13,635 research outputs found
New VLT observations of the Fermi pulsar PSR J1048-5832
PSR J1048-5832 is a Vela-like (P=123.6 ms; tau~20.3 kyr) gamma-ray pulsar
detected by Fermi, at a distance of ~2.7 kpc and with a rotational energy loss
rate dot{E}_{SD} ~2 x 10^{36} erg/s. The PSR J1048-5832 field has been observed
with the VLT in the V and R bands. We used these data to determine the colour
of the object detected closest to the Chandra position (Star D) and confirm
that it is not associated with the pulsar. For the estimated extinction along
the line of sight, inferred from a re-analysis of the Chandra and XMM-Newton
spectra, the fluxes of Star D (V~26.7; R~25.8) imply a -0.13 < (V-R)_0 < 0.6.
This means that the PSR J1048-5832 spectrum would be unusually red compared to
the Vela pulsar.Moreover, the ratio between the unabsorbed optical and X-ray
flux of PSR J1048-5832 would be much higher than for other young pulsars. Thus,
we conclude that Star D is not the PSR J1048-5832 counterpart. We compared the
derived R and V-band upper limits (R>26.4; V>27.6) with the extrapolation of
the X and gamma-ray spectra and constrained the pulsar spectrum at
low-energies. In particular, the VLT upper limits suggest that the pulsar
spectrum could be consistent with a single power-law, stretching from the
gamma-rays to the optical.Comment: 5 pages, 2 figures, accepted for publication on Monthly Notices of
the Royal Astronomical Society Main Journa
Unifying Einstein and Palatini gravities
We consider a novel class of gravity theories where the connection is
related to the conformally scaled metric with
a scaling that depends on the scalar curvature only. We call them
C-theories and show that the Einstein and Palatini gravities can be obtained as
special limits. In addition, C-theories include completely new physically
distinct gravity theories even when . With nonlinear ,
C-theories interpolate and extrapolate the Einstein and Palatini cases and may
avoid some of their conceptual and observational problems. We further show that
C-theories have a scalar-tensor formulation, which in some special cases
reduces to simple Brans-Dicke-type gravity. If matter fields couple to the
connection, the conservation laws in C-theories are modified. The stability of
perturbations about flat space is determined by a simple condition on the
lagrangian.Comment: 17 pages, no figure
Pre-selection in cointegration-based pairs trading
This paper compares the final profitability of a cointegration-based pairs trading strategy
when pairs of stocks are pre-selected using seven different measures. Pre-selection matters,
since the excess returns remarkably vary, in terms of both average and variability, depending
on the metrics used. Differences in profitability by pre-selection metrics are retrieved even
after considering commissions and cut rules, market impact, and a stricter definition of the
Spread reversion to the equilibrium. Besides, the profitability of the pairs trading strategy is
also found heterogeneous across the different pre-selection metrics considered in terms of
exposure to the traditional risk-factors
Sensitivity of profitability in cointegration-based pairs trading
The cointegrated-based pair trading crucially depends on two key parameters: the length of the formation period and the divergence signal (or opening trigger), which are generally arbitrarily or statistically determined in the literature. In this article, we perform a sensitivity analysis ofthe pairs trading profitability to its parametrization, employing the daily closing prices of the S&P 500 constituent stocks.
We found that that not only the measures of performance (i.e. average excess returns, Sharpe ratios and percentage of positive excess returns), but also strategy characteristics and trades features (i.e. average trades’ duration and number of trades) are highly sensitive to the choice of the parameters
Pre‑selection in cointegration‑based pairs trading
The paper compares the final profitability of a cointegration-based pairs trading
strategy when pairs of stocks are pre-selected by means of seven different measures.
Some of the measures considered have been extensively used in the pairs trading
literature, while others represent a novelty in this type of application. We find
that pre-selection matters, since the excess returns remarkably vary, in terms of both
average and variability, depending on the metrics used. Differences in profitability
by pre-selection metrics are retrieved even after considering commissions and cut
rules, market impact, a stricter definition of the Spread reversion to the equilibrium
and alternative cointegration tests. Besides, the pairs trading profitability is found to
be heterogeneous across the different pre-selection metrics also in terms of exposure
to the systematic stock-market risk factors
Radio-quiet and radio-loud pulsars: similar in Gamma-rays but different in X-rays
We present new Chandra and XMM-Newton observations of a sample of eight
radio-quiet Gamma-ray pulsars detected by the Fermi Large Area Telescope. For
all eight pulsars we identify the X-ray counterpart, based on the X-ray source
localization and the best position obtained from Gamma-ray pulsar timing. For
PSR J2030+4415 we found evidence for an about 10 arcsec-long pulsar wind
nebula. Our new results consolidate the work from Marelli et al. 2011 and
confirm that, on average, the Gamma-ray--to--X-ray flux ratios (Fgamma/Fx) of
radio-quiet pulsars are higher than for the radio-loud ones. Furthermore, while
the Fgamma/Fx distribution features a single peak for the radio-quiet pulsars,
the distribution is more dispersed for the radio-loud ones, possibly showing
two peaks. We discuss possible implications of these different distributions
based on current models for pulsar X-ray emission.Comment: Accepted for publication in The Astrophysical Journal; 12 pages, 3
figures, 2 table
An overview of the EXTraS project: Exploring the X-ray Transient and Variable Sky
The EXTraS project (Exploring the X-ray Transient and variable Sky) will
harvest the hitherto unexplored temporal domain information buried in the
serendipitous data collected by the European Photon Imaging Camera (EPIC)
instrument onboard the ESA XMM-Newton X-ray observatory since its launch. This
will include a search for fast transients, as well as a search and
characterization of variability (both periodic and aperiodic) in hundreds of
thousands of sources spanning more than nine orders of magnitude in time scale
and six orders of magnitude in flux. X-ray results will be complemented by
multiwavelength characterization of new discoveries. Phenomenological
classification of variable sources will also be performed. All our results will
be made available to the community. A didactic program in selected High Schools
in Italy, Germany and the UK will also be implemented. The EXTraS project
(2014-2016), funded within the EU/FP7 framework, is carried out by a
collaboration including INAF (Italy), IUSS (Italy), CNR/IMATI (Italy),
University of Leicester (UK), MPE (Germany) and ECAP (Germany).Comment: 6 pages, 1 figure. Proceedings of "Swift: 10 years of Discovery", to
appear in Po
Observations of three young gamma-ray pulsars with the Gran Telescopio Canarias
We report the analysis of the first deep optical observations of three
isolated -ray pulsars detected by the {\em Fermi Gamma-ray Space
Telescope}: the radio-loud PSR\, J0248+6021 and PSR\, J0631+1036, and the
radio-quiet PSR\, J0633+0632. The latter has also been detected in the X rays.
The pulsars are very similar in their spin-down age (40--60 kyrs),
spin-down energy ( erg s), and dipolar surface
magnetic field (-- G). These pulsars are promising
targets for multi-wavelength observations, since they have been already
detected in rays and in radio or X-rays. None of them has been
detected yet in the optical band. We observed the three pulsar fields in 2014
with the Spanish 10.4m Gran Telescopio Canarias (GTC). We could not find any
candidate optical counterpart to the three pulsars close to their most recent
radio or {\em Chandra} positions down to limits of ,
, for PSR\, J0248+6021, J0631+1036, and J0633+0632,
respectively. From the inferred optical upper limits and estimated distance and
interstellar extinction, we derived limits on the pulsar optical luminosity. We
also searched for the X-ray counterpart to PSR\, J0248+6021 with \chan\ but we
did not detect the pulsar down to a 3 flux limit of
erg cm s (0.3--10 keV). For all these pulsars, we compared the
optical flux upper limits with the extrapolations in the optical domain of the
-ray spectra and compared their multi-wavelength properties with those
of other -ray pulsars of comparable age.Comment: 12 pages, 5 figures, accepted for publication in MNRA
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