aim to measure the degree of ionisation of the IGM
between 5.02<z<5.84 and to study the chemical abundance pattern and dust
content of its host galaxy. We measured the flux decrement due to absorption at
Ly, and wavelength regions. The hydrogen and metal
absorption lines formed in the host galaxy were fitted with Voigt profiles to
obtain column densities. Our measurements of the Ly-forest optical
depth are consistent with previous measurements of QSOs, but have a much
smaller uncertainty. The analysis of the red damping wing yields a neutral
fraction (3). We obtain column density measurements of
several elements. The ionisation corrections due to the GRB is estimated to be
negligible (<0.03 dex), but larger corrections may apply due to the
pre-existing radiation field (up to 0.4 dex based on sub-DLA studies). Our
measurements confirm that the Universe is already predominantly ionised over
the redshift range probed in this work, but was slightly more neutral at z>5.6.
GRBs are useful probes of the ionisation state of the IGM in the early
Universe, but because of internal scatter we need a larger statistical sample
to draw robust conclusions. The high [Si/Fe] in the host can be due to dust
depletion, alpha-element enhancement, or a combination of both. The very high
value of [Al/Fe]=2.40+/-0.78 might connected to the stellar population history.
We estimate the host metallicity to be -1.7<[M/H]<-0.9 (2%-13% of solar).
(trunc.)Comment: 15 pages, 12 figure
The mysterious optical afterglow spectrum of GRB140506A at z=0.889
Context. Gamma-ray burst (GRBs) afterglows probe sightlines to star-forming
regions in distant star-forming galaxies. Here we present a study of the
peculiar afterglow spectrum of the z = 0.889 Swift GRB 140506A. Aims. Our aim
is to understand the origin of the very unusual properties of the absorption
along the line-of-sight. Methods. We analyse spectroscopic observations
obtained with the X-shooter spectrograph mounted on the ESO/VLT at two epochs
8.8 h and 33 h after the burst as well as imaging from the GROND instrument. We
also present imaging and spectroscopy of the host galaxy obtained with the
Magellan telescope. Results. The underlying afterglow appears to be a typical
afterglow of a long-duration GRB. However, the material along the line-of-
sight has imprinted very unusual features on the spectrum. Firstly, there is a
very broad and strong flux drop below 8000 AA (4000 AA in the rest frame),
which seems to be variable between the two spectroscopic epochs. We can
reproduce the flux-drops both as a giant 2175 AA extinction bump and as an
effect of multiple scattering on dust grains in a dense environment. Secondly,
we detect absorption lines from excited H i and He i. We also detect molecular
absorption from CH+ . Conclusions. We interpret the unusual properties of these
spectra as reflecting the presence of three distinct regions along the
line-of-sight: the excited He i absorption originates from an H ii-region,
whereas the Balmer absorption must originate from an associated
photodissociation region. The strong metal line and molecular absorption and
the dust extinction must originate from a third, cooler region along the
line-of-sight. The presence of (at least) three separate regions is reflected
in the fact that the different absorption components have different velocities
relative to the systemic redshift of the host galaxy.Comment: 8 pages, 4 figures. Accepted for publications in A&
Hybrid Stars in a Strong Magnetic Field
We study the effects of high magnetic fields on the particle population and
equation of state of hybrid stars using an extended hadronic and quark SU(3)
non-linear realization of the sigma model. In this model the degrees of freedom
change naturally from hadrons to quarks as the density and/or temperature
increases. The effects of high magnetic fields and anomalous magnetic moment
are visible in the macroscopic properties of the star, such as mass, adiabatic
index, moment of inertia, and cooling curves. Moreover, at the same time that
the magnetic fields become high enough to modify those properties, they make
the star anisotropic.Comment: Revised version with updated reference
Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers
On 2020 February 24, during their third observing run ("O3"), the Laser
Interferometer Gravitational-wave Observatory and Virgo Collaboration (LVC)
detected S200224ca: a candidate gravitational wave (GW) event produced by a
binary black hole (BBH) merger. This event was one of the best-localized
compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72
deg), and so the Neil Gehrels Swift Observatory performed rapid
near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately
79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH
event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No
likely EM counterparts to the GW event were found by the Swift BAT, XRT, or
UVOT, nor by other observatories. Here we report on the results of our searches
for an EM counterpart, both in the BAT data near the time of the merger, and in
follow-up UVOT/XRT observations. We also discuss the upper limits we can place
on EM radiation from S200224ca, and the implications these limits have on the
physics of BBH mergers. Namely, we place a shallow upper limit on the
dimensionless BH charge, , and an upper limit on
the isotropic-equivalent energy of a blast wave erg
(assuming typical GRB parameters).Comment: 14 pages, 6 figures, accepted for publication in Ap
The Spectral Energy Distribution of Fermi bright blazars
(Abridged) We have conducted a detailed investigation of the broad-band
spectral properties of the \gamma-ray selected blazars of the Fermi LAT Bright
AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray
spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray
data, collected within three months of the LBAS data taking period, we were
able to assemble high-quality and quasi-simultaneous Spectral Energy
Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is
similar to that of blazars discovered at other wavelengths, clearly showing, in
the usual Log - Log F representation, the typical broad-band
spectral signatures normally attributed to a combination of low-energy
synchrotron radiation followed by inverse Compton emission of one or more
components. We have used these SEDs to characterize the peak intensity of both
the low and the high-energy components. The results have been used to derive
empirical relationships that estimate the position of the two peaks from the
broad-band colors (i.e. the radio to optical and optical to X-ray spectral
slopes) and from the gamma-ray spectral index. Our data show that the
synchrotron peak frequency is positioned between 10 and
10 Hz in broad-lined FSRQs and between and Hz in
featureless BL Lacertae objects.We find that the gamma-ray spectral slope is
strongly correlated with the synchrotron peak energy and with the X-ray
spectral index, as expected at first order in synchrotron - inverse Compton
scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton
(SSC) models cannot explain most of our SEDs, especially in the case of FSRQs
and low energy peaked (LBL) BL Lacs. (...)Comment: 85 pages, 38 figures, submitted to Ap
Looking Into the Fireball: ROTSE-III and Swift Observations of Early GRB Afterglows
We report on a complete set of early optical afterglows of gamma-ray bursts
(GRBs) obtained with the ROTSE-III telescope network from March 2005 through
June 2007. This set is comprised of 12 afterglows with early optical and
Swift/XRT observations, with a median ROTSE-III response time of 45 s after the
start of gamma-ray emission (8 s after the GCN notice time). These afterglows
span four orders of magnitude in optical luminosity, and the contemporaneous
X-ray detections allow multi-wavelength spectral analysis. Excluding X-ray
flares, the broadband synchrotron spectra show that the optical and X-ray
emission originate in a common region, consistent with predictions of the
external forward shock in the fireball model. However, the fireball model is
inadequate to predict the temporal decay indices of the early afterglows, even
after accounting for possible long-duration continuous energy injection. We
find that the optical afterglow is a clean tracer of the forward shock, and we
use the peak time of the forward shock to estimate the initial bulk Lorentz
factor of the GRB outflow, and find 100<Gamma_0<1000, consistent with
expectations.Comment: 31 pages, 5 figures, submitted to Ap
Comparing emission- and absorption-based gas-phase metallicities in GRB host galaxies at using JWST
JWST/NIRSpec is providing sensitive spectroscopic observations of distant
galaxies, extending our view of cosmic chemical evolution out to the epoch of
reionization and down to galaxy masses 1-2 dex lower than previously possible
at . These observations nevertheless remain heavily dominated by light
from luminous star forming regions. An alternative and sensitive probe of the
metallicity of galaxies is through absorption lines imprinted on the luminous
afterglow spectra of long gamma ray burst (GRBs) from intervening material
within their host galaxy. However, these two independent but complementary
probes need to be cross-calibrated before they can be combined. We present the
first results from a cycle-1 JWST program to investigate the relation between
the metallicity of the neutral gas measured with GRB afterglow absorption lines
to the emission line metallicity of the star forming regions of the GRB host
measured with NIRSpec. Using an initial sample of seven GRB host galaxies at
, we find a tight relation between absorption and emission line
metallicities when using the recent Laseter et al. (2023) metallicity
diagnostic, implying a relatively chemically-homogeneous multi-phase
interstellar medium, and indicating that absorption and emission line probes
can be directly combined to investigate the chemical enrichment of galaxies.
However, the relation is less clear when using other diagnostics, such as
and . Ultimate confirmation of the relation between absorption
and emission line metallicities will require a more direct determination of the
emission line metallicity via the detection of temperature-sensitive auroral
lines in our GRB host galaxy sample.Comment: For submission to MNRAS; comments welcome. 21 pages, 13 figure
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