arXiv subjects

N. Kylafis

Publications and source records attributed to N. Kylafis.

23 records · Page 2Linked to original sources

arXiv2014

The RoboPol optical polarization survey of gamma-ray - loud blazars

We present first results from RoboPol, a novel-design optical polarimeter operating at the Skinakas Observatory in Crete. The data, taken during the May - June 2013 commissioning of the instrument, constitute a single-epoch linear polarization survey of a sample of gamma-ray - loud blazars, defined according to unbiased and objective selection criteria, easily reproducible in simulations, as well as a comparison sample of, otherwise similar, gamma-ray - quiet blazars. As such, the results of this survey are appropriate for both phenomenological population studies and for tests of theoretical population models. We have measured polarization fractions as low as $0.015$ down to $R$ magnitude of 17 and as low as $0.035$ down to 18 magnitude. The hypothesis that the polarization fractions of gamma-ray - loud and gamma-ray - quiet blazars are drawn from the same distribution is rejected at the $10^{-3}$ level. We therefore conclude that gamma-ray - loud and gamma-ray - quiet sources have different optical polarization properties. This is the first time this statistical difference is demonstrated in optical wavelengths. The polarization fraction distributions of both samples are well-described by exponential distributions with averages of $\langle p \rangle =6.4 ^{+0.9}_{-0.8}\times 10^{-2}$ for gamma-ray--loud blazars, and $\langle p \rangle =3.2 ^{+2.0}_{-1.1}\times 10^{-2}$ for gamma-ray--quiet blazars. The most probable value for the difference of the means is $3.4^{+1.5}_{-2.0}\times 10^{-2}$. The distribution of polarization angles is statistically consistent with being uniform.

astro-ph.HE↗

arXiv2014

The RoboPol Pipeline and Control System

We describe the data reduction pipeline and control system for the RoboPol project. The RoboPol project is monitoring the optical $R$-band magnitude and linear polarization of a large sample of active galactic nuclei that is dominated by blazars. The pipeline calibrates and reduces each exposure frame, producing a measurement of the magnitude and linear polarization of every source in the $13'\times 13'$ field of view. The control system combines a dynamic scheduler, real-time data reduction, and telescope automation to allow high-efficiency unassisted observations.

astro-ph.IM↗

arXiv2011

GRIPS - Gamma-Ray Imaging, Polarimetry and Spectroscopy

We propose to perform a continuously scanning all-sky survey from 200 keV to 80 MeV achieving a sensitivity which is better by a factor of 40 or more compared to the previous missions in this energy range. The Gamma-Ray Imaging, Polarimetry and Spectroscopy (GRIPS) mission addresses fundamental questions in ESA's Cosmic Vision plan. Among the major themes of the strategic plan, GRIPS has its focus on the evolving, violent Universe, exploring a unique energy window. We propose to investigate $γ$-ray bursts and blazars, the mechanisms behind supernova explosions, nucleosynthesis and spallation, the enigmatic origin of positrons in our Galaxy, and the nature of radiation processes and particle acceleration in extreme cosmic sources including pulsars and magnetars. The natural energy scale for these non-thermal processes is of the order of MeV. Although they can be partially and indirectly studied using other methods, only the proposed GRIPS measurements will provide direct access to their primary photons. GRIPS will be a driver for the study of transient sources in the era of neutrino and gravitational wave observatories such as IceCUBE and LISA, establishing a new type of diagnostics in relativistic and nuclear astrophysics. This will support extrapolations to investigate star formation, galaxy evolution, and black hole formation at high redshifts.

astro-ph.HE↗

arXiv2000

Phase lag variability associated with the 0.5-10 Hz QPO in GRS 1915+105

We have used Rossi X-ray Timing Explorer data to measure the lags between soft (2-5 keV) and hard (5-13 keV) photons and to study the aperiodic variability of the superluminal black hole candidate GRS 1915+105 during low-flux states. The power density spectra exhibit quasi-periodic oscillations (QPO) whose frequency increases with increasing count rate and varies in the frequency range 0.6-8 Hz. A correlation between the QPO frequency and the phase lag spectra is reported for the first time. This correlation is found for both the phase lag continuum and the phase lag at the QPO frequency. We find that as the QPO frequency moves to higher values the phase lags reverse sign from positive to negative. The absolute value of the lag always increases with photon energy. The negative (soft) lags are associated with a softer energy spectrum, whereas the positive (hard) lags are seen when the source is harder. We describe a possible scenario that may account for the change in the sign of the lags.

astro-ph↗

arXiv2000

Dust properties of external galaxies; NGC 891 revisited

We compare 850um SCUBA images of NGC 891 with the corresponding V-band optical depth predicted from radiation transfer simulations. These two tracers of dust show a very similar distribution along the minor axis and a reasonable agreement along the major axis. Assuming that the grains responsible for optical extinction are also the source of 850um emission we derive a submillimeter emissivity (emission efficiency) for dust in the NGC 891 disk. This quantity is found to be a factor of 2-3 higher than the generally-accepted (but highly uncertain) values adopted for the Milky Way. It should be stated, however, that if a substantial fraction of dust in NGC 891 is clumped, the emissivity in the two galaxies may be quite similar. We use our newly-acquired emissivity to convert our 850um images into detailed maps of dust mass and, utilizing 21cm and CO-emission data for NGC 891, derive the gas-to-dust ratio along the disk. We compute an average ratio of 260 -- a value consistent with the Milky Way and external spirals within the uncertainties in deriving both the dust mass and the quantity of molecular gas. The bulk of dust in NGC 891 appears to be closely associated with the molecular gas phase although it may start to follow the distribution of atomic hydrogen at radii >9 kpc (i.e. >0.5 R_25). Using the optical depth of the NGC 891 disk, we quantify how light emitted at high redshift is attenuated by dust residing in foreground spirals. For B-band observations of galaxies typically found in the Hubble Deep Field, the amount of light lost is expected to be small (~ 5%). This value depends critically on the maximum radial extent of cold dust in spiral disks (which is poorly known). It may also represent a lower limit if galaxies expel dust over time into the intergalactic medium.

astro-ph↗