Papers by Svetlana Jorstad
Galaxies, Apr 27, 2021
Time-variable polarization is an extremely valuable observational tool to probe the dynamical phy... more Time-variable polarization is an extremely valuable observational tool to probe the dynamical physical conditions of blazar jets. Since 2008, we have been monitoring the flux and linear polarization of a sample of gamma-ray bright blazars at optical frequencies. Some of the observations were performed on nightly or intra-night time-scales in four optical bands, providing information on the frequency and time dependence of the polarization. The observed behavior is similar to that found in simulations of turbulent plasma in a relativistic jet that contains a standing shock and/or a helical background magnetic field. Similar simulations predict the characteristics of X-ray synchrotron polarization of blazars that will be measured in the future by the Imaging X-ray Polarimetry Explorer (IXPE).
Galaxies, Sep 3, 2016
We analyze the behavior of the BL Lac object S4 0954+658 during an unprecedented bright optical f... more We analyze the behavior of the BL Lac object S4 0954+658 during an unprecedented bright optical flare in early 2015. The optical flare was accompanied by a powerful γ-ray flare and the detection of very-high-energy γ-ray emission. We analyze total and polarized intensity images obtained with the VLBA at 43 GHz and discover a new bright polarized superluminal knot, which was ejected from the VLBI-core during the peak of the flare.
HAL (Le Centre pour la Communication Scientifique Directe), 2009
Context. BL Lacertae is the prototype of the blazar subclass named after it. Yet, it has occasion... more Context. BL Lacertae is the prototype of the blazar subclass named after it. Yet, it has occasionally shown a peculiar behaviour that has questioned a simple interpretation of its broad-band emission in terms of synchrotron plus synchrotron self-Compton (SSC) radiation. Aims. In the 2007-2008 observing season we carried out a new multiwavelength campaign of the Whole Earth Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the XMM-Newton satellite in July and December 2007, and January 2008, to study its emission properties, particularly in the optical-X-ray energy range. Methods. The source was monitored in the optical-to-radio bands by 37 telescopes. The brightness level was relatively low. Some episodes of very fast variability were detected in the optical bands. Flux changes had larger amplitude at the higher radio frequencies than at longer wavelengths. Results. The X-ray spectra acquired by the EPIC instrument onboard XMM-Newton are well fitted by a power law with photon index Γ ∼ 2 and photoelectric absorption exceeding the Galactic value. However, when taking into account the presence of a molecular cloud on the line of sight, the EPIC data are best fitted by a double power law, implying a concave X-ray spectrum. The spectral energy distributions (SEDs) built with simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations suggest that the peak of the synchrotron emission lies in the near-IR band, and show a prominent UV excess, besides a slight soft-X-ray excess. A comparison with the SEDs corresponding to previous observations with X-ray satellites shows that the X-ray spectrum is very variable, since it can change from extremely steep to extremely hard, and can be more or less curved in intermediate states. We ascribe the UV excess to thermal emission from the accretion disc, and the other broad-band spectral features to the presence of two synchrotron components, with their related SSC emission. We fit the thermal emission with a black body law and the non-thermal components by means of a helical jet model. The fit indicates a disc temperature > ∼ 20000 K and a luminosity > ∼ 6 × 10 44 erg s -1 .
Galaxies, 2021
Time-variable polarization is an extremely valuable observational tool to probe the dynamical phy... more Time-variable polarization is an extremely valuable observational tool to probe the dynamical physical conditions of blazar jets. Since 2008, we have been monitoring the flux and linear polarization of a sample of gamma-ray bright blazars at optical frequencies. Some of the observations were performed on nightly or intra-night time-scales in four optical bands, providing information on the frequency and time dependence of the polarization. The observed behavior is similar to that found in simulations of turbulent plasma in a relativistic jet that contains a standing shock and/or a helical background magnetic field. Similar simulations predict the characteristics of X-ray synchrotron polarization of blazars that will be measured in the future by the Imaging X-ray Polarimetry Explorer (IXPE).
Galaxies, 2017
Blazar CTA 102 underwent exceptional optical and high-energy outbursts in 2012 and 2016-2017. We ... more Blazar CTA 102 underwent exceptional optical and high-energy outbursts in 2012 and 2016-2017. We analyze its behaviour during these events, focusing on polarimetry as a tool that allows us to trace changes in the physical conditions and geometric configuration of the emission source close to the central black hole. We also use Fermi γ-ray data in conjunction with optical photometry in an effort to localize the origin of the outbursts.
The Astrophysical Journal, 2019
The quasar 3C454.3 underwent a uniquely-structured multi-frequency outburst in June 2016. The bla... more The quasar 3C454.3 underwent a uniquely-structured multi-frequency outburst in June 2016. The blazar was observed in the optical R band by several ground-based telescopes in photometric and polarimetric modes, at γ-ray frequencies by the Fermi Large Area Telescope, and at 43 GHz with the Very Long Baseline Array. The maximum flux density was observed on 2016 June 24 at both optical and γ-ray frequencies, reaching S max opt = 18.91 ± 0.08 mJy and S max γ = 22.20 ± 0.18 × 10 -6 ph cm -2 s -1 , respectively. The June 2016 outburst possessed a precipitous decay at both γ-ray and optical frequencies, with the source decreasing in flux density by a factor of 4 over a 24-hour period in R band. Intraday variability was observed throughout the outburst, with flux density changes between 1 and 5 mJy over the course of a night. The precipitous decay featured statistically significant quasi-periodic micro-variability oscillations with an amplitude of ∼ 2-3% about the mean trend and a characteristic period of 36 minutes. The optical degree of polarization jumped from ∼ 3% to nearly 20% during the outburst, while the position angle varied by ∼ 120 • . A knot was ejected from the 43 GHz core on 2016 Feb 25, moving at an apparent speed v app = 20.3c ± 0.8c. From the observed minimum timescale of variability τ min opt ≈ 2 hr and derived Doppler factor δ = 22.6, we find a size of the emission region r 2.6 × 10 15 cm. If the quasi-periodic micro-variability oscillations are caused by periodic variations of the Doppler factor of emission from a turbulent vortex, we derive a rotational speed of the vortex ∼ 0.2c.
Nature, 2017
Blazar spectral variability as explained by a twisted inhomogeneous jet Article LJMU has develope... more Blazar spectral variability as explained by a twisted inhomogeneous jet Article LJMU has developed LJMU Research Online for users to access the research output of the University more effectively.
Monthly Notices of the Royal Astronomical Society, 2017
We report on long-term multiwavelength monitoring of blazar Mrk 421 by the GLAST-AGILE Support Pr... more We report on long-term multiwavelength monitoring of blazar Mrk 421 by the GLAST-AGILE Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in the period 2007-2015, characterized by several extreme flares. The ratio between the optical, X-ray and γ -ray fluxes is very variable. The γ -ray flux variations show a fair correlation with the optical ones starting from 2012. We analyse spectropolarimetric data and find wavelengthdependence of the polarization degree (P), which is compatible with the presence of the host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA). Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of the EVPA. We build broad-band spectral energy distributions with simultaneous near-infrared and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite. They show strong variability in both flux and X-ray spectral shape and suggest a shift of the synchrotron peak up to a factor of ∼50 in frequency. The interpretation of the flux and spectral variability is compatible with jet models including at least two emitting regions that can change their orientation with respect to the line of sight.
Galaxies, 2016
We analyze the broadband activity of the flat spectrum radio quasar PKS 1222+216 from 2008 to 201... more We analyze the broadband activity of the flat spectrum radio quasar PKS 1222+216 from 2008 to 2015 using multi-frequency monitoring which involves γ-ray data from the Fermi Large Area Telescope, total intensity and linear polarization observations from different optical telescopes in R band, and imaging of the inner jet structure with the Very Long Baseline Array (VLBA) at 43 GHz. During the observations, the source showed several dramatic flares at γ rays and optical bands, with the rising branch of a γ-ray flare accompanied by a rapid rotation of the polarization position angle (EVPA), a fast increase of the degree of polarization in the optical band, brightening of the VLBI core, and appearance of a new superluminal component in the parsec-scale jet. The rapid variability of the optical linear polarization may be explained by a strong turbulence in the jet plasma. We find a correlation between the γ rays, optical R band, and 43 GHz variability on a long-term scale (months and years), and a good general alignment between EVPAs in R band and at 43 GHz, while the correlation between short-term variations (days and weeks) is weaker. Synchronous activity across the bands supports the idea that the emission regions responsible for the γ-ray and optical flares are co-spatial and located in the vicinity of the mm-wave core of the parsec-scale jet. However, these connections do not completely explain the challenging behaviour of PKS 1222+216, since there are some γ-ray flares which are not accompanied by jet events, and vice versa. We need a continuation of multi-frequency monitoring along with high resolution imaging of the parsec-scale jet to understand in detail the origin of high energy emission in blazars.
Galaxies, 2016
We analyze the behavior of the BL Lac object S4 0954+658 during an unprecedented bright optical f... more We analyze the behavior of the BL Lac object S4 0954+658 during an unprecedented bright optical flare in early 2015. The optical flare was accompanied by a powerful γ-ray flare and the detection of very-high-energy γ-ray emission. We analyze total and polarized intensity images obtained with the VLBA at 43 GHz and discover a new bright polarized superluminal knot, which was ejected from the VLBI-core during the peak of the flare.
We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and gamma-... more We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO 0235+164 in 2008. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array (VLBA) images at 7mm with ~0.15 milliarcsecond resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte-Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary core and in the superluminal knot, both at >12 parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a dist...
Monthly Notices of the Royal Astronomical Society, 2014
The blazar 1156+295 was active at γ-ray energies, exhibiting three prominent flares during the ye... more The blazar 1156+295 was active at γ-ray energies, exhibiting three prominent flares during the year 2010. Here, we present results using the combination of broadband (X-ray through mm single dish) monitoring data and radio band imaging data at 43 GHz on the connection of γ-ray events to the ejections of superluminal components and other changes in the jet of 1156+295. The kinematics of the jet over the interval 2007.0-2012.5 using 43 GHz Very Long Baseline Array observations, reveal the presence of four moving and one stationary component in the inner region of the blazar jet. The propagation of the third and fourth components in the jet corresponds closely in time to the active phase of the source in γ rays. We briefly discuss the implications of the structural changes in the jet for the mechanism of γ-ray production during bright flares. To localise the γ-ray emission site in the blazar, we performed the correlation analysis between the 43 GHz radio core and the γ-ray light curve. The time lag obtained from the correlation constrains the γ-ray emitting region in the parsec-scale jet.
Proceedings of the International Astronomical Union, 2014
We present an analysis of the parsec-scale jet structure of the quasar 4C+21.35 with a resolution... more We present an analysis of the parsec-scale jet structure of the quasar 4C+21.35 with a resolution of 0.1 milliarcseconds based on 63 epochs of Very Long Baseline Array observations at 43 GHz from 2007 June to 2014 May along with the Fermi LAT γ-ray light curve and multi-frequency optical photometric and polarimetric data. We find that the innermost jet of the quasar consists of a very compact core of size ~0.03 mas, as well as feature A1 located 0.16 ± 0.03 mas from the core. The distance of A1 remains fairly stable, but its position angle with respect to the core changes from -10 to +10 deg. We detect 4 superluminal knots in the inner jet with apparent speeds ranging from 10c to 20c. The first two components appeared in the jet during the high γ-ray state of the quasar from mid-2010 to early 2011, while the fourth knot appears to be connected with the γ-ray active state in late 2013 - early 2014. The first knot can be associated with the dramatic VHE flare in 2010 June and possesse...
The Astrophysical Journal, 2012
We present multiwavelength data of the blazar 3C 454.3 obtained during an extremely bright outbur... more We present multiwavelength data of the blazar 3C 454.3 obtained during an extremely bright outburst from November 2010 through January 2011. These include flux density measurements with the Herschel Space Observatory at five submillimeter-wave and far-infrared bands, the Fermi Large Area Telescope at γ-ray energies, Swift at X-ray, ultraviolet (UV), and optical frequencies, and the Submillimeter Array at 1.3 mm. From this dataset, we form a series of 52 spectral energy distributions (SEDs) spanning nearly two months that are unprecedented in time coverage and breadth of frequency. Discrete correlation anlaysis of the millimeter, far-infrared, and γ-ray light curves show that the variations were essentially simultaneous, indicative of co-spatiality of the emission, at these wavebands. In contrast, differences in short-term fluctuations at various wavelengths imply the presence of inhomegeneities in physical conditions across the source. We locate the site of the outburst in the parsec-scale "core", whose flux density as measured on 7 mm Very Long Baseline Array images increased by 70% during the first five weeks of the outburst. Based on these considerations and guided by the SEDs, we propose a model in which turbulent plasma crosses a conical standing shock in the parsec-scale region of the jet. Here, the high-energy emission in the model is produced by inverse Compton scattering of seed photons supplied by either nonthermal radiation from a Mach disk, thermal emission from hot dust, or (for X-rays) synchrotron radiation from plasma that crosses the standing shock. For the two dates on which we fitted the model SED to the data, the model corresponds very well to the observations at all bands except at X-ray energies, where the spectrum is flatter than observed. Subject headings: galaxies: active-quasars: general-quasars: individual: 3C 454.3 -2. Observations Our Herschel target of opportunity observations began on 2010 November 19 (MJD 55519). Near-daily Swift pointings were arranged by us as well as by other groups starting on 2010 November 2, with a few gaps caused by moon avoidance, γ-ray burst observations, and other incidents. Observations at other wavebands were already underway or were initiated shortly thereafter. The Large Area Telescope (LAT) instrument on Fermi operated in standard survey mode, scanning the entire sky every three hours. Twice-weekly observations of 3C 454.3 were carried out at the Submillimeter Array (SMA) on Mauna Kea, HI. VLBA observations took place on 2010 November 1, 6, and 13 and December 4 as part of the ongoing Boston University (BU) γ-ray blazar monitoring program. We obtained limited mid-IR flux measurements of 3C 454.3 with the NASA Infrared Telescope Facility (IRTF) on Mauna Kea on 2010 November 3, during the early stage of the outburst. 2.1. Herschel Observations Our Herschel observing plan was designed to obtain both time delays and spectral energy distributions with intensive observing on every possible day when the PACS-Photometer and SPIRE-Photometer modes were operating during the 51-day visibility window. PACS and SPIRE observations are normally carried out in separate instrument campaigns 10-14 days long; on changeover days, both instruments can observe within a 24 hour period. Each PACS-Photometer observation was carried out in "scan map" mode with 3-arcminute legs and a cross-scan step of 4 arcseconds with a total on-source time of 72 seconds. Each PACS-Photometer observation included "70 µm" (60-85 µm bandpass) and "160 µm" (130-210 µm bandpass) bands. The SPIRE-Photometer observations were done
The Astrophysical Journal, 2010
We report on 18 months of multiwavelength observations of the blazar 3C 454.3 (Crazy Diamond) car... more We report on 18 months of multiwavelength observations of the blazar 3C 454.3 (Crazy Diamond) carried out in the period 2007 July–2009 January. In particular, we show the results of the AGILE campaigns which took place on 2008 May–June, 2008 July–August, and 2008 October–2009 January. During the 2008 May–2009 January period, the source average flux was highly variable, with a clear fading trend toward the end of the period, from an average γ-ray flux F E>100 MeV ≳ 200 × 10−8 photons cm−2 s−1 in 2008 May–June, to F E>100 MeV ∼ 80 × 10−8 photons cm−2 s−1 in 2008 October–2009 January. The average γ-ray spectrum between 100 MeV and 1 GeV can be fit by a simple power law, showing a moderate softening (from ΓGRID ∼ 2.0 to ΓGRID ∼ 2.2) toward the end of the observing campaign. Only 3σ upper limits can be derived in the 20–60 keV energy band with Super-AGILE, because the source was considerably off-axis during the whole time period. In 2007 July–August and 2008 May–June, 3C 454.3 wa...
The Astrophysical Journal, 2013
We analyze the multifrequency behavior of the quasar 3C 454.3 during three prominent γ-ray outbur... more We analyze the multifrequency behavior of the quasar 3C 454.3 during three prominent γ-ray outbursts: 2009 Autumn, 2010 Spring, and 2010 Autumn. The data reveal a repeating pattern, including a triple flare structure, in the properties of each γ-ray outburst, which implies similar mechanism(s) and location for all three events. The multi-frequency behavior indicates that the lower frequency events are co-spatial with the γ-ray outbursts, although the γ-ray emission varies on the shortest timescales. We determine that the variability from UV to IR wavelengths during an outburst results from a single synchrotron component whose
The Astrophysical Journal, 2011
We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and γ-ray ... more We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and γ-ray wavelengths of the BL Lacertae object AO 0235+164. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array (VLBA) images at λ = 7 mm with ∼ 0.15 milliarcsecond resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte-Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7 mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary "core" and in the superluminal knot, both parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a disturbance, elongated along the line of sight by light-travel time delays, that passes through a standing recollimation shock in the core and propagates down the jet to create the superluminal knot. The multi-wavelength light curves vary together on long time-scales (months/years), but the correspondence is poorer on shorter time-scales. This, as well as the variability of the polarization and the dual location of the outburst, agrees with the expectations of a multi-zone emission model in which turbulence plays a major role in modulating the synchrotron and inverse Compton fluxes.
The Astrophysical Journal, 2010
We combine time-dependent multi-waveband flux and linear polarization observations with submillia... more We combine time-dependent multi-waveband flux and linear polarization observations with submilliarcsecond-scale polarimetric images at λ = 7 mm of the BL Lacertae-type blazar OJ287 to locate the γ-ray emission in prominent flares in the jet of the source > 14 pc from the central engine. We demonstrate a highly significant correlation between the strongest γ-ray and millimeter-wave flares through Monte-Carlo simulations. The two reported γ-ray peaks occurred near the beginning of two major mm-wave outbursts, each of which is associated with a linear polarization maximum at millimeter wavelengths. Our Very Long Baseline Array observations indicate that the two mm-wave flares originated in the second of two features in the jet that are separated by > 14 pc. The simultaneity of the peak of the higher-amplitude γ-ray flare and the maximum in polarization of the second jet feature implies that the γ-ray and mm-wave flares are co-spatial and occur > 14 pc from the central engine. We also associate two optical flares, accompanied by sharp polarization peaks, with the two γ-ray events. The multi-waveband behavior is most easily explained if the γ-rays arise from synchrotron self-Compton scattering of optical photons from the flares. We propose that flares are triggered by interaction of moving plasma blobs with a standing shock. The γ-ray and optical emission is quenched by inverse Compton losses as synchrotron photons from the newly shocked plasma cross the emission region. The mm-wave polarization is high at the onset of a flare, but decreases as the electrons emitting at these wavelengths penetrate less polarized regions.
I present total and polarized intensity images at ultra-high resolution (0.1 milliarcseconds) of ... more I present total and polarized intensity images at ultra-high resolution (0.1 milliarcseconds) of a sample of 33 gamma-ray blazars obtained monthly with the Very Long Baseline Array (VLBA) at 43 GHz, starting in Summer 2008 when the Fermi Gamma-Ray Space Telescope began to operate. The VLBA observations determine the flux and polarization of the millimeter-wave core and other components of the jet, as well as the kinematics of bright superluminal knots. I will compare the gamma-ray light curves of the blazars, obtained with the Fermi Large Area Telescope, with flux variations in the core and knots, the times when new knots pass through the core, changes in the flux at millimeter, optical, and X-ray wavelengths, and variations in the optical and millimeter-wave polarization. The results of the analysis show that, for a number of the blazars (e.g. AO 0235+164, 3C 279, and PKS 1510-089), strong gamma-ray activity lasting more than a month can include multiple flares with different durat...
Journal of Physics: Conference Series, 2008
We report on recent near-infrared (NIR) and X-ray observations of Sagittarius A* (Sgr A*), the el... more We report on recent near-infrared (NIR) and X-ray observations of Sagittarius A* (Sgr A*), the electromagnetic manifestation of the ∼4×10 6 M⊙ super-massive black hole (SMBH) at the Galactic Center. The goal of these coordinated multi-wavelength observations is to investigate the variable emission from Sgr A* in order to obtain a better understanding of the underlying physical processes in the accretion flow/outflow. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra Xray Observatory (July 2005). We report on a polarized NIR flare synchronous to a 8×10 33 erg/s X-ray flare in July 2005, and a further flare in May 2007 that shows the highest sub-flare to flare contrast observed until now. The observations can be interpreted in the framework of a model involving a temporary disk with a short jet. In the disk component flux density variations can be explained due to hot spots on relativistic orbits around the central SMBH. The variations of the sub-structures of the May 2007 flare are interpreted as a variation of the hot spot structure due to differential rotation within the disk.
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Papers by Svetlana Jorstad