arXiv subjects

J. Sitarek

Publications and source records attributed to J. Sitarek.

31 records · Page 2Linked to original sources

arXiv2013

Gamma-rays from nebulae around binary systems containing energetic rotation powered pulsars

We consider nebulae which are created around binary systems containing rotation powered pulsars and companion stars with strong stellar winds. It is proposed that the stellar and pulsar winds have to mix at some distance from the binary system, defined by the orbital period of the companion stars and the velocity of the stellar wind. The mixed pulsar-stellar wind expands with a specific velocity determined by the pulsar power and the mass loss rate of the companion star. Relativistic particles, either from the inner pulsar magnetosphere and/or accelerated at the shocks between stellar and pulsar winds, are expected to be captured and isotropized in the reference frame of the mixed wind. Therefore, they can efficiently comptonize stellar radiation producing GeV-TeV $γ$-rays in the inverse Compton process. We calculate the $γ$-ray spectra expected in such scenario for the two example binary systems: J1816+4510 which is the redback type millisecond binary and LS 5039 which is supposed to contain energetic pulsar. It is concluded that the steady TeV $γ$-ray emission from J1816+4510 should be on the 100 hr sensitivity limit of the planned Cherenkov Telescope Array, provided that $ε\sim 10%$ of the rotational energy lost by the pulsar is transferred to TeV electrons. On the other hand, the comparison of the predicted steady TeV $γ$-ray emission, expected from $γ$-ray binary LS 5039, with the observations of the TeV emission in a low state, reported by the H.E.S.S. Collaboration, allows us to put stringent upper limit on the product of the part of the hemisphere in which the mixed pulsar-stellar wind is confined, $Δ_{\rm mix}$, and the energy conversion efficiency, $ε$, from the supposed pulsar to the TeV electrons injected in this system, $Δ_{\rm mix}\cdot ε<1%$.

astro-ph.HE↗

arXiv2012

High energy emission from the nebula around the Black Widow binary system containing millisecond pulsar B1957+20

The features of pulsed $γ$-ray emission from classical and millisecond pulsars indicate that the high energy radiation processes in their inner magnetospheres occur in a similar way. In the last decade several TeV $γ$-ray nebulae have been discovered around classical pulsars. The above facts suggest that $γ$-rays should be produced also in the surroundings of millisecond pulsars. We discuss a model for the bow shock nebula around the well known Black Widow binary system containing the millisecond pulsar B1957+20. This model predicts the existence of a synchrotron X-ray and inverse Compton $γ$-ray nebula around this system. We want to find out whether $γ$-ray emission from the nebula around B1957+20 could be detected by the future and present Cherenkov telescopes. Using the Monte Carlo method we followed the propagation of relativistic electrons in the vicinity of the pulsar. We calculated the very high energy radiation produced by them in the synchrotron process and the inverse Compton scattering of the Microwave Background Radiation and of the infrared radiation from the galactic disk. We also computed the X-ray emission produced by the electrons in the synchrotron process. We show that the hard X-ray tail emission observed from the vicinity of B1957+20 can be explained by our model. Moreover, we predict that the TeV $γ$-ray emission produced by the electrons in the inverse Compton process should be detectable by the future Cherenkov Telescope Array and possibly by the long term observations with the present Cherenkov arrays such as MAGIC and VERITAS. The $γ$-ray emission from B1957+20 is expected to be extended, inhomogeneous, and shifted from the present location of the binary system by a distance comparable to the radius of the nebula.

astro-ph.HE↗

arXiv2012

Application of a generalized likelihood ratio test statistic to MAGIC data

The commonly used detection test statistic for Cherenkov telescope data is Li & Ma (1983), Eq. 17. It evaluates the compatibility of event counts in an on-source region with those in a representative off-region. It does not exploit the typically known gamma-ray point spread function (PSF) of a system, and in practice its application requires either assumptions on the symmetry of the acceptance across the field of view, orMonte Carlo simulations.MAGIC has an azimuth-dependent, asymmetric acceptance which required a careful review of detection statistics. Besides an adapted Li & Ma based technique, the recently presented generalized LRT statistic of [1] is now in use. It is more flexible, more sensitive and less systematics-affected, because it is highly customized for multi-pointing Cherenkov telescope data with a known PSF. We present the application of this new method to archival MAGIC data and compare it to the other, Li&Ma-based method.

astro-ph.HE↗

arXiv2011

Performance of the MAGIC Stereo System

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes sensitive above ~60 GeV, and located on the Canary Island of La Palma at the height of 2200 m.a.s.l. Since Autumn 2009 both telescopes are working together in stereoscopic mode. We use both Crab Nebula observations and Monte Carlo simulations to evaluate the performance of the system. Advanced stereo analysis allows MAGIC to achieve a sensitivity better than 0.8% of the Crab Nebula flux in 50 h of observations in the medium energy range (around a few hundred GeV). At those energies the angular resolution is better than 0.07{\circ}, and the energy resolution is as good as 16%. We perform also a detailed study of possible systematics effects for the MAGIC telescopes.

astro-ph.HE↗

arXiv2011

Testing the emission models of blazar jets with the MAGIC telescopes

The MAGIC telescopes discovered very high energy (VHE, E>100 GeV) gamma-ray emission coming from the distant Flat Spectrum Radio Quasar (FSRQ) PKS 1222+21 (4C +21.35, z=0.432). It is the second most distant VHE gamma-ray source, with well measured redshift, detected until now. The observation was performed on 2010 June 17 (MJD 55364.9) using the two 17 m diameter imaging Cherenkov telescopes on La Palma (Canary Islands, Spain). The MAGIC detection coincides with high energy MeV/GeV gamma-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The averaged integral flux above 100 GeV is equivalent to 1 Crab Nebula flux. The VHE flux measured by MAGIC varies significantly within the 30 minutes of exposure implying a flux doubling time of about 10 minutes. The VHE and MeV/GeV spectra, corrected for the absorption by the extragalactic background light, can be described by a single power law with photon index 2.72+/-0.34 between 3 GeV and 400 GeV, consistent with gamma-ray emission belonging to a single component in the jet. The absence of a spectral cutoff at 30-60 GeV (indeed, one finds a strict lower limit Ec>130 GeV) constrains the gamma-ray emission region to lie outside the broad line region, which would otherwise absorb the VHE gamma-rays. Together with the detected fast variability, this challenges present emission models from jets in FSRQs.

astro-ph.HE↗

arXiv2010

Time dependent gamma-ray production in the anisotropic IC $e^\pm$ pair cascade initiated by electrons in active galaxies

New high energy emission features have been recently discovered by the Cherenkov telescopes from active galaxies e.g., a few minutes variability time scale of TeV emission from Mrk 501 and PKS 2155-304, sub-TeV $γ$-ray emission from GeV peaked blazar 3C 279, and TeV emission from two nearby active galaxies, M87 and Cen A, which jets are inclined at a relatively large angle to the line of sight. These results have put a new light on the high energy processes occurring in central parts of active galaxies stimulating more detailed studies of $γ$-ray emission models. Here we report the results of a detailed analysis concerning the most general version of the model for the $γ$-ray production by leptons injected in the jet which interact with the thermal radiation from an accretion disk (the so called {\it external inverse Compton model}). We investigate the $γ$-ray spectra produced in an anisotropic Inverse Compton (IC) $e^\pm$ pair cascade in the whole volume above the accretion disk. The cascade $γ$-ray spectra are obtained for different locations of the observer in respect to the direction of the jet. We also study the time evolution of this $γ$-ray emission caused by the propagation of the relativistic leptons along the jet and the delays resulting from different places of the origin of $γ$-rays above the accretion disk. We discuss the main features of such a cascade model assuming constant injection rate of electrons along the jet. We are investigating two models for their different maximum energies: constant value independent on the distance along the jet or limited by the synchrotron energy losses considered locally in the jet. The model is discussed in the context of blazars observed at small and large inclination angles taking as an example the parameters of the two famous sources Cen A and 3C 279.

astro-ph.HE↗

arXiv2009

Gamma-rays from the IC $e^\pm$ pair cascade in the radiation field of an accretion disk: Application to CenA

The very short time scale variability of TeV $γ$-ray emission from active galaxies suggests that the acceleration process of particles and the production of primary $γ$-rays likely occurs relatively close to the accretion disk. We calculate the $γ$-ray spectra produced in an Inverse Compton $e^\pm$ pair cascade initiated by primary $γ$-rays which are injected close to the surface of the accretion disk. Possible synchrotron energy losses of secondary cascade $e^\pm$ pairs are also taken into account. Since the soft radiation field is anisotropic, the resulting $γ$-ray spectra strongly depend on the observation angle. We investigate their basic properties for different parameters describing such a model. The model is applied to the misaligned blazar Cen A recently detected in the TeV $γ$-rays. We conclude on the site of the $γ$-ray emission region in Cen A based on the comparison of the model with the observations of this source in the GeV-TeV energy range.

astro-ph.HE↗

arXiv2009

MARS, the MAGIC Analysis and Reconstruction Software

With the commissioning of the second MAGIC gamma-ray Cherenkov telescope situated close to MAGIC-I, the standard analysis package of the MAGIC collaboration, MARS, has been upgraded in order to perform the stereoscopic reconstruction of the detected atmospheric showers. MARS is a ROOT-based code written in C++, which includes all the necessary algorithms to transform the raw data recorded by the telescopes into information about the physics parameters of the observed targets. An overview of the methods for extracting the basic shower parameters is presented, together with a description of the tools used in the background discrimination and in the estimation of the gamma-ray source spectra.

astro-ph.IM↗

arXiv2009

Performance of the MAGIC telescopes in stereoscopic mode

The MAGIC gamma-ray observatory has recently been upgraded by a second Cherenkov telescope at a distance of 85 m from the first one. Simultaneous observation of air showers with the two MAGIC telescopes (stereoscopic mode) will improve the reconstruction of the shower axis and solve the ambiguity in the impact point occurring in single-telescope mode. Also, the stereo observation will result in a better angular resolution, energy estimation and cosmic-ray background rejection. It is expected that the sensitivity of MAGIC improves significantly over the full energy range (60 GeV - 20 TeV). Here, we present the performance estimated from Monte Carlo simulations.

astro-ph.IM↗

arXiv2009

MAGIC observation of Globular Cluster M13 and its millisecond pulsars

Based on MAGIC observations from June and July 2007, we present upper limits to the E>140 GeV emission from the globular cluster M13. Those limits allow us to constrain the population of millisecond pulsars within M13 and to test models for acceleration of leptons inside their magnetospheres and/or surrounding. We conclude that in M13 either millisecond pulsars are fewer than expected or they accelerate leptons less efficiently than predicted.

astro-ph.HE↗

arXiv2008

Modification of the gamma-ray spectra by internal absorption in OVV blazars: The example cases of 3C 273 and 3C 279

Recent observations with the low threshold Cherenkov telescopes proved that sub-TeV gamma-rays are able to arrive from active galaxies at relatively large distances in spite of expected severe absorption in the extragalactic background light (EBL). We calculate the gamma-ray spectra at TeV energies from two example OVV quasars, 3C 273 and 3C 279, assuming that gamma-rays are injected in the inner parts of the jets launched by the accretion disks. It is assumed that gamma-rays in the broad energy range (from MeV up to TeV) are produced in these blazars with the power law spectrum with the spectral index as observed from these objects by the EGRET telescope at GeV energies. We take into account the internal absorption of these gamma-rays by considering a few different models for the radiation field surrounding the jet. The classical picture of the relativistic blob in jet model for the injection of primary gamma-rays is considered with the injection rate of gamma-rays as observed by the EGRET telescope in the GeV energy range. The results of calculations are compared with positive detection and the upper limits on the sub-TeV gamma-ray fluxes from these two sources. It is concluded that even with the Stecker EBL model, the level of the gamma-ray emission from 3C~279 is close to the recent measurements in the sub-TeV gamma-ray energies provided that the injected gamma-ray spectrum extends from the GeV energies over the next two decades with this same spectral index. We also suggest that a few day time scale flare from 3C 273 can be detected by the MAGIC II stereo telescopes.

astro-ph↗

arXiv2008

Monte Carlo Studies of Geomagnetic Field Effects on the Imaging Air Cherenkov Technique for the MAGIC Telescope Site

Imaging air Cherenkov telescopes (IACTs) detect the Cherenkov light from extensive air showers (EAS) initiated by very high energy (VHE) gamma-rays impinging on the Earth's atmosphere. Due to the overwhelming background from hadron induced EAS, the discrimination of the rare gamma-like events is vital. The influence of the geomagnetic field (GF) on the development of EAS can further complicate the imaging air Cherenkov technique. The amount and the angular distribution of Cherenkov light from EAS can be obtained by means of Monte Carlo (MC) simulations. Here we present the results from dedicated MC studies of GF effects on images from gamma-ray initiated EAS for the MAGIC telescope site, where the GF strength is ~40 micro Tesla. The results from the MC studies suggest that GF effects degrade not only measurements of very low energy gamma-rays below ~100 GeV but also those at TeV-energies.

astro-ph↗

arXiv2007

High Energy Gamma-rays from Globular Clusters

It is expected that specific globular clusters can contain up to a hundred of millisecond pulsars. These pulsars can accelerate leptons at the shock waves originated in collisions of the pulsar winds and/or inside the pulsar magnetospheres. Energetic leptons diffuse gradually through the globular cluster comptonizing stellar and microwave background radiation. We calculate the GeV-TeV $γ$-ray spectra for different models of injection of leptons and parameters of the globular clusters assuming reasonable, of the order of 1%, efficiency of energy conversion from the pulsar winds into the relativistic leptons. It is concluded that leptons accelerated in the globular cluster cores should produce well localized $γ$-ray sources which are concentric with these globular clusters. The results are shown for four specific globular clusters (47 Tuc, Ter 5, M13, and M15), in which significant population of millisecond pulsars have been already discovered. We argue that the best candidates, which might be potentially detected by the present Cherenkov telescopes and the planned satellite telescopes (AGILE, GLAST), are 47 Tuc on the southern hemisphere, and M13 on the northern hemisphere. We conclude that detection (or non-detection) of GeV-TeV $γ$-ray emission from GCs by these instruments put important constraints on the models of acceleration of leptons by millisecond pulsars.

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