Omar Kurtanidze - Academia.edu (original) (raw)

Papers by Omar Kurtanidze

The Astronomical Journal, 2006

arXiv (Cornell University), Feb 3, 2010

As part of our comprehensive long-term multi-waveband monitoring of 34 blazars, we followed the a... more As part of our comprehensive long-term multi-waveband monitoring of 34 blazars, we followed the activity in the jet of the blazar PKS 1510−089 during major outbursts during the first half of 2009. The most revealing event was a two-month long outburst that featured a number of γ-ray flares. During the outburst, the position angle of optical linear polarization rotated by about 720 • , which implies that a single emission feature was responsible for all of the flares during the outburst. At the end of the rotation, a new superluminal knot (∼ 22c) passed through the "core" seen on 43 GHz VLBA images at essentially the same time as an extremely sharp, high-amplitude γ-ray and optical flare occurred. We associate the entire multi-flare outburst with this knot. The ratio of γ-ray to synchrotron integrated flux indicates that some of the γ-ray flares resulted from inverse Compton scattering of seed photons outside the ultra-fast spine of the jet. Because many of the flares occurred over time scales of days or even hours, there must be a number of sources of IR-optical-UV seed photonsprobably synchrotron emission-surrounding the spine, perhaps in a slower sheath of the jet.

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015), 2016

high significance above ∼2 TeV in ∼13.1 h livetime. The observations comprise low flux states and... more high significance above ∼2 TeV in ∼13.1 h livetime. The observations comprise low flux states and strong flaring events, which in 2014 show a flux level comparable to the 1997 historical maximum. Such high flux states enable spectral variability and flux variability studies down to a timescale of four minutes in the 2-20 TeV energy range. During the 2014 flare, the source is clearly detected in each of these bins. The intrinsic spectrum is well described by a power law of index Γ = 2.15 ± 0.06 and does not show curvature in this energy range. Flux dependent spectral analyses show a clear harder-when-brighter behaviour. The high flux levels and the high sensitivity of H.E.S.S. allow studies in the unprecedented combination of short timescales and an energy coverage that extends significantly above 10 TeV. The high energies allow us to probe the effect of EBL absorption at low redshifts, jet physics and LIV. The multiwavelength context of these VHE observations is presented as well.

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015), 2016

The high-frequency peaked BL Lac object 1ES 0229+200 (z = 0.14) was first detected in very high e... more The high-frequency peaked BL Lac object 1ES 0229+200 (z = 0.14) was first detected in very high energy (VHE, E > 100 GeV) γ-rays by the H.E.S.S. (High Energy Stereoscopic System) collaboration in 2006 [1]. No flux variability was reported in the initial study and its spectral characteristics have been used to derive constraints on the extragalactic background light (EBL, [1]) and on the intergalactic magnetic field (IGMF, e.g. [2, 3, 4, 5]). 1ES 0229+200 has been observed with H.E.S.S. for ∼130 hours from 2004 to 2013: the full dataset analysed with a more sensitive method will be presented here. The results indicate that the source is not constant and displays flux variability on yearly and monthly timescales. The existence of flux variability affects the derivation of the constraints on the IGMF. The H.E.S.S. observations cover several simultaneous multi-frequency campaigns and the VHE variations are compared with those reported in different bands.

Proceedings of 7th International Fermi Symposium — PoS(IFS2017), 2017

The flat spectrum radio quasar (FSRQ) PKS 1510-089 (z=0.361) is known for its complex multiwavele... more The flat spectrum radio quasar (FSRQ) PKS 1510-089 (z=0.361) is known for its complex multiwavelength behaviour. Since 2015, it has been very active across the entire electromagnetic spectrum. This has lead to joint observation campaigns including Fermi-LAT, Cherenkov telescopes and several instrument covering the synchrotron branch. Observations resulted in a range of remarkable measurements, including rapid flares above 200 GeV with peak-fluxes exceeding the long-term average by up to a factor of 30 and unprecedented optical flares peaking in R-band at 13.6 magnitudes. The comparison of the various multi-wavelength light-curves also show that different events follow different spectral evolution within the gamma-ray band and display different relationships to the synchrotron emission. We discuss different selection effects as well as the effect of pair-absorption on flares originating at different distances from the core and conclude that absorption in the BLR is not the sole reason for the broad-band diversity. This diversity of multi-frequency correlations during different flares also reflects a diversity in physical properties of the emission regions and-possibly-even different dominating radiation mechanisms. This has implications for interpretations of orphan flares and searches for potential correlations to neutrino emission in AGN.

Symposium - International Astronomical Union, 2000

Proceedings of 7th International Fermi Symposium — PoS(IFS2017), 2017

AIP Conference Proceedings, 2017

The BL Lac type object Mrk 501 was observed at very high energies (E > 100 GeV) in 2014 with the ... more The BL Lac type object Mrk 501 was observed at very high energies (E > 100 GeV) in 2014 with the upgraded H.E.S.S. (High Energy Stereoscopic System) phase 2 array. The data collected with the central 28 m telescope allow for a broader energy range extending to lower energies when compared to the one obtained with the four small telescopes alone. A strong flaring event with a flux level comparable to the 1997 historical maximum has been detected as a consequence of target of opportunity observations triggered by alerts from the FACT collaboration. The First G-APD Cherenkov Telescope (FACT) is continuously monitoring bright blazars at TeV energies providing important pre-and post-flare information. For the first time, the data and lightcurves from H.E.S.S. and FACT are compared. These contemporaneous observations allow for a better characterization of the source emission. In a multiwavelength context, more precise correlation studies between VHE and lower energies are possible thanks to the dense sampling of the FACT observations. The hard intrinsic spectrum detected by H.E.S.S. during the flare allows the derivation of strong constraints on the scale of Lorentz invariance violation via the non-detection of EBL opacity modifications and from time-of-flight studies.

Symposium - International Astronomical Union, 2001

We give a brief summary of the ongoing Abastumani Blazar Monitoring Program (ABMP) started in May... more We give a brief summary of the ongoing Abastumani Blazar Monitoring Program (ABMP) started in May 1997. More than ~50,000 frames of data have been collected during 370 nights of observations on about 50 target objects. Preliminary results of two years of monitoring of ten selected AGNs carried out in the framework of ABMP are presented. All observations were done in the BVRI bands using the CCD ST-6 based photometer attached to the 70-cm meniscus telescope's Newtonian focus. Image reductions were made using different software packages of image reduction systems such as IRAF, MIDAS and STARLINK. All objects under study show light variations exceeding one magnitude. The largest variation was observed for AO 0235+116, amounting to 4.0 mag in the R band.

Symposium - International Astronomical Union, 2001

We present a brief summary of the ongoing Abastumani Blazar Monitoring Program started in the May... more We present a brief summary of the ongoing Abastumani Blazar Monitoring Program started in the May of 1997. More than 50 000 frames have been collected during 498 observing nights for about 50 blazars.

Galaxies, 2016

In this work, we present optical R band observations of AO 0235+164 carried out during the period... more In this work, we present optical R band observations of AO 0235+164 carried out during the period of November 2006 to December 2012 using the Ap6E CCD camera attached to the primary focus of the 70 cm meniscus telescope at Abastumani Observatory, Georgia. It shows a large variation of ∆R = 4.88 mag (14.19-19.07 mag) and a short time scale of ∆T v = 73.5 min during our monitoring period. When periodicity analysis methods are applied to the R-band data from both historic and our observations, periods P 1 = 8.26 yr and P 2 = 0.54 yr are found.

International Astronomical Union Colloquium, 1995

In this short review we briefly discuss all the extensive low dispersion objective-prism spectral... more In this short review we briefly discuss all the extensive low dispersion objective-prism spectral surveys carried out in the last fifty years for the study of the surface and space distributions of late-type giant stars, namely C stars. In the light of data obtained and new discoveries, the expediency of undertaking a deep mI low dispersion spectral survey in the near-infrared spectral region is considered.

AIP Conference Proceedings, 2004

We have performed two campaigns of intensive monitoring of the blazars 3C 273, 3C 279, and PKS 15... more We have performed two campaigns of intensive monitoring of the blazars 3C 273, 3C 279, and PKS 1510−089 in the X-ray, optical, near-IR, and radio regions in March and April 2002. The quasar 3C 279 reveals significant interday variability at all wavelengths. The quasar 3C 273 shows smooth variations in the X-ray and near-IR regions on time scales of 2-3 days. For both quasars the results indicate close connection between X-ray and near-IR emission with a delay of X-rays from 0 to 3 days. The quasar PKS 1510-089 was in a low emission state at all frequencies during the campaigns.

Chinese J Astron Astrophys, 2004

The photometry is reported for galaxies in two clusters A1983, 2065 with redshifts 0.046, 0.072 r... more The photometry is reported for galaxies in two clusters A1983, 2065 with redshifts 0.046, 0.072 respectively. The luminosity segregation is observed only within a magnitude from the brightest galaxy. The alingment of the galaxy major axis is observed in the Corona Borealis cluster. The intermediate distance clusters (0.05<z<0.15) will be studied by CCD ounted on 125cm RCh and 70cm meniscus

The Astrophysical Journal, 2015

The occurrence of low-amplitude flux variations in blazars on hourly timescales, commonly known a... more 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 ∼ (40−60)% ±(2−10)%, and the electric vector position angle ∼ (10−20) deg ±(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 counterclockwise 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.

New Eyes to See Inside the Sun and Stars, 1998

The programme of long-term high-speed photometric survey about two hundred Normal and Peculiar A-... more The programme of long-term high-speed photometric survey about two hundred Normal and Peculiar A-type stars with 125cm RC telescope equipped by Two-Star Photometer is presented.

The Astronomical Journal, 2006

arXiv (Cornell University), Feb 3, 2010

As part of our comprehensive long-term multi-waveband monitoring of 34 blazars, we followed the a... more As part of our comprehensive long-term multi-waveband monitoring of 34 blazars, we followed the activity in the jet of the blazar PKS 1510−089 during major outbursts during the first half of 2009. The most revealing event was a two-month long outburst that featured a number of γ-ray flares. During the outburst, the position angle of optical linear polarization rotated by about 720 • , which implies that a single emission feature was responsible for all of the flares during the outburst. At the end of the rotation, a new superluminal knot (∼ 22c) passed through the "core" seen on 43 GHz VLBA images at essentially the same time as an extremely sharp, high-amplitude γ-ray and optical flare occurred. We associate the entire multi-flare outburst with this knot. The ratio of γ-ray to synchrotron integrated flux indicates that some of the γ-ray flares resulted from inverse Compton scattering of seed photons outside the ultra-fast spine of the jet. Because many of the flares occurred over time scales of days or even hours, there must be a number of sources of IR-optical-UV seed photonsprobably synchrotron emission-surrounding the spine, perhaps in a slower sheath of the jet.

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015), 2016

high significance above ∼2 TeV in ∼13.1 h livetime. The observations comprise low flux states and... more high significance above ∼2 TeV in ∼13.1 h livetime. The observations comprise low flux states and strong flaring events, which in 2014 show a flux level comparable to the 1997 historical maximum. Such high flux states enable spectral variability and flux variability studies down to a timescale of four minutes in the 2-20 TeV energy range. During the 2014 flare, the source is clearly detected in each of these bins. The intrinsic spectrum is well described by a power law of index Γ = 2.15 ± 0.06 and does not show curvature in this energy range. Flux dependent spectral analyses show a clear harder-when-brighter behaviour. The high flux levels and the high sensitivity of H.E.S.S. allow studies in the unprecedented combination of short timescales and an energy coverage that extends significantly above 10 TeV. The high energies allow us to probe the effect of EBL absorption at low redshifts, jet physics and LIV. The multiwavelength context of these VHE observations is presented as well.

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015), 2016

The high-frequency peaked BL Lac object 1ES 0229+200 (z = 0.14) was first detected in very high e... more The high-frequency peaked BL Lac object 1ES 0229+200 (z = 0.14) was first detected in very high energy (VHE, E > 100 GeV) γ-rays by the H.E.S.S. (High Energy Stereoscopic System) collaboration in 2006 [1]. No flux variability was reported in the initial study and its spectral characteristics have been used to derive constraints on the extragalactic background light (EBL, [1]) and on the intergalactic magnetic field (IGMF, e.g. [2, 3, 4, 5]). 1ES 0229+200 has been observed with H.E.S.S. for ∼130 hours from 2004 to 2013: the full dataset analysed with a more sensitive method will be presented here. The results indicate that the source is not constant and displays flux variability on yearly and monthly timescales. The existence of flux variability affects the derivation of the constraints on the IGMF. The H.E.S.S. observations cover several simultaneous multi-frequency campaigns and the VHE variations are compared with those reported in different bands.

Proceedings of 7th International Fermi Symposium — PoS(IFS2017), 2017

The flat spectrum radio quasar (FSRQ) PKS 1510-089 (z=0.361) is known for its complex multiwavele... more The flat spectrum radio quasar (FSRQ) PKS 1510-089 (z=0.361) is known for its complex multiwavelength behaviour. Since 2015, it has been very active across the entire electromagnetic spectrum. This has lead to joint observation campaigns including Fermi-LAT, Cherenkov telescopes and several instrument covering the synchrotron branch. Observations resulted in a range of remarkable measurements, including rapid flares above 200 GeV with peak-fluxes exceeding the long-term average by up to a factor of 30 and unprecedented optical flares peaking in R-band at 13.6 magnitudes. The comparison of the various multi-wavelength light-curves also show that different events follow different spectral evolution within the gamma-ray band and display different relationships to the synchrotron emission. We discuss different selection effects as well as the effect of pair-absorption on flares originating at different distances from the core and conclude that absorption in the BLR is not the sole reason for the broad-band diversity. This diversity of multi-frequency correlations during different flares also reflects a diversity in physical properties of the emission regions and-possibly-even different dominating radiation mechanisms. This has implications for interpretations of orphan flares and searches for potential correlations to neutrino emission in AGN.

Symposium - International Astronomical Union, 2000

Proceedings of 7th International Fermi Symposium — PoS(IFS2017), 2017

AIP Conference Proceedings, 2017

The BL Lac type object Mrk 501 was observed at very high energies (E > 100 GeV) in 2014 with the ... more The BL Lac type object Mrk 501 was observed at very high energies (E > 100 GeV) in 2014 with the upgraded H.E.S.S. (High Energy Stereoscopic System) phase 2 array. The data collected with the central 28 m telescope allow for a broader energy range extending to lower energies when compared to the one obtained with the four small telescopes alone. A strong flaring event with a flux level comparable to the 1997 historical maximum has been detected as a consequence of target of opportunity observations triggered by alerts from the FACT collaboration. The First G-APD Cherenkov Telescope (FACT) is continuously monitoring bright blazars at TeV energies providing important pre-and post-flare information. For the first time, the data and lightcurves from H.E.S.S. and FACT are compared. These contemporaneous observations allow for a better characterization of the source emission. In a multiwavelength context, more precise correlation studies between VHE and lower energies are possible thanks to the dense sampling of the FACT observations. The hard intrinsic spectrum detected by H.E.S.S. during the flare allows the derivation of strong constraints on the scale of Lorentz invariance violation via the non-detection of EBL opacity modifications and from time-of-flight studies.

Symposium - International Astronomical Union, 2001

We give a brief summary of the ongoing Abastumani Blazar Monitoring Program (ABMP) started in May... more We give a brief summary of the ongoing Abastumani Blazar Monitoring Program (ABMP) started in May 1997. More than ~50,000 frames of data have been collected during 370 nights of observations on about 50 target objects. Preliminary results of two years of monitoring of ten selected AGNs carried out in the framework of ABMP are presented. All observations were done in the BVRI bands using the CCD ST-6 based photometer attached to the 70-cm meniscus telescope's Newtonian focus. Image reductions were made using different software packages of image reduction systems such as IRAF, MIDAS and STARLINK. All objects under study show light variations exceeding one magnitude. The largest variation was observed for AO 0235+116, amounting to 4.0 mag in the R band.

Symposium - International Astronomical Union, 2001

We present a brief summary of the ongoing Abastumani Blazar Monitoring Program started in the May... more We present a brief summary of the ongoing Abastumani Blazar Monitoring Program started in the May of 1997. More than 50 000 frames have been collected during 498 observing nights for about 50 blazars.

Galaxies, 2016

In this work, we present optical R band observations of AO 0235+164 carried out during the period... more In this work, we present optical R band observations of AO 0235+164 carried out during the period of November 2006 to December 2012 using the Ap6E CCD camera attached to the primary focus of the 70 cm meniscus telescope at Abastumani Observatory, Georgia. It shows a large variation of ∆R = 4.88 mag (14.19-19.07 mag) and a short time scale of ∆T v = 73.5 min during our monitoring period. When periodicity analysis methods are applied to the R-band data from both historic and our observations, periods P 1 = 8.26 yr and P 2 = 0.54 yr are found.

International Astronomical Union Colloquium, 1995

In this short review we briefly discuss all the extensive low dispersion objective-prism spectral... more In this short review we briefly discuss all the extensive low dispersion objective-prism spectral surveys carried out in the last fifty years for the study of the surface and space distributions of late-type giant stars, namely C stars. In the light of data obtained and new discoveries, the expediency of undertaking a deep mI low dispersion spectral survey in the near-infrared spectral region is considered.

AIP Conference Proceedings, 2004

We have performed two campaigns of intensive monitoring of the blazars 3C 273, 3C 279, and PKS 15... more We have performed two campaigns of intensive monitoring of the blazars 3C 273, 3C 279, and PKS 1510−089 in the X-ray, optical, near-IR, and radio regions in March and April 2002. The quasar 3C 279 reveals significant interday variability at all wavelengths. The quasar 3C 273 shows smooth variations in the X-ray and near-IR regions on time scales of 2-3 days. For both quasars the results indicate close connection between X-ray and near-IR emission with a delay of X-rays from 0 to 3 days. The quasar PKS 1510-089 was in a low emission state at all frequencies during the campaigns.

Chinese J Astron Astrophys, 2004

The photometry is reported for galaxies in two clusters A1983, 2065 with redshifts 0.046, 0.072 r... more The photometry is reported for galaxies in two clusters A1983, 2065 with redshifts 0.046, 0.072 respectively. The luminosity segregation is observed only within a magnitude from the brightest galaxy. The alingment of the galaxy major axis is observed in the Corona Borealis cluster. The intermediate distance clusters (0.05<z<0.15) will be studied by CCD ounted on 125cm RCh and 70cm meniscus

The Astrophysical Journal, 2015

The occurrence of low-amplitude flux variations in blazars on hourly timescales, commonly known a... more 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 ∼ (40−60)% ±(2−10)%, and the electric vector position angle ∼ (10−20) deg ±(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 counterclockwise 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.

New Eyes to See Inside the Sun and Stars, 1998

The programme of long-term high-speed photometric survey about two hundred Normal and Peculiar A-... more The programme of long-term high-speed photometric survey about two hundred Normal and Peculiar A-type stars with 125cm RC telescope equipped by Two-Star Photometer is presented.