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We study simultaneous soft ($0.7 - 7$ keV) and hard ($7 - 20$ keV) X-ray light curves at a total of eight epochs during $2016 - 2019$ of two TeV blazars Mrk 421 and 1ES 1959+650 observed by the SXT and LAXPC instruments onboard AstroSat. The light curves are $45 - 450$ ks long and may be sampled with time bins as short as $600 - 800$ sec with high signal to noise ratio. The blazars show a harder when brighter trend at all epochs. Discrete cross-correlation functions indicate that the hard and soft X-ray variability are strongly correlated. The time lag is consistent with zero in some epochs, and indicates hard or soft lag of a few hours in the rest. In the leptonic model of blazar emission, soft lag may be due to slower radiative cooling of lower energy electrons while hard lag may be caused by gradual acceleration of the high energy electrons emitting at the hard X-ray band. Assuming the above scenario and the value of the Doppler factor ($\delta$) to be $10 - 20$, the hard and soft lags may be used to estimate the magnetic field to be $\sim 0.1$ Gauss and the acceleration parameter to be $\sim 10^4$ in the emission region. Due to the availability of the high time resolution ($\sim$ minutes to hours) light curves from AstroSat, the value of the illusive acceleration parameter could be estimated, which provides a stringent constraint on the theories of particle acceleration in blazar jets., Comment: 13 Pages, 4 Figures, 3 Tables, Accepted on July 10, 2023 in Monthly Notices of the Royal Astronomical Society (MNRAS)

We present the results of studying the accretion disk vs jet power for a large fraction of all the blazars detected by the Fermi Gamma-Ray Space Telescope. The disk power is inferred from the emission line luminosities obtained from published results. As indicators of jet power, we use low frequency radio luminosity from the extended jet, maximum speed of radio knots observed in the VLBA monitoring of the pc-scale jets, kinetic energy of electrons in the jet deduced from the best-fit theoretical models of their spectral energy distribution, and gamma-ray luminosity with and without beaming correction. We obtain a significant correlation in most of those cases. However, we find that the correlations are often driven by the common redshift dependence of the compared quantities. In order to remove the redshift bias and probe the intrinsic correlation between the disk and jet power, we compute the partial correlation coefficient as well as the correlation in small redshift bins, and find that the intrinsic disk-jet correlation is still present but weaker. In the cases, in which the common redshift dependence does not affect the result, we find that blazars do not exhibit high jet power for low disk luminosities while there are both high and low jet power for high disk luminosities. This result indicates that a powerful disk is a necessary but not sufficient condition to produce a powerful jet., Comment: 11 pages, 5 figures, Published in Physical Review D in the 15 September 2022 issue (Vol. 106, No. 6)

We have studied the nature and origin of the soft X-ray excess detected in the interesting changing-look AGN (CLAGN) Mrk~590 using two decades of multi-wavelength observations from \xmm{}, \suzaku{}, \swift{} and \nustar{}. In the light of vanishing soft excess in this CLAGN, we test two models, "the warm Comptonization" and "the ionized disk reflection" using extensive UV/X-ray spectral analysis. Our main findings are: (1) the soft X-ray excess emission, last observed in 2004, vanished in 2011, and never reappeared in any of the later observations, (2) we detected a significant variability ($\sim300\%$) in the observed optical-UV and power-law flux between observations with the lowest state ($L_{\rm bol} = 4.4\times 10^{43}\, erg\, s^{-1}$, in 2016) and the highest state ($L_{\rm bol} = 1.2\times 10^{44}\, erg\, s^{-1}$, in 2018), (3) the UV and power-law fluxes follow same temporal pattern, (4) the photon index showed a significant variation ($\Gamma=1.88^{+0.02}_{-0.08}$ and $\Gamma=1.58^{+0.02}_{-0.03}$ in 2002 and 2021 respectively) between observations, (5) no Compton hump was detected in the source spectra but a narrow Fe$K_{\alpha}$ line is present in all observations, (6) we detected a high-energy cut-off in power-law continuum ($92^{+55}_{-25} \rm keV$ and $60^{+10}_{-08} \rm keV$) with the latest \nustar{} observations, (7) the warm Comptonization model needs an additional diskbb component to describe the source UV bump, (8) there is no correlation between the Eddington rate and the soft excess as found in other changing-look AGNs. We conclude that given the spectral variability in UV/X-rays, the ionized disk reflection or the warm Comptonization models may not be adequate to describe the vanishing soft excess feature observed in Mrk~590., Comment: Resubmitted to ApJ following minor comments from the referee

It is well-known that the $\gamma$-ray emission in blazars originate in the relativistic jet pointed at the observers. However, it is not clear whether the exact location of the GeV emission is less than a pc from the central engine, such that it may receive sufficient amount of photons from the broad line region (BLR) or farther out at 1-100 pc range. The former assumption has been successfully used to model the spectral energy distribution of many blazars. However, simultaneous detection of TeV $\gamma$-rays along with GeV outbursts in some cases indicate that the emission region must be outside the BLR. In addition, GeV outbursts have sometimes been observed to be simultaneous with the passing of a disturbance through the so called "VLBI core," which is located tens of pc away from the central engine. Hence, the exact location of $\gamma$-ray emission remains ambiguous. Here we present a method we have developed to constrain the location of the emission region. We identify simultaneous months-timescale GeV and optical outbursts in the light curves spanning over 8 years of a sample of eleven blazars. Using theoretical jet emission models we show that the energy ratio of simultaneous optical and GeV outbursts is strongly dependent on the location of the emission region. Comparing the energy dissipation of the observed multi-wavelength outbursts and that of the simulated flares in our theoretical model we find that most of the above outbursts originate beyond the BLR at approximately a few pc from the central engine.

Recent analysis of blazar variability has revealed a proportionality between the mean flux and the root mean squared (rms) fluctuations about the mean flux. Although such rms-flux relation has been previously observed in the accretion disc/corona variability of X-ray binaries and Seyfert galaxies, and has been extensively modelled, its emergence in the jet light curves of blazars calls for a revised theoretical understanding of this feature. In this work, we analyse the time variability properties of realistic multi-wavelength jet light curves, simulated in the context of a simplified version of the internal shocks model, particularly focusing on the rms-flux relation. These shocks accelerate the jet electrons to relativistic energies, which then cool radiatively via synchrotron and inverse-Compton processes. We find that the rms-flux relation may be consistently recovered in the cases, in which the shocks have different amplitudes based on the speed of the colliding blobs generating them as opposed to all shocks having the same amplitude. We observe that the slope of the rms-flux relation depends on the wavelength at which the variability is observed and the energy distribution of the electron population. We find that the accretion disc and the jet variability are anti-correlated, with the latter lagging that of the disc. Our results provide crucial constraints on the physical properties of the jet, and the mode of connection through which the accretion disc and jet may be related., Comment: 14 pages, 7 figures, accepted for publication in MNRAS

We study the multi-wavelength variability of the blazar Mrk 421 at minutes to days timescales using simultaneous data at $\gamma$-rays from Fermi, 0.7-20 keV energies from AstroSat, and optical and near-infrared (NIR) wavelengths from ground-based observatories. We compute the shortest variability timescales at all of the above wavebands and find its value to be ~1.1 ks at the hard X-ray energies and increasingly longer at soft X-rays, optical and NIR wavelengths as well as at the GeV energies. We estimate the value of the magnetic field to be 0.5 Gauss and the maximum Lorentz factor of the emitting electrons ~1.6 x $10^5$ assuming that synchrotron radiation cooling drives the shortest variability timescale. Blazars vary at a large range of timescales often from minutes to years. These results, as obtained here from the very short end of the range of variability timescales of blazars, are a confirmation of the leptonic scenario and in particular the synchrotron origin of the X-ray emission from Mrk 421 by relativistic electrons of Lorentz factor as high as $10^5$. This particular mode of confirmation has been possible using minutes to days timescale variability data obtained from AstroSat and simultaneous multi-wavelength observations., Comment: 8 pages, 2 figures, accepted for publication in the Journal of Astrophysics and Astronomy

Half of the 2020 Nobel Prize is awarded for discovering a super-massive black hole at the centre of our Galaxy. One of the first indications of the existence of a black hole at the centre of a galaxy was found by Jennison and Das Gupta in 1953 while carrying on a radio observation of the source Cygnus A. Mrinal Das Gupta, who was doing his PhD at Manchester University at the time of this discovery, spent the major part of his professional career at Calcutta University. We give an outline of Das Gupta's life and explain the scientific significance of his discovery., Comment: Invited paper submitted to Science and Culture

We analyze X-ray light curves of the blazars Mrk 421, PKS 2155-304, and 3C 273 using observations by the Soft X-ray Telescope on board AstroSat and archival XMM-Newton data. We use light curves of length 30-90 ks each from 3-4 epochs for all three blazars. We apply the autoregressive integrated moving average (ARIMA) model which indicates the variability is consistent with short memory processes for most of the epochs. We show that the power spectral density (PSD) of the X-ray variability of the individual blazars are consistent within uncertainties across the epochs. This implies that the construction of broadband PSD using light curves from different epochs is accurate. However, using certain properties of the variance of the light curves and its segments, we show that the blazars exhibit hints of non-stationarity beyond that due to their characteristic red noise nature in some of those observations. We find a linear relationship between the root-mean-squared amplitude of variability at shorter timescales and the mean flux level at longer timescales for light curves of Mrk 421 across epochs separated by decades as well as light curves spanning 5 days and $\sim$10 yr. The presence of flux-rms relation over very different timescales may imply that, similar to the X-ray binaries and Seyfert galaxies, longer and shorter timescale variability are connected in blazars., Comment: 12 pages, 4 figures. Accepted for publication in the Astrophysical Journal

We present cross-correlation studies of gamma-ray (0.1-300 GeV), X-ray (0.2-10 keV) and optical (R-band) variability of a sample of 26 blazars during 2008-2016. The light curves are from Fermi-LAT, Swift-XRT, and the Yale-SMARTS blazar monitoring program. We stack the discrete cross-correlation functions of the blazars such that the features that are consistently present in a large fraction of the sample become more prominent in the final result. We repeat the same analysis for two subgroups, namely, low synchrotron peaked (LSP) and high synchrotron peaked (HSP) blazars. We find that, on average, the variability at multiple bands is correlated, with a time lag consistent with zero in both subgroups. We describe this correlation with a leptonic model of non-thermal emission from blazar jets. By comparing the model results with those from the actual data we find that the inter-band cross-correlations are consistent with an emission region of size nearly 0.1 pc within the broad line region for LSP blazars. We rule out large changes of magnetic field (> 0.5 Gauss) across the emission region or small values of magnetic field (e.g. 0.2 Gauss) for this population. We also find that the observed variability of the HSP blazars can be explained if the emission region is much larger than the distance to the broad line region from the central black hole.

The power spectral density (PSD) of the X-ray emission variability from the accretion disc-corona region of black hole X-ray binaries and active galactic nuclei has a broken power law shape with a characteristic break time-scale. If the disc and the jet are connected, the jet variability may also contain a characteristic time-scale related to that of the disc-corona. Recent observations of the blazar Mrk 421 have confirmed the broken power law shape of the PSD of its jet X-ray variability. We model the time variability of a blazar, in which emitting particles are assumed to be accelerated by successive shock waves flowing down the jet with a varying inter-shock time-scale. We investigate the possible relation between the characteristic time-scales in the disc and jet variability based on the above model, along with mathematically and physically simulated disc variability. We find that both the PSD of the jet and disc variability may have a broken power law shape but the break time-scales are not related in general except only in systems with a small range of BH mass. The break in the jet and the disc PSD are connected to the interval between large amplitude outbursts in the jet (inter-shock time-scale) and to the viscous time-scale in the disc, respectively. In frequency bands where multiple emission processes are involved or emission is from lower energy particles, the break in the PSD may not be prominent enough for detection., Comment: 9 pages, 6 figures. Accepted for publication in the Monthly Notices of the Royal Astronomical Society

We compare the rise and decay timescales of $\sim$200 long-term ($\sim$weeks-months) GeV and R-band outbursts and $\sim$25 short-term ($\sim$hr-day) GeV flares in a sample of 10 blazars using light curves from the Fermi-LAT and the Yale/SMARTS monitoring project. We find that most of the long-term outbursts are symmetric, indicating that the observed variability is dominated by the crossing timescale of a disturbance, e.g., a shock. A larger fraction of short-term flares are asymmetric with an approximately equal fraction of longer and shorter decay than rise timescale. We employ the MUlti-ZOne Radiation Feedback (MUZORF) model to interpret the above results. We find that the outbursts with slow rise times indicate a gradual acceleration of the particles to GeV energy. A change in the bulk Lorentz factor of the plasma or the width of the shocked region can lead to an increase of the cooling time causing a faster rise than decay time. Parameters such as the luminosity or the distance of the broad line region (BLR) affects the cooling time strongly if a single emission mechanism, e.g., external Compton scattering of BLR photons is considered but may not if other mechanisms, e.g., synchrotron self-compton and external Compton scattering of the torus photon are included. This work carries out a systematic study of the symmetry of flares, which can be used to estimate relevant geometric and physical parameters of blazar jets in the context of the MUZORF model., Comment: 22 pages, 14 figures. Accepted for publication in MNRAS

We analyze X-ray light curves of the blazar Mrk 421 obtained from the Soft X-ray Imaging Telescope and the Large Area X-Ray Proportional Counter instrument onboard the Indian space telescope $AstroSat$ and archival observations from $Swift$. We show that the X-ray power spectral density (PSD) is a piece-wise power-law with a break, i.e., the index becomes more negative below a characteristic "break-timescale". Galactic black hole X-ray binaries and Seyfert galaxies exhibit a similar characteristic timescale in their X-ray variability that is proportional to their respective black hole mass. X-rays in these objects are produced in the accretion disk or corona. Hence, such a timescale is believed to be linked to the properties of the accretion flow. Any relation observed between events in the accretion disk and those in the jet can be used to characterize the disk-jet connection. However, evidence of such link have been scarce and indirect. Mrk 421 is a BL Lac object which has a prominent jet pointed towards us and a weak disk emission, and it is assumed that most of its X-rays are generated in the jet. Hence, existence of the break in its X-ray PSD may indicate that changes in the accretion disk, which may be the source of the break timescale are translating into the jet, where the X-rays are produced., Comment: 7 pages, 3 figures, 1 table. Accepted for publication in the Astrophysical Journal Letters

We perform time-dependent, spatially-resolved simulations of blazar emission to evaluate several flaring scenarios related to magnetic-field amplification and enhanced particle acceleration. The code explicitly accounts for light-travel-time effects and is applied to flares observed in the flat spectrum radio quasar (FSRQ) PKS 0208-512, which show optical/{\gamma}-ray correlation at some times, but orphan optical flares at other times. Changes in both the magnetic field and the particle acceleration efficiency are explored as causes of flares. Generally, external Compton emission appears to describe the available data better than a synchrotron self-Compton scenario, and in particular orphan optical flares are difficult to produce in the SSC framework. X-ray soft-excesses, {\gamma}-ray spectral hardening, and the detections at very high energies of certain FSRQs during flares find natural explanations in the EC scenario with particle acceleration change. Likewise, optical flares with/without {\gamma}-ray counterparts can be explained by different allocations of energy between the magnetization and particle acceleration, which may be related to the orientation of the magnetic field relative to the jet flow. We also calculate the degree of linear polarization and polarization angle as a function of time for a jet with helical magnetic field. Tightening of the magnetic helix immediately downstream of the jet perturbations, where flares occur, can be sufficient to explain the increases in the degree of polarization and a rotation by >= 180 degree of the observed polarization angle, if light-travel-time effects are properly considered., Comment: 12 pages, 9 figures. Accepted for publication in MNRAS

We examine ~10 years of photometric data and find that the black hole X-ray binary V4641 Sgr has two optical states, passive and active, during X-ray quiescence. The passive state is dominated by ellipsoidal variations and is stable in the shape and variability of the light curve. The active state is brighter and more variable. Emission during the active state varies over the course of the orbital period and is redder than the companion star. These optical/infrared states last for weeks or months. V4641 Sgr spends approximately 85% of X-ray quiescence in the passive state and 15% in the active. We analyze passive colors and spectroscopy of V4641 Sgr and show that they are consistent with a reddened B9III star (with E(B-V) = 0.37 +/- 0.19) with little or no contribution from the accretion disk. We use X-ray observations with an updated ephemeris to place an upper limit on the duration of an X-ray eclipse of <8.3 deg in phase (~1.6 hours). High resolution spectroscopy yields a greatly improved measurement of the rotational velocity of the companion star of V_rot_sin(i) = 100.9 +/- 0.8 km s^-1. We fit ellipsoidal models to the passive state data and find an inclination angle of i = 72.3 +/- 4.1 deg, a mass ratio of Q = 2.2 +/- 0.2, and component masses for the system of M_BH = 6.4 +/- 0.6 M_sun and M_2 = 2.9 +/- 0.4 M_sun. Using these values we calculate an updated distance to V4641 Sgr of 6.2 +/- 0.7 kpc., Comment: 21 pages, 16 figures, 7 tables. Accepted by Astrophysical Journal

We report the results of a new 60 ks Chandra X-ray Observatory Advanced CCD Imaging Spectrometer S-array (ACIS-S) observation of the reddened, radio-selected, highly polarized `FeLoBAL' quasar FIRST J1556+3517. We investigated a number of models of varied sophistication to fit the 531-photon spectrum. These models ranged from simple power laws to power laws absorbed by hydrogen gas in differing ionization states and degrees of partial covering. Preferred fits indicate that the intrinsic X-ray flux is consistent with that expected for quasars of similarly high luminosity, i.e., an intrinsic, dereddened and unabsorbed optical to X-ray spectral index of -1.7. We cannot tightly constrain the intrinsic X-ray power-law slope, but find indications that it is flat (photon index Gamma = 1.7 or flatter at a >99% confidence for a neutral hydrogen absorber model). Absorption is present, with a column density a few times 10^23 cm^-2, with both partially ionized models and partially covering neutral hydrogen models providing good fits. We present several lines of argument that suggest the fraction of X-ray emissions associated with the radio jet is not large. We combine our Chandra data with observations from the literature to construct the spectral energy distribution of FIRST J1556+3517 from radio to X-ray energies. We make corrections for Doppler beaming for the pole-on radio jet, optical dust reddening, and X-ray absorption, in order to recover a probable intrinsic spectrum. The quasar FIRST J1556+3517 seems to be an intrinsically normal radio-quiet quasar with a reddened optical/UV spectrum, a Doppler-boosted but intrinsically weak radio jet, and an X-ray absorber not dissimilar from that of other broad absorption line quasars., Comment: to be published in MNRAS

We present multi-epoch optical observations of the blazar 3C 454.3 (z = 0.859) from 2008 August through 2011 December, using the SMARTS Consortium 1.5m+RCSpectrograph and 1.3m+ANDICAM in Cerro Tololo, Chile. The spectra reveal that the broad optical emission lines Mg II, H-beta and H-gamma are far less variable than the optical or gamma-ray continuum. Although, the gamma-rays varied by a factor of 100 above the EGRET era flux, the lines generally vary by a factor of 2 or less. Smaller variations in the gamma-ray flux did not produce significant variation in any of the observed emission lines. Therefore, to first order, the ionizing flux from the disk changes only slowly during large variations of the jet. However, two exceptions in the response of the broad emission lines are reported during the largest gamma-ray flares in 2009 December and 2010 November, when significant deviations from the mean line flux in H-gamma and Mg II were observed. H-gamma showed a maximum 3-sigma and 4-sigma deviation in each flare, respectively, corresponding to a factor of 1.7 and 2.5 increase in flux. Mg II showed a 2-sigma deviation in both flares; no variation was detected in H-beta during either flare. These significant deviations from the mean line flux also coincide with 7mm core ejections reported previously (Jorstad et al. 2012). The correlation of the increased emission line flux with mm core ejections, and gamma-ray, optical and UV flares suggests that the broad line region extends beyond the gamma-emitting region during the 2009 and 2010 flares., Comment: 12 pages, 6 figures, Accepted to the Astrophysical Journal

Recent multiwavelength observations of PKS 0208-512 by SMARTS, Fermi, and Swift revealed that gamma-ray and optical light curves of this flat spectrum radio quasars are highly correlated, but with an exception of one large optical flare having no corresponding gamma-ray activity or even detection. On the other hand, recent advances in SNRs observations and plasma simulations both reveal that magnetic field downstream of astrophysical shocks can be largely amplified beyond simple shock compression. These amplifications, along with their associated particle acceleration, might contribute to blazar flares, including the peculiar flare of PKS 0208-512. Using our time dependent multizone blazar emission code, we evaluate several scenarios that may represent such phenomena. This code combines Monte Carlo method that tracks the radiative processes including inverse Compton scattering, and Fokker-Planck equation that follows the cooling and acceleration of particles. It is a comprehensive time dependent code that fully takes into account the light travel time effects. In this study, both the changes of the magnetic field and acceleration efficiency are explored as the cause of blazar flares. Under these assumption, synchrotron self-Compton and external Compton scenarios produce distinct features that favor the external Compton scenario. The optical flares with/without gamma-ray counterparts can be explained by different allocations of energy between the magnetization and particle acceleration, which in turn can be affected by the relative orientation between the magnetic field and the shock flow. We compare the details of the observations and simulation, and highlight what implications this study has on our understanding of relativistic jets., Comment: 6 pages. To appear in the proceeding of 'The Innermost Regions of Relativistic Jets and Their Magnetic Fields', Granada, Spain

The flat spectrum radio quasar (FSRQ) PKS 0208-512 underwent three outbursts at the optical-near-infrared (OIR) wavelengths during 2008-2011. The second OIR outburst did not have a gamma-ray counterpart despite being comparable in brightness and temporal extent to the other two. We model the time variable spectral energy distribution of PKS 0208-512 during those three flaring episodes with leptonic models to investigate the physical mechanism that can produce this anomalous flare. We show that the redder-when-brighter spectral trend in the OIR bands can be explained by the superposition of a fixed thermal component from the accretion disk and a synchrotron component of fixed shape and variable normalization. We estimate the accretion disk luminosity at L_d ~8 X 10^45 erg/s. Using the observed variability timescale in the OIR band t_{var,obs} ~2 d and the X-ray luminosity L_X ~3.5 X 10^45 erg/s, we constrain the location of the emitting region to distance scales that are broadly comparable with the dusty torus. We show that variations in the Compton dominance parameter by a factor of ~4 --- which may result in the anomalous outburst --- can be relatively easily accounted for by moderate variations in the magnetic field strength or the location of the emission region. Since such variations appear to be rare among FSRQs, we propose that most gamma-ray/OIR flares in these objects are produced in jet regions where the magnetic field and external photon fields vary similarly with distance along the jet, e.g., u_B ~u_ext ~r^{-2}., Comment: 6 pages, 2 figures, 1 table. Accepted for publication in the Astrophysical Journal Letters. The published ApJL version is slightly abridged due to word limit. This is a follow-up of http://xxx.lanl.gov/abs/1212.2629

The Yale/SMARTS optical-near-IR monitoring program has followed the variations in emission of the Fermi-LAT monitored blazars in the southern sky with closely spaced observations since 2008. We report the discovery of an optical-near-IR (OIR) outburst with no accompanying gamma-rays in the blazar PKS 0208-512, one of the targets of this program. While the source undergoes three outbursts of 1 mag or more at OIR wavelengths lasting for longer than 3 months during 2008-2011, only interval 1 and 3 have corresponding bright phases in GeV energies lasting longer than 1 month. The OIR outburst during interval 2 is comparable in brightness and temporal extent to the OIR flares during intervals 1 and 3 which do have gamma-ray counterparts. Gamma-ray and OIR variability are very well-correlated in most cases in the Fermi blazars and the lack of correlation in this case is anomalous. By analyzing the gamma-ray, OIR, and supporting multi-wavelength variability data in details, we speculate that the location of the outburst in the jet during interval 2 was closer to the black hole where the jet is more compact and the magnetic field strength is higher, and the bulk Lorentz factor of the material in the jet is smaller. These result in a much lower Compton dominance and no observable gamma-ray outburst during interval 2., Comment: 6 pages, 3 figures. 2012 Fermi Symposium proceedings - eConf C121028

We report the discovery of an anomalous flare in a bright blazar, namely, PKS 0208-512, one of the targets of the Yale/SMARTS optical-near-infrared (OIR) monitoring program of Fermi blazars. We identify three intervals during which PKS 0208-512 undergoes outbursts at OIR wavelengths lasting for longer than 3 months. Its brightness increases and then decreases again by at least 1 magnitude in these intervals. In contrast, the source undergoes bright phases in GeV energies lasting for longer than 1 month during intervals 1 and 3 only. The OIR outburst during interval 2 is comparable in brightness and temporal extent to the OIR flares during intervals 1 and 3 which do have gamma-ray counterparts. By analyzing the gamma-ray, OIR, and supporting multi-wavelength variability data in details, we speculate that the OIR outburst during interval 2 was caused by a change in the magnetic field without any change in the total number of emitting electrons or Doppler factor of the emitting region. Alternatively, it is possible that the location of the outburst in the jet during interval 2 was closer to the black hole where the jet is more compact and the bulk Lorentz factor of the material in the jet is smaller. We also discuss the complex OIR spectral behavior during these three intervals., Comment: 7 pages, 4 figures, Accepted for publication in ApJ Letters

We present the optical/infra-red lightcurve (O/IR) of the black hole X-ray binary GX 339-4 collected at the SMARTS 1.3m telescope from 2002 to 2010. During this time the source has undergone numerous state transitions including hard-to-soft state transitions when we see large changes in the near-IR flux accompanied by modest changes in optical flux, and three rebrightening events in 2003, 2005 and 2007 after GX 339-4 transitioned from the soft state to the hard. All but one outburst show similar behavior in the X-ray hardness-intensity diagram. We show that the O/IR colors follow two distinct tracks that reflect either the hard or soft X-ray state of the source. Thus, either of these two X-ray states can be inferred from O/IR observations alone. From these correlations we have constructed spectral energy distributions of the soft and hard states. During the hard state, the near-IR data have the same spectral slope as simultaneous radio data when GX 339-4 was in a bright optical state, implying that the near-IR is dominated by a non-thermal source, most likely originating from jets. Non-thermal emission dominates the near-IR bands during the hard state at all but the faintest optical states, and the fraction of non-thermal emission increases with increasing optical brightness. The spectral slope of the optical bands indicate that a heated thermal source is present during both the soft and hard X-ray states, even when GX 339-4 is at its faintest optical state. We have conducted a timing analysis of the light curve for the hard and soft states and find no evidence of a characteristic timescale within the range of 4-230 days., Comment: Accepted for publication in AJ, Table 3 can be viewed at http://www.astro.yale.edu/buxton/GX339/

We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 111 between 2004 and 2010 at X-ray (2.4--10 keV), optical (R band), and radio (14.5, 37, and 230 GHz) wave bands, as well as multi-epoch imaging with the Very Long Baseline Array (VLBA) at 43 GHz. Over the six years of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. This shows a clear connection between the radiative state near the black hole, where the X-rays are produced, and events in the jet. The X-ray continuum flux and Fe line intensity are strongly correlated, with a time lag shorter than 90 days and consistent with zero. This implies that the Fe line is generated within 90 light-days of the source of the X-ray continuum. The power spectral density function of X-ray variations contains a break, with steeper slope at shorter timescales. The break timescale of 13 (+12,-6) days is commensurate with scaling according to the mass of the central black hole based on observations of Seyfert galaxies and black hole X-ray binaries (BHXRBs). The data are consistent with the standard paradigm, in which the X-rays are predominantly produced by inverse Compton scattering of thermal optical/UV seed photons from the accretion disk by a distribution of hot electrons --- the corona --- situated near the disk. Most of the optical emission is generated in the accretion disk due to reprocessing of the X-ray emission. The relationships that we have uncovered between the accretion disk and the jet in 3C 111, as well as in the FR I radio galaxy 3C 120 in a previous paper, support the paradigm that active galactic nuclei and Galactic BHXRBs are fundamentally similar, with characteristic time and size scales proportional to the mass of the central black hole, Comment: Accepted for publication in ApJ. 18 pages, 17 figures, 11 tables (full machine readable data-tables online in ApJ website)

We present the time variability properties of a sample of six blazars, AO 0235+164, 3C 273, 3C 279, PKS 1510-089, PKS 2155-304, and 3C 454.3, at optical-IR as well as gamma-ray energies. These observations were carried out as a part of the Yale/SMARTS program during 2008-2010 that has followed the variations in emission of the bright Fermi-LAT-monitored blazars in the southern sky with closely-spaced observations at BVRJK bands. We find the optical/IR time variability properties of these blazars to be remarkably similar to those at the gamma-ray energies. The power spectral density (PSD) functions of the R-band variability of all six blazars are fit well by simple power-law functions with negative slope such that there is higher amplitude variability on longer timescales. No clear break is identified in the PSD of any of the sources. The average slope of the PSD of R-band variability of these blazars is similar to what was found by the Fermi team for the gamma-ray variability of a larger sample of bright blazars. This is consistent with leptonic models where the optical-IR and gamma-ray emission is generated by the same population of electrons through synchrotron and inverse-Compton processes, respectively. The prominent flares present in the optical-IR as well as the gamma-ray light curves of these blazars are predominantly symmetric, i.e., have similar rise and decay timescales, indicating that the long-term variability is dominated by the crossing time of radiation or a disturbance through the emission region rather than by the acceleration or energy-loss timescales of the radiating electrons. In the blazar 3C 454.3, which has the highest-quality light curves, the location of a large gamma-ray outburst during 2009 December is consistent with being in the jet at ~18 pc from the central engine. This poses strong constraints on the models of high energy emission in the jets of blazars., Comment: Accepted for publication in ApJ. 15 pages (ApJ format), 6 tables, 11 figures. More discussion, explanation of analysis, and references have been added. Major conclusions remain unchanged

We analyze the behavior of the parsec-scale jet of the quasar 3C~454.3 during pronounced flaring activity in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors $\Gamma>$10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the $R$-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long-lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 Autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and $\gamma$-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the mm-wave core lies at the end of the jet's acceleration and collimation zone., Comment: 57 pages, 23 figures, 8 tables (submitted to ApJ)

We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 120 between 2002 and 2007 at X-ray, optical, and radio wave bands, as well as imaging with the Very Long Baseline Array (VLBA). Over the 5 yr of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. Consistent with this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the radio variations. This implies that, in this radio galaxy, the radiative state of accretion disk plus corona system, where the X-rays are produced, has a direct effect on the events in the jet, where the radio emission originates. The X-ray power spectral density of 3C 120 shows a break, with steeper slope at shorter timescale and the break timescale is commensurate with the mass of the central black hole based on observations of Seyfert galaxies and black hole X-ray binaries. These findings provide support for the paradigm that black hole X-ray binaries and active galactic nuclei are fundamentally similar systems, with characteristic time and size scales linearly proportional to the mass of the central black hole. The X-ray and optical variations are strongly correlated in 3C 120, which implies that the optical emission in this object arises from the same general region as the X-rays, i.e., in the accretion disk-corona system. We numerically model multi-wavelength light curves of 3C 120 from such a system with the optical-UV emission produced in the disk and the X-rays generated by scattering of thermal photons by hot electrons in the corona. From the comparison of the temporal properties of the model light curves to that of the observed variability, we constrain the physical size of the corona and the distances of the emitting regions from the central BH., Comment: Accepted for publication in the Astrophysical Journal. 28 pages, 21 figures, 2 tables