Disk-Jet Connection in Agns and Microquasars: The Possibility of Thermal Flares in the Center

We discuss the possibility of thermal flares in centers of AGNs and microquasars. We present preliminary results of an ongoing study trying to assess the feasibility of a hypothesis suggesting that certain flares observed in these sources originate in the very centers of the systems and not in the relativistic jets. Using a simple toy model we reproduce optical flares with lightcurves very similar to those observed in the sources. The model suits especially well those cases where only the latter peak of a double-peaked optical flare has a radio counterpart.Comment: 6 pages, 1 figure, to appear in the proceedings of the HEPRO II conference, Buenos Aires, October 26-30 200

Radio variability of active galactic nuclei : analysis of long-term multifrequency data

A large sample of Active Galactic Nuclei (AGN) has been monitored on a regular basis in Metsähovi Radio Observatory since the 1980s. These observations form an extensive database of about 100 brightest Northern compact extragalactic objects at high radio frequencies. In this thesis the Metsähovi observations are complemented with lower frequency radio data from the University of Michigan Radio Astronomy Observatory, and with higher frequency data from the literature. The main focus in this thesis is to understand the long-term radio variability behaviour of AGN by using various methods. Several statistical methods, and their capability to detect variability timescales, are examined in detail. In addition, the flux curves are examined visually to study the flare characteristics, and to verify the timescales obtained. The results show that variability behaviour of AGN is complex, and multiple characteristic timescales are common. Flares in the radio regime are long-lasting, on average, 2.5 years at 37 GHz. In addition, large outbursts occur quite rarely, only once in every four to six years. None of the sources were seen to exhibit strict periodicities. These results together show that long-term monitoring is essential in understanding the true behaviour of AGN. In this thesis, differences between various statistical timeseries analysis methods are investigated in detail for the first time using a large sample of sources and long datasets. Especially the wavelet method has never before been used to study a large sample of AGN at high radio frequencies. A useful property of wavelets is that they show when the timescale has been present and how it has changed. Thus, it is ideal for studying sources which are not strictly periodic and which change their behaviour. The long-term total flux density observations are also used to calculate the Doppler boosting factors, Lorentz factors and viewing angles of the jets. Quasars are found to be more Doppler boosted and to have faster jets than BL Lacertae objects. The various parameters obtained in this thesis, describing the characteristics of AGN, can be further used to study the physical processes and the relationships between different emission regions in these sources

A connection between γ\gamma-ray and parsec-scale radio flares in the blazar 3C 273

We present a comprehensive 5-43 GHz VLBA study of the blazar 3C 273 initiated after an onset of a strong $\gamma$-ray flare in this source. We have analyzed the kinematics of new-born components, light curves, and position of the apparent core to pinpoint the location of the $\gamma$-ray emission. Estimated location of the $\gamma$-ray emission zone is close to the jet apex, 2 pc to 7 pc upstream from the observed 7 mm core. This is supported by ejection of a new component. The apparent core position was found to be inversely proportional to frequency. The brightness temperature in the 7 mm core reached values up to at least $10^{13}$ K during the flare. This supports the dominance of particle energy density over that of magnetic field in the 7 mm core. Particle density increased during the radio flare at the apparent jet base, affecting synchrotron opacity. This manifested itself as an apparent core shuttle along the jet during the 7 mm flare. It is also shown that a region where optical depth decreases from $\tau\sim1$ to $\tau<<1$ spans over several parsecs along the jet. The jet bulk flow speed estimated at the level of 12c on the basis of time lags between 7 mm light curves of stationary jet features is 1.5 times higher than that derived from VLBI apparent kinematics analysis.Comment: Accepted for publication in MNRAS. 17 pages, 15 figures, 10 tables, with supplementary materials attache

Constraints on Blazar Jet Conditions During Gamma-Ray Flaring from Radiative Transfer Modeling

As part of a program to investigate jet flow conditions during GeV gamma-ray flares detected by Fermi, we are using UMRAO multi-frequency, centimeter-band total flux density and linear polarization monitoring observations to constrain radiative transfer models incorporating propagating shocks orientated at an arbitrary angle to the flow direction. We describe the characteristics of the model, illustrate how the data are used to constrain the models, and present results for three program sources with diverse characteristics: PKS 0420-01, OJ 287, and 1156+295. The modeling of the observed spectral behavior yields information on the sense, strength and orientation of the shocks producing the radio-band flaring; on the energy distribution of the radiating particles; and on the observer's viewing angle with respect to the jet independent of VLBI data. We present evidence that, while a random component dominates the jet magnetic field, a distinguishing feature of those radio events with an associated gamma-ray flare is the presence of a weak but non-negligible ordered magnetic field component along the jet axis.Comment: 6 pages, 4 figures. To appear in the proceedings of "The Innermost Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spai

ALMA/NICER observations of GRS 1915+105 indicate a return to a hard state

Context. GRS 1915 +105 is a transient black hole X-ray binary consistently emitting 10-100% of the Eddington luminosity in the X-ray band over the last three decades until mid-2018 when the source luminosity suddenly decreased by an order of magnitude. This phase was followed by a change to a state with even lower average X-ray fluxes never seen before during the outburst but presenting renewed flaring activity at di fferent wavelengths, albeit with mean fluxes still in decline.Aims. GRS 1915 +105 has the longest orbital period known among low-mass X-ray binaries, the largest accretion disk size, and therefore the largest mass supply for accretion. The high inclination of the disk allows the study of geometrical e ffects of the accretion flow such as changes in the height-to-radius ratio or the e ffect of accretion disk winds on the intrinsic emission that is expected during the outburst decay. In addition, the transient jet is expected to change to a compact, self-absorbed, steady jet.Methods. We conducted two full polarization Atacama Large Millimeter Array observations to study the jet properties during the outburst decay by analyzing the spectral, polarization, and intra-epoch variability for both observation epochs. In addition, we analyzed almost daily Neutron Star Interior Composition Explorer pointing observations, modeling X-ray power spectral densities, spectral energy distributions, and light curves with a physically motivated model to follow the changing accretion disk properties throughout the outburst decay and relating them to the jet emission.Results. We show that the X-ray and millimeter (mm) spectral, timing, and polarization properties are consistent with those of a typical decaying X-ray binary outburst and that GRS 1915 +105 has descended into the low-luminosity hard X-ray state. The jet emission in the mm is consistent with a compact, steady jet with similar to 1% linear polarization, and the magnetic field is likely aligned with the jet position angle. Relating the mm emission to the X-ray emission reveals that the source has changed from a higher radio /X-ray correlation index to a lower one; L-radio proportional to L-X(0.6)

Simultaneous long-term monitoring of LS I +61°303 by OVRO and Fermi-LAT

Previous long-term monitorings of the γ-ray-loud X-ray binary LS I +61°303 have revealed the presence of a long-term modulation of ∼4.5 yr. After 9 yr of simultaneous monitoring of LS I +61°303 by the Owens Valley Radio Observatory and the Fermi-LAT, two cycles of the long-term period are now available. Here we perform timing analysis on the radio and the γ-ray light curves. We confirm the presence of previously detected periodicities at both radio and GeV γ-ray wavelengths. Moreover, we discover an offset of the long-term modulation between radio and γ-ray data which could imply different locations of the radio (15 GHz) and GeV emission along the precessing jet

Simultaneous long-term monitoring of LS I +61{\deg}303 by OVRO and Fermi-LAT

Previous long-term monitorings of the gamma-ray-loud X-ray binary LS I +61{\deg}303 have revealed the presence of a long-term modulation of ~4.5 years. After nine years of simultaneous monitoring of LS I +61{\deg}303 by the Owens Valley Radio Observatory and the Fermi-LAT, two cycles of the long-term period are now available. Here we perform timing-analysis on the radio and the gamma-ray light curves. We confirm the presence of previously detected periodicities at both radio and GeV gamma-ray wavelengths. Moreover, we discover an offset of the long-term modulation between radio and gamma-ray data which could imply different locations of the radio (15 GHz) and GeV emission along the precessing jet.Comment: 8 pages, 7 figures, 2 tables, accepted for publication in MNRA

Simultaneous long-term monitoring of LS I +61°303 by OVRO and Fermi-LAT

Previous long-term monitorings of the γ-ray-loud X-ray binary LS I +61°303 have revealed the presence of a long-term modulation of ∼4.5 yr. After 9 yr of simultaneous monitoring of LS I +61°303 by the Owens Valley Radio Observatory and the Fermi-LAT, two cycles of the long-term period are now available. Here we perform timing analysis on the radio and the γ-ray light curves. We confirm the presence of previously detected periodicities at both radio and GeV γ-ray wavelengths. Moreover, we discover an offset of the long-term modulation between radio and γ-ray data which could imply different locations of the radio (15 GHz) and GeV emission along the precessing jet

Diagnosing Magnetic Field Geometry in Blazar Jets Using Multi-Frequency, Centimeter-Band Polarimetry and Radiative Transfer Modeling

We use multi-frequency linear polarization observations from the University of Michigan blazar program (UMRAO), in combination with radiative transfer simulations of emission from a relativistic jet, to investigate the time-dependent flow conditions, including magnetic field geometry, in an example blazar OT 081. We adopt a scenario incorporating relativistic shocks during flaring, and both ordered axial and helical magnetic field components and magnetic turbulence in the underlying flow; these constituents are consistent with the observed periods of ordered behavior in the polarization intermixed with stochastic variations. The simulations are able to reproduce the global features of the observed light curves, including amplitude and spectral evolution of the linear polarization, during four time periods spanning 25 years. From the simulations, we identify the signature of a weak-to-strong helical magnetic field on the polarization, but conclude that a dominant helical magnetic field is not consistent with the UMRAO polarization data. The modeling identifies time-dependent changes in the ratio of the ordered-to-turbulent magnetic field, and changes in the flow direction and Lorentz factor. These suggest the presence of jet-like structures within a broad envelope seen at different orientations