arXiv subjects

Paul J. Wiita

Publications and source records attributed to Paul J. Wiita.

At least 91 records · Page 5Linked to original sources

arXiv2017

Origin of X-rays in the low state of the FSRQ 3C 273: Evidence of inverse Compton emission

We analyze the 2.5--10 keV X-ray spectra of the luminous quasar 3C 273 and simultaneous observations in UV wavelengths from XMM--Newton between 2000 and 2015. The lowest flux level ever was observed in 2015. The continuum emission from 3C 273 is generally best described by an absorbed power-law but during extremely low states the addition of fluorescence from the K-shell iron line improves the fit. We study the spectral evolution of the source during its extended quiescent state and also examine connections between the X-ray and ultraviolet emissions, which have been seen in some, but not all, previous work. We detect a possible anti-correlation between these two bands during the low state that characterized 3C 273 for most of this period; however, this was not present during a flaring state. A harder-when-brighter trend for the X-ray spectrum was observed in these long-term observations of 3C 273 for the first time. We suggest that the X-ray emission in 3C 273 is the result of inverse Compton scattering of soft UV seed photons (emitted from the local environment of the AGN), most likely in a thermal corona. We can explain the significant temporal variation of the spectral continuum as an outcome of changing optical depth of the comptonizing medium, along the lines of the wind-shock model proposed by Courvoisier and Camenzind (1989).

astro-ph.HE↗

arXiv2017

X-ray Intraday Variability of Five TeV Blazars with NuSTAR

We have examined 40 NuSTAR light curves (LCs) of five TeV emitting high synchrotron peaked blazars: 1ES 0229+200, Mrk 421, Mrk 501, 1ES 1959+650 and PKS 2155-304. Four of the blazars showed intraday variability in the NuSTAR energy range of 3-79 keV. Using an auto correlation function analysis we searched for intraday variability timescales in these LCs and found indications of several between 2.5 and 32.8 ks in eight LCs of Mrk 421, a timescale around 8.0 ks for one LC of Mrk 501, and timescales of 29.6 ks and 57.4 ks in two LCs of PKS 2155-304. The other two blazars' LCs do not show any evidence for intraday variability timescales shorter than the lengths of those observations, however, the data was both sparser and noisier, for them. We found positive correlations with zero lag between soft (3-10 keV) and hard (10-79 keV) bands for most of the LCs, indicating that their emissions originate from the same electron population. We examined spectral variability using a hardness ratio analysis and noticed a general "harder-when-brighter" behavior. The 22 LCs of Mrk 421 observed between July 2012 and April 2013 show that this source was in a quiescent state for an extended period of time and then underwent an unprecedented double peaked outburst while monitored on a daily basis during 10 - 16 April 2013. We briefly discuss models capable of explaining these blazar emissions.

astro-ph.HE↗

arXiv2017

Multi-wavelength variability study of the classical BL Lac object PKS 0735+178 on timescales ranging from decades to minutes

We present the results of our power spectral analysis for the BL Lac object PKS 0735+178 utilizing the Fermi-LAT survey at high-energy $γ$-rays, several ground-based optical telescopes, and single-dish radio telescopes operating at GHz frequencies. The novelty of our approach is that, by combining long-term and densely sampled intra-night light curves in the optical regime, we were able to construct for the first time the optical power spectrum of the blazar for a time domain extending from 23 years down to minutes. Our analysis reveals that: (i) the optical variability is consistent with a pure red noise, for which the power spectral density can well be approximated by a single power-law throughout the entire time domain probed; (ii) the slope of power spectral density at high-energy $γ$-rays ($\sim 1$), is significantly flatter than that found at radio and optical frequencies ($\sim 2$) within the corresponding time variability range; (iii) for the derived power spectra we did not detect any low-frequency flattening, nor do we see any evidence for cut-offs at the highest frequencies down to the noise floor levels due to measurement uncertainties. We interpret our findings in terms of a model where the blazar variability is generated by the underlying single stochastic process (at radio and optical frequencies), or a linear superposition of such processes (in the $γ$-ray regime). Along with the detailed PSD analysis, we also present the results of our extended (1998-2015) intra-night optical monitoring program and newly acquired optical photo-polarimetric data for the source.

astro-ph.HE↗

arXiv2016

Variability in Active Galactic Nuclei from Propagating Turbulent Relativistic Jets

We use the Athena hydrodynamics code to model propagating two-dimensional relativistic jets as approximations to the growth of radio-loud active galactic nuclei for various input jet velocities and jet-to-ambient matter density ratios. Using results from these simulations we estimate the changing synchrotron emission by summing the fluxes from a vertical strip of zones behind the reconfinement shock, which is nearly stationary, and from which a substantial portion of the flux variability should arise. We explore a wide range of time scales by considering two light curves from each simulation; one uses a relativistic turbulence code with bulk velocities taken from our simulations as input, while the other uses the bulk velocity data to compute fluctuations caused by variations in the Doppler boosting due to changes in the direction and the speed of the flow through all zones in the strip. We then calculate power spectral densities (PSDs) from the light curves for both turbulent and bulk velocity origins for variability. The range of the power-law slopes of the PSDs for the turbulence induced variations is -1.8 to -2.3, while for the bulk velocity produced variations this range is -2.1 to -2.9; these are in agreement with most observations. When superimposed, these power spectra span a very large range in frequency (about five decades), with the turbulent fluctuations yielding most of the shorter timescale variations and the bulk flow changes dominating the longer periods.

astro-ph.HE↗

arXiv2016

Multi-band optical variability of three TeV Blazars on Diverse Timescales

We present our optical photometric observations of three TeV blazars, PKS 1510-089, PG 1553+113 and Mrk 501 taken using two telescopes in India, one in Bulgaria, one in Greece and one in Serbia during 2012 - 2014. These observations covered a total of 95 nights with a total of 202 B filter frames, 247 images in V band, 817 in R band while 229 images were taken in the I filter. This work is focused on multi-band flux and colour variability studies of these blazars on diverse timescales which are useful in understanding the emission mechanisms. We studied the variability characteristics of above three blazars and found all to be active over our entire observational campaigns. We also searched for any correlation between the brightness of the sources and their colour indices. During the times of variability, no significant evidence for the sources to display spectral changes correlated with magnitude was found on timescales of a few months. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux variability.

astro-ph.HE↗

arXiv2015

Flux and spectral variability of the blazar PKS 2155-304 with XMM-Newton: Evidence of Particle Acceleration and Synchrotron Cooling

We have analyzed XMM-Newton observations of the high energy peaked blazar, PKS 2155-304, made on 24 May 2002 in the 0.3 - 10 keV X-ray band. These observations display a mini-flare, a nearly constant flux period and a strong flux increase. We performed a time-resolved spectral study of the data, by dividing the data into eight segments. We fitted the data with a power-law and a broken power-law model, and in some of the segments we found a noticeable spectral flattening of the source's spectrum below 10 keV. We also performed time-resolved cross-correlation analyses and detected significant hard and soft lags (for the first time in a single observation of this source) during the first and last parts of the observation, respectively. Our analysis of the spectra, the variations of photon-index with flux as well as the correlation and lags between the harder and softer X-ray bands indicate that both the particle acceleration and synchrotron cooling processes make an important contribution to the emission from this blazar. The hard lags indicate a variable acceleration process. We also estimated the magnetic field value using the soft lags. The value of the magnetic field is consistent with the values derived from the broad-band SED modeling of this source.

astro-ph.HE↗

arXiv2015

Multi-band optical variability of the Blazar S5 0716+714 in outburst state during 2014-2015

We analyzed the multi-band optical behaviour of the BL Lacertae object, S5 0716+714, during its outburst state from 2014 November - 2015 March. We took data on 23 nights at three observatories, one in India and two in Bulgaria, making quasi-simultaneous observations in B, V, R, and I bands. We measured multi-band optical fluxes, colour and spectral variations for this blazar on intraday and short timescales. The source was in a flaring state during the period analyzed and displayed intense variability in all wavelengths. R band magnitude of 11.6 was attained by the target on 18 Jan 2015, which is the brightest value ever recorded for S5 0716+714. The discrete correlation function method yielded good correlation between the bands with no measurable time lags, implying that radiation in these bands originate from the same region and by the same mechanism. We also used the structure function technique to look for characteristic timescales in the light curves. During the times of rapid variability, no evidence for the source to display spectral changes with magnitude was found on either of the timescales. The amplitude of variations tends to increase with increasing frequency with a maximum of $\sim$ 22% seen during flaring states in B band. A mild trend of larger variability amplitude as the source brightens was also found. We found the duty cycle of our source during the analyzed period to be $\sim$ 90%. We also investigated the optical spectral energy distribution of S5 0716+714 using B, V, R, and I data points for 21 nights. We briefly discuss physical mechanisms most likely responsible for its flux and spectral variations.

astro-ph.HE↗

arXiv2015

Optical and Radio Variability of BL Lacertae

We observed the prototype blazar, BL Lacertae, extensively in optical and radio bands during an active phase in the period 2010--2013 when the source showed several prominent outbursts. We searched for possible correlations and time lags between the optical and radio band flux variations using multifrequency data to learn about the mechanisms producing variability. During an active phase of BL Lacertae, we searched for possible correlations and time lags between multifrequency light curves of several optical and radio bands. We tried to estimate any possible variability timescales and inter-band lags in these bands. We performed optical observations in B, V, R and I bands from seven telescopes in Bulgaria, Georgia, Greece and India and obtained radio data at 36.8, 22.2, 14.5, 8 and 4.8 GHz frequencies from three telescopes in Ukraine, Finland and USA. Significant cross-correlations between optical and radio bands are found in our observations with a delay of cm-fluxes with respect to optical ones of ~250 days. The optical and radio light curves do not show any significant timescales of variability. BL Lacertae showed many optical 'mini-flares' on short time-scales. Variations on longer term timescales are mildly chromatic with superposition of many strong optical outbursts. In radio bands, the amplitude of variability is frequency dependent. Flux variations at higher radio frequencies lead the lower frequencies by days or weeks. The optical variations are consistent with being dominated by a geometric scenario where a region of emitting plasma moves along a helical path in a relativistic jet. The frequency dependence of the variability amplitude supports an origin of the observed variations intrinsic to the source.

astro-ph.HE↗

arXiv2015

Nature of Intra-night Optical Variability of BL Lacertae

We present the results of extensive multi-band intra-night optical monitoring of BL Lacertae during 2010--2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them observations were done quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with an average sampling interval of around 8 minutes. BL Lacertae showed significant variations on hour-like timescales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states deviates from the linear trend suggesting different jet components contributing to the emission at different times.

astro-ph.HE↗

arXiv2015

Frequency dependent core shifts and parameter estimation for the blazar 3C 454.3

We study the core shift effect in the parsec scale jet of the blazar 3C 454.3 using the 4.8 GHz - 36.8 GHz radio light curves obtained from three decades of continuous monitoring. From a piecewise Gaussian fit to each flare, time lags $Δt$ between the observation frequencies $ν$ and spectral indices $α$ based on peak amplitudes $A$ are determined. From the fit $Δt \propto ν^{1/k_r}$, $k_r = 1.10 \pm 0.18$ indicating equipartition between the magnetic field energy density and the particle energy density. From the fit $A \propto ν^α$, $α$ is in the range $-0.24$ to $1.52$. A mean magnetic field strength at 1 pc, $B_1 = 0.5 \pm 0.2$ G, and at the core, $B_{\rm core} = 46 \pm 16$ mG, are inferred, consistent with previous estimates. The measure of core position offset is $Ω_{rν} = 6.4 \pm 2.8$ pc GHz$^{1/k_r}$ when averaged over all frequency pairs. Based on the statistical trend shown by the measured core radius $r_{\rm core}$ as a function of $ν$, we infer that the synchrotron opacity model may not be valid for all cases. A Fourier periodogram analysis yields power law slopes in the range $-1.6$ to $-3.5$ describing the power spectral density shape and gives bend timescales in the range $0.52 - 0.66~$yr. This result, and both positive and negative $α$, indicate that the flares originate from multiple shocks in a small region. Important objectives met in our study include: the demonstration of the computational efficiency and statistical basis of the piecewise Gaussian fit; consistency with previously reported results; evidence for the core shift dependence on observation frequency and its utility in jet diagnostics in the region close to the resolving limit of very long baseline interferometry observations.

astro-ph.HE↗

arXiv2015

Multi-band variability in the blazar 3C 273 with XMM-Newton

We have undertaken a nearly simultaneous optical/UV and X-ray variability study of the flat spectrum radio quasar, 3C 273 using data available from the XMM$-$Newton satellite mission from June 2000 to July 2012. Here we focus on the multi-wavelength flux variability on both intra-day and long time scales of this very well known radio-loud source. We found high flux variability over long time scales in all bands for which observations were made. The optical/UV variability amplitude was more than twice than that in the X-ray bands. There is some frequency dependence of the variability in optical/UV bands in the sense that the variability amplitude increases with increasing frequency; however, the X-ray emissions disagree with this trend as the variability amplitude decreases from soft to hard X-ray bands. On intra-day time scales 3C 273 showed small amplitude variability in X-ray bands. A hardness ratio analysis in the X-ray regime indicates that the particle acceleration mechanism dominates the cooling mechanism during most of the ~12 year span of these observations.

astro-ph.GA↗

arXiv2015

Multi-band optical-NIR variability of blazars on diverse timescales

To search for optical variability on a wide range of timescales, we have carried out photometric monitoring of two flat spectrum radio quasars, 3C 454.3 and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting remarkably high activity and pronounced variability at all wavelengths. CCD magnitudes in B, V, R and I pass-bands were determined for $\sim$ 7000 new optical observations from 114 nights made during 2011 - 2014, with an average length of $\sim$ 4 h each, at seven optical telescopes: four in Bulgaria, one in Greece, and two in India. We measured multiband optical flux and colour variations on diverse timescales. Discrete correlation functions were computed among B, V, R, and I observations, to search for any time delays. We found weak correlations in some cases with no significant time lags. The structure function method was used to estimate any characteristic time-scales of variability. We also investigated the spectral energy distribution of the three blazars using B, V, R, I, J and K pass-band data. We found that the sources almost always follows a bluer-when-brighter trend. We discuss possible physical causes of the observed spectral variability.

astro-ph.GA↗

arXiv2015

Modeling the Emission from Turbulent Relativistic Jets in Active Galactic Nuclei

We present a numerical model developed to calculate observed fluxes of relativistic jets in active galactic nuclei. The observed flux of each turbulent eddy is dependent upon its variable Doppler boosting factor, computed as a function of the relativistic sum of the individual eddy and bulk jet velocities and our viewing angle to the jet. The total observed flux is found by integrating the radiation from the eddies over the turbulent spectrum. We consider jets that contain turbulent eddies that have either standard Kolmogorov or recently derived relativistic turbulence spectra. We also account for the time delays in receiving the emission of the eddies due to their different simulated positions in the jet, as well as due to the varying beaming directions as they turn over. We examine these theoretical light curves and compute power spectral densities (PSDs) for a range of viewing angles, bulk velocities of the jet, and turbulent velocities. These PSD slopes depend significantly on the turbulent velocity and are essentially independent of viewing angle and bulk velocity. The flux variations produced in the simulations for realistic values of the parameters tested are consistent with the types of variations observed in radio-loud AGN as, for example, recently measured with the Kepler satellite, as long as the turbulent velocities are not too high.

astro-ph.HE↗

arXiv2014

Black Hole spin dependence of general relativistic multi-transonic accretion close to the horizon

We introduce a novel formalism to investigate the role of the spin angular momentum of astrophysical black holes in influencing the behaviour of low angular momentum general relativistic accretion. We propose a metric independent analysis of axisymmetric general relativistic flow, and consequently formulate the space and time dependent equations describing the general relativistic hydrodynamic accretion flow in the Kerr metric. The associated stationary critical solutions for such flow equations are provided and the stability of the stationary transonic configuration is examined using an elegant linear perturbation technique. We examine the properties of infalling material for both prograde and retrograde accretion as a function of the Kerr parameter at extremely close proximity to the event horizon. Our formalism can be used to identify a new spectral signature of black hole spin, and has the potential of performing the black hole shadow imaging corresponding to the low angular momentum accretion flow.

astro-ph.HE↗

arXiv2014

Spectral energy distributions of the BL Lac PKS 2155-304 from XMM-Newton

We have used all 20 archival XMM-Newton observations of PKS 2155-304 with simultaneous X-ray and UV/optical data to study its long term flux and spectral variability. We find significant variations, in all bands, on time-scales of years with an rms amplitude of ~ 35-45 per cent, through the optical/UV variations are not correlated with those in the X-ray. We constructed SEDs that span more than three orders of magnitude in frequency and we first fitted them with a log-parabolic model; such models have been applied many times in the past for this, and other, blazars. These fits were poor, so we then examined combined power-law and log-parabolic fits that are improvements. These models indicate that the optical/UV and X-ray flux variations are mainly driven by model normalization variations, but the X-ray band flux is also affected by spectral variations, as parametrized with the model 'curvature' parameter, b. Overall, the energy at which the emitted power is maximum correlates positively with the total flux. As the spectrum shifts to higher frequencies, the spectral 'curvature' increase, in contrast to what is expected if a single log-parabolic model were an acceptable representation of the broad band SEDs. Our results suggest that the optical/UV and X-ray emissions in this source may arise from different lepton populations.

astro-ph.HE↗

arXiv2014

Extragalactic radio sources with sharply inverted spectrum at metre wavelengths

We present the first results of a systematic search for the rare extragalactic radio sources showing an inverted (integrated) spectrum, with spectral index $α\ge +2.0$, a previously unexplored spectral domain. The search is expected to yield strong candidates for $α\ge +2.5$, for which the standard synchrotron self-absorption (characterized by a single power-law energy distribution of relativistic electron population) would not be a plausible explanation, even in an ideal case of a perfectly homogeneous source of incoherent synchrotron radiation. Such sharply inverted spectra, if found, would require alternative explanations, e.g., free-free absorption, or non-standard energy distribution of relativistic electrons which differs from a power-law (e.g., Maxwellian). The search was carried out by comparing two sensitive low-frequency radio surveys made with sub-arcminute resolution, namely, the WISH survey at 352 MHz and TGSS/DR5 at 150 MHz. The overlap region between these two surveys contains 7056 WISH sources classified as `single' and brighter than 100 mJy at 352 MHz. We focus here on the seven of these sources for which we find $α> +2.0$. Two of these are undetected at 150 MHz and are particularly good candidates for $α> +2.5$. Five of the seven sources exhibit a `Gigahertz-Peaked-Spectrum' (GPS).

astro-ph.HE↗

arXiv2014

Investigating Active Galactic Nuclei Variability Using Combined Multi-Quarter Kepler Data

We have used photometry from the Kepler satellite to characterize the variability of four radio-loud active galactic nuclei (AGN) on timescales from years to minutes. The Kepler satellite produced nearly continuous high precision data sets which provided better temporal coverage than possible with ground based observations. We have now accumulated eleven quarters of data, eight of which were reported in our previous paper. In addition to constructing power spectral densities (PSDs) and characterizing the variability of the last three quarters, we have linked together the individual quarters using a multiplicative scaling process, providing data sets spanning ~2.8 years with >98% coverage at a 30 minute sampling rate. We compute PSDs on these connected data sets that yield power law slopes at low frequencies in the approximate range of -1.5 to -2.0, with white noise seen at higher frequencies. These PSDs are similar to those of both the individual quarters and to those of ground based optical observations of other AGN. We also have explored a PSD binning method intended to reduce a bias toward shallow slope fits by evenly distributing the points within the PSDs. This tends to steepen the computed PSD slopes, especially when the low frequencies are relatively poorly fit. We detected flares lasting several days in which the brightness increased by ~15-20% in one object, as well a smaller flare in another. Two AGN showed only small, ~1-2%, fluctuations in brightness.

astro-ph.GA↗

arXiv2013

Anti-correlated Optical Flux and Polarization Variability in BL Lac

We present the results of photometric (V band) and polarimetric observations of the blazar BL Lac during 2008--2010 using TRISPEC attached to the KANATA 1.5-m telescope in Japan. The data reveal a great deal of variability ranging from days to months with detection of strong variations in fractional polarization. The V band flux strongly anti-correlates with the degree of polarization during the first of two observing seasons but not during the second. The direction of the electric vector, however, remained roughly constant during all our observations. These results are consistent with a model with at least two emission regions being present, with the more variable component having a polarization direction nearly perpendicular to that of the relatively quiescent region so that a rising flux can produce a decline in degree of polarization. We also computed models involving helical jet structures and single transverse shocks in jets and show that they might also be able to agree with the anti-correlations between flux and fractional polarization.

astro-ph.HE↗