54 research outputs found
Constraining models of twin peak quasi-periodic oscillations with realistic neutron star equations of state
Twin-peak quasi-periodic oscillations (QPOs) are observed in the X-ray
power-density spectra of several accreting low-mass neutron star (NS) binaries.
In our previous work we have considered several QPO models. We have identified
and explored mass-angular-momentum relations implied by individual QPO models
for the atoll source 4U 1636-53. In this paper we extend our study and confront
QPO models with various NS equations of state (EoS). We start with simplified
calculations assuming Kerr background geometry and then present results of
detailed calculations considering the influence of NS quadrupole moment
(related to rotationally induced NS oblateness) assuming Hartle-Thorne
spacetimes. We show that the application of concrete EoS together with a
particular QPO model yields a specific mass-angular-momentum relation. However,
we demonstrate that the degeneracy in mass and angular momentum can be removed
when the NS spin frequency inferred from the X-ray burst observations is
considered. We inspect a large set of EoS and discuss their compatibility with
the considered QPO models. We conclude that when the NS spin frequency in 4U
1636-53 is close to 580Hz we can exclude 51 from 90 of the considered
combinations of EoS and QPO models. We also discuss additional restrictions
that may exclude even more combinations. Namely, there are 13 EOS compatible
with the observed twin peak QPOs and the relativistic precession model.
However, when considering the low frequency QPOs and Lense-Thirring precession,
only 5 EOS are compatible with the model.Comment: 12 pages, 9 figures, 3 tables, accepted for publication in The
Astrophysical Journa
FRAM telescopes and their measurements of aerosol content at the Pierre Auger Observatory and at future sites of the Cherenkov Telescope Array
A FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope is a system of
a robotic mount, a large-format CCD camera and a fast telephoto lens that can
be used for atmospheric monitoring at any site when information about the
atmospheric transparency is required with high spatial or temporal resolution
and where continuous use of laser-based methods for this purpose would
interfere with other observations. The original FRAM has been operated at the
Pierre Auger Observatory in Argentina for more than a decade, while three more
FRAMs are foreseen to be used by the Cherenkov Telescope Array (CTA). The CTA
FRAMs are being deployed ahead of time to characterize the properties of the
sites prior to the operation of the CTA telescopes; one FRAM has been running
on the planned future CTA site in Chile for a year while two others are
expected to become operational before the end of 2018. We report on the
hardware and current status of operation and/or deployment of all the FRAM
instruments in question as well as on some of the preliminary results of
integral aerosol measurements by the FRAMs in Argentina and ChileComment: Proceedings of AtmoHEAD 201
New developments in aerosol measurements using stellar photometry
The idea of using stellar photometry for atmospheric monitoring for optical experiments in highenergy astrophysics is seemingly straightforward, but reaching high precision of the order of 0.01 in the determination of the vertical aerosol optical depth (VAOD) has proven difficult. Wide-field photometry over a large span of altitudes allows a fast determination of VAOD independently of the absolute calibration of the system, while providing this calibration as a useful by-product. Using several years of data taken by the FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope at the Pierre Auger Observatory in Argentina and about a year of data taken by a similar instrument deployed at the planned future Southern site of the Cherenkov Telescope Array in Chile, we have developed methods to improve the precision of this measurement technique towards and possibly beyond the 0.01 mark. Detailed laboratory measurements of the response of the whole system to both the spectrum and intensity of incoming light have proven indispensable in this analysis as the usual assumption of linearity of the CCD detectors is not valid anymore for the conditions of the observations
A search for ultra-high-energy photons at the Pierre Auger Observatory exploiting air-shower universality
The Pierre Auger Observatory is the most sensitive detector to primary photons with energies above ∼0.2 EeV. It measures extensive air showers using a hybrid technique that combines a fluorescence detector (FD) with a ground array of particle detectors (SD). The signatures of a photon-induced air shower are a larger atmospheric depth at the shower maximum (X) and a steeper lateral distribution function, along with a lower number of muons with respect to the bulk of hadron-induced background. Using observables measured by the FD and SD, three photon searches in different energy bands are performed. In particular, between threshold energies of 1-10 EeV, a new analysis technique has been developed by combining the FD-based measurement of X with the SD signal through a parameter related to its muon content, derived from the universality of the air showers. This technique has led to a better photon/hadron separation and, consequently, to a higher search sensitivity, resulting in a tighter upper limit than before. The outcome of this new analysis is presented here, along with previous results in the energy ranges below 1 EeV and above 10 EeV. From the data collected by the Pierre Auger Observatory in about 15 years of operation, the most stringent constraints on the fraction of photons in the cosmic flux are set over almost three decades in energy
Study on multi-ELVES in the Pierre Auger Observatory
Since 2013, the four sites of the Fluorescence Detector (FD) of the Pierre Auger Observatory record ELVES with a dedicated trigger. These UV light emissions are correlated to distant lightning strikes. The length of recorded traces has been increased from 100 μs (2013), to 300 μs (2014-16), to 900 μs (2017-present), to progressively extend the observation of the light emission towards the vertical of the causative lightning and beyond. A large fraction of the observed events shows double ELVES within the time window, and, in some cases, even more complex structures are observed. The nature of the multi-ELVES is not completely understood but may be related to the different types of lightning in which they are originated. For example, it is known that Narrow Bipolar Events can produce double ELVES, and Energetic In-cloud Pulses, occurring between the main negative and upper positive charge layer of clouds, can induce double and even quadruple ELVES in the ionosphere. This report shows the seasonal and daily dependence of the time gap, amplitude ratio, and correlation between the pulse widths of the peaks in a sample of 1000+ multi-ELVES events recorded during the period 2014-20. The events have been compared with data from other satellite and ground-based sensing devices to study the correlation of their properties with lightning observables such as altitude and polarity
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