arXiv subjects

Łukasz Stawarz

Publications and source records attributed to Łukasz Stawarz.

25 records · Page 2Linked to original sources

arXiv2016

First detection in gamma-rays of a young radio galaxy: Fermi-LAT observations of the Compact Symmetric Object PKS 1718-649

We report the $γ$-ray detection of a young radio galaxy, PKS 1718$-$649, belonging to the class of Compact Symmetric Objects (CSOs), with the Large Area Telescope (LAT) on board the {\it Fermi} satellite. The third {\it Fermi} Gamma-ray LAT catalog (3FGL) includes an unassociated $γ$-ray source, 3FGL J1728.0$-$6446, located close to PKS 1718$-$649. Using the latest Pass 8 calibration, we confirm that the best fit $1 σ$ position of the $γ$-ray source is compatible with the radio location of PKS 1718$-$649. Cross-matching of the $γ$-ray source position with the positions of blazar sources from several catalogs yields negative results. Thus, we conclude that PKS 1718$-$649 is the most likely counterpart to the unassociated LAT source. We obtain a detection test statistics TS$\sim 36$ ($>$5$σ$) with a best fit photon spectral index $Γ=$2.9$\pm$0.3 and a 0.1-100 GeV photon flux density $F_{\rm 0.1-100GeV}=$(11.5$\pm$0.3)$\times{\rm 10^{-9}}$ ph cm$^{-2}$ s$^{-1}$. We argue that the linear size ($\sim$2 pc), the kinematic age ($\sim$100 years), and the source distance ($z=0.014$) make PKS 1718$-$649 an ideal candidate for $γ$-ray detection in the framework of the model proposing that the most compact and the youngest CSOs can efficiently produce GeV radiation via inverse-Compton scattering of the ambient photon fields by the radio lobe non-thermal electrons. Thus, our detection of the source in $γ$-rays establishes young radio galaxies as a distinct class of extragalactic high-energy emitters, and yields an unique insight on the physical conditions in compact radio lobes interacting with the interstellar medium of the host galaxy.

astro-ph.HE↗

arXiv2015

Discovery of VHE gamma-rays from the radio galaxy PKS 0625-354 with H.E.S.S

Most of the extragalactic objects detected so far in the very high energy (VHE) regime are blazars, but the discovered nearby radio galaxies: M87, Cen A and NGC 1275 of type FRI seem to constitute a new class of VHE emitters. The radio galaxy PKS 0625-354 was observed and detected ($\sim$6$σ$) with the H.E.S.S. phase I telescopes in 2012, above an energy threshold of 250 GeV. The time-averaged VHE energy spectrum is well characterized by a power law model. The broad-band light curve, including the available multiwavelength data, as well as the VHE data gathered with H.E.S.S. will be presented.

astro-ph.HE↗

arXiv2015

Discovery of a Highly Polarized Optical Microflare in the Blazar S5 0716+714 During 2014 WEBT Campaign

The occurrence of low-amplitude flux variations in blazars on hourly timescales, commonly known as microvariability, is still a widely debated subject in high-energy astrophysics. Several competing scenarios have been proposed to explain such occurrences, including various jet plasma instabilities leading to the formation of shocks, magnetic reconnection sites, and turbulence. In this letter we present the results of our detailed investigation of a prominent, five-hour-long optical microflare detected during recent WEBT campaign in 2014, March 2-6 targeting the blazar 0716+714. After separating the flaring component from the underlying base emission continuum of the blazar, we find that the microflare is highly polarized, with the polarization degree $\sim (40-60)\%$$\pm (2-10)\%$, and the electric vector position angle $\sim (10 - 20)$deg$\pm (1-8)$deg slightly misaligned with respect to the position angle of the radio jet. The microflare evolution in the $(Q,\,U)$ Stokes parameter space exhibits a looping behavior with a counter-clockwise rotation, meaning polarization degree decreasing with the flux (but higher in the flux decaying phase), and approximately stable polarization angle. The overall very high polarization degree of the flare, its symmetric flux rise and decay profiles, and also its structured evolution in the $Q-U$ plane, all imply that the observed flux variation corresponds to a single emission region characterized by a highly ordered magnetic field. As discussed in the paper, a small-scale but strong shock propagating within the outflow, and compressing a disordered magnetic field component, provides a natural, though not unique, interpretation of our findings.

astro-ph.HE↗

arXiv2014

Longterm Optical Monitoring of Bright BL Lacertae Objects with ATOM: Spectral Variability and Multiwavelength Correlations

Blazars are the established sources of an intense and variable non-thermal radiation extending from radio wavelengths up to HE and VHE gamma-rays. Understanding the spectral evolution of blazars in selected frequency ranges, as well as multi-frequency correlations in various types of blazar sources, is of a primary importance for constraining the blazar physics. Here we present the results of a long-term optical monitoring of a sample of 30 blazars of the BL Lac type. We study the optical color-magnitude correlation patterns emerging in the analyzed sample, and compare the optical properties of the targets with the high-energy gamma-ray and high-frequency radio data. The optical observations were carried out in R and B filters using ATOM telescope. Each object was observed during at least 20 nights in the period 2007-2012. We find significant global color-magnitude correlations in 40 % of the sample. The sources which do not display any clear chromatism often do exhibit bluer-when-brighter (bwb) behavior but only in isolated shorter time intervals. We also discovered spectral state transitions at optical wavelengths in several sources. Finally, we find that the radio, optical, and gamma-ray luminosities of the sources obey almost linear correlations, which seem however induced, at least partly, by the redshift dependance, and may be also affected by non-simultaneousness of the analyzed dataset. We argue that the observed bwb behavior is intrinsic to the jet emission regions, at least for some of the analyzed blazars, rather than resulting from the contamination of the measured flux by the starlight of host galaxies. We also conclude that the significance of color-magnitude scalings does not correlate with the optical color, but instead seems to depend on the source luminosity, in a sense that these are the lowest-luminosity BL Lac objects which display the strongest correlations.

astro-ph.HE↗

arXiv2012

Infrared and hard X-ray diagnostics of AGN identification from the Swift/BAT and AKARI all-sky surveys

We combine data from two all-sky surveys in order to study the connection between the infrared and hard X-ray (>10keV) properties for local active galactic nuclei (AGN). The Swift/Burst Alert Telescope all-sky survey provides an unbiased, flux-limited selection of hard X-ray detected AGN. Cross-correlating the 22-month hard X-ray survey with the AKARI all-sky survey, we studied 158 AGN detected by the AKARI instruments. We find a strong correlation for most AGN between the infrared (9, 18, and 90 micron) and hard X-ray (14-195 keV) luminosities, and quantify the correlation for various subsamples of AGN. Partial correlation analysis confirms the intrinsic correlation after removing the redshift contribution. The correlation for radio galaxies has a slope and normalization identical to that for Seyfert 1s, implying similar hard X-ray/infrared emission processes in both. In contrast, Compton-thick sources show a large deficit in the hard X-ray band, because high gas column densities diminish even their hard X-ray luminosities. We propose two photometric diagnostics for source classification: one is an X-ray luminosity vs. infrared color diagram, in which type 1 radio-loud AGN are well isolated from the others in the sample. The other uses the X-ray vs. infrared color as a useful redshift-independent indicator for identifying Compton-thick AGN. Importantly, Compton-thick AGN and starburst galaxies in composite systems can also be differentiated in this plane based upon their hard X-ray fluxes and dust temperatures. This diagram may be useful as a new indicator to classify objects in new and upcoming surveys such as WISE and NuSTAR.

astro-ph.HE↗

arXiv2012

Deep Chandra X-ray Imaging of a Nearby Radio Galaxy 4C+29.30: X-ray/Radio Connection

We report results from our deep Chandra X-ray observations of a nearby radio galaxy, 4C+29.30 (z=0.0647). The Chandra image resolves structures on sub-arcsec to arcsec scales, revealing complex X-ray morphology and detecting the main radio features: the nucleus, a jet, hotspots, and lobes. The nucleus is absorbed (N(H)=3.95 (+0.27/-0.33)x10^23 atoms/cm^2) with an unabsorbed luminosity of L(2-10 keV) ~ (5.08 +/-0.52) 10^43 erg/s characteristic of Type 2 AGN. Regions of soft (<2 keV) X-ray emission that trace the hot interstellar medium (ISM) are correlated with radio structures along the main radio axis indicating a strong relation between the two. The X-ray emission beyond the radio source correlates with the morphology of optical line-emitting regions. We measured the ISM temperature in several regions across the galaxy to be kT ~ 0.5 with slightly higher temperatures (of a few keV) in the center and in the vicinity of the radio hotspots. Assuming these regions were heated by weak shocks driven by the expanding radio source, we estimated the corresponding Mach number of 1.6 in the southern regions. The thermal pressure of the X-ray emitting gas in the outermost regions suggest the hot ISM is slightly under-pressured with respect to the cold optical-line emitting gas and radio-emitting plasma, which both seem to be in a rough pressure equilibrium. We conclude that 4C+29.30 displays a complex view of interactions between the jet-driven radio outflow and host galaxy environment, signaling feedback processes closely associated with the central active nucleus.

astro-ph.CO↗

arXiv2011

Suzaku X-ray Follow-up Observations of Seven Unassociated Fermi-LAT Gamma-ray Sources at High Galactic Latitudes

We report on our second-year campaign of X-ray follow-up observations of unidentified Fermi-LAT γ-ray sources at high Galactic latitudes (|b|>10 degree) using the X-ray Imaging Spectrometer onboard the Suzaku X-ray Observatory. In this second year of the project, seven new targets were selected from the First Fermi-LAT Catalog, and studied with 20-40 ks effective Suzaku exposures. We detected an X-ray point source coincident with the position of the recently discovered millisecond pulsar PSR J2302+4442 within the 95% confidence error circle of 1FGL J2302.8+4443. The X-ray spectrum of the detected counterpart was well fit by a blackbody model with temperature of kT ~0.3 keV, consistent with an origin of the observed X-ray photons from the surface of a rotating magnetized neutron star. For four other targets which were also recently identified with a normal pulsar (1FGL J0106.7+4853) and millisecond pulsars (1FGL J1312.6+0048, J1902.0-5110, and J2043.2+1709), only upper limits in the 0.5-10 keV band were obtained at the flux levels of ~10^{-14} erg cm^{-2} s^{-1}. A weak X-ray source was found in the field of 1FGL J1739.4+8717, but its association with the variable γ-ray emitter could not be confirmed with the available Suzaku data alone. For the remaining Fermi-LAT object 1FGL J1743.8-7620 no X-ray source was detected within the LAT 95% error ellipse. We briefly discuss the general properties of the observed high Galactic-latitude Fermi-LAT objects by comparing their multiwavelength properties with those of known blazars and millisecond pulsars.

astro-ph.HE↗