Detection of the host galaxy of S5 0716+714 Nilsson, K.; Pursimo, T.; Sillanpää, A. ...
Astronomy and astrophysics (Berlin),
08/2008, Letnik:
487, Številka:
2
Journal Article
Recenzirano
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We have acquired a deep i-band image of the BL Lacertae object S5 0716+714 while the target was in an low optical state. Due to the faintness of the nucleus, we were able to detect the underlying ...host galaxy. The host galaxy is measured to have an I-band magnitude of $17.5 \pm 0.5$ and an effective radius of ($2.7 \pm 0.8$) arcsec. Using the host galaxy as a “standard candle”, we derive $z = 0.31 \pm 0.08$ (1σ error) for the host galaxy of S5 0716+714. This redshift is consistent with the redshift $z = 0.26$ determined by spectroscopy for 3 galaxies close to S5 0716+714. The effective radius at $z = 0.31$ would be $12 \pm 4$ kpc, which is consistent with values obtained for BL Lac host galaxies. An optical spectrum acquired during the same epoch shows no identifiable spectral lines.
Tests of Einstein's general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton's theory. ...Binary pulsars provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system. In the absence of gravitational wave emission the outburst would have happened 20 days later.
Context. Photometric monitoring of active galactic nuclei is often complicated by the presence of a strong host galaxy component, which adds unwanted flux to the measurement and introduces a ...seeing-dependence to the flux that can plaque e.g. microvariability studies. We are currently monitoring a sample of 24 TeV candidate BL Lacertae objects, many of which exhibit a prominent host galaxy component, using differential aperture photometry. Aims. In order to study our light curves free from the above effects, we have derived the host galaxy flux in differential aperture photometry as a function of aperture radius and FWHM for 20 resolved sources in our sample. Methods. We created accurate surface brightness models of the targets and any significant nearby sources using high-resolution R-band imaging obtained at the Nordic Optical Telescope (NOT) and performed differential aperture photometry of the models over a grid of aperture radii and FWHM values. Results. The results are given as correction tables, that list the fluxes (in mJy) of all “contaminating” sources (host galaxy + significant nearby objects) as a function of aperture radius and FWHM. We found that the derived fluxes depend strongly on aperture radius, but the FWHM has only a minor effect (a few percent). We also discuss the implications of our findings to optical monitoring programs and potential sources of error in our derived fluxes. During this work we have also constructed new calibration star sequences for 9 objects and present the finding charts and calibrated magnitudes.
Abstract
The X-ray variability of the BL Lacertae source Mrk 501 was studied during 11.5 yr of monitoring with Swift. Here, we report the results of this study pertaining to the epoch of 2014 ...March–October, when our target showed the most powerful and long-lasting X-ray flaring activity. This epoch was characterized by X-ray flares varying in amplitude by factors of 2–5 on time-scales of a few weeks or shorter. We detected 35 instances of the intraday variability, sometimes occurring within the 1 ks observational runs. The X-ray flux was generally correlated with the TeV flux, while the 0.3–300 GeV and optical-UV fluxes did not show a significant correlation. Some notable incidences of more complicated variability patterns could also be recognized, indicating that the high-energy emission in Mrk 501 arose from an emission region more complex than a single zone. The best fits of the 0.3–10 keV spectra were mainly obtained using the logparabola model. Strong spectral variability was detected, affecting the slope but not the curvature of the spectrum. In strong flares, the spectral index became harder than 1.70. The spectral evolution was characterized by a harder-when-brighter behaviour, shifting the peak of the spectral energy distribution by about 20 keV that happens rarely in blazars.
We compare the variability properties of very high energy gamma-ray emitting BL Lac objects in the optical and radio bands. We use variability information to distinguish multiple emission components ...in the jet, to be used as a guidance for spectral energy distribution modelling. Our sample includes 32 objects in the northern sky for which we have data for at least two years in both bands. We use optical R-band data from the Tuorla blazar monitoring program and 15 GHz radio data from the Owens Valley Radio Observatory blazar monitoring program. We estimate the variability amplitudes using the intrinsic modulation index, and study the time-domain connection by cross-correlating the optical and radio light curves assuming power law power spectral density. Our sample objects are in general more variable in the optical than radio. We find correlated flares in about half of the objects, and correlated long-term trends in more than 40% of the objects. In these objects we estimate that at least 10−50% of the optical emission originates in the same emission region as the radio, while the other half is due to faster variations not seen in the radio. This implies that simple single-zone spectral energy distribution models are not adequate for many of these objects.
OJ 287 is a BL Lac object at redshift z= 0.306 that has shown double-peaked bursts at regular intervals of ∼12 yr during the last ∼40 yr. We analyse optical photopolarimetric monitoring data from ...2005 to 2009, during which the latest double-peaked outburst occurred. The aim of this study is twofold: firstly, we aim to analyse variability patterns and statistical properties of the optical polarization light curve. We find a strong preferred position angle in optical polarization. The preferred position angle can be explained by separating the jet emission into two components: an optical polarization core and chaotic jet emission. The optical polarization core is stable on time-scales of years and can be explained as emission from an underlying quiescent jet component. The chaotic jet emission sometimes exhibits a circular movement in the Stokes plane. We find six such events, all on the time-scales of 10–20 d. We interpret these events as a shock front moving forwards and backwards in the jet, swiping through a helical magnetic field. Secondly, we use our data to assess different binary black hole models proposed to explain the regularly appearing double-peaked bursts in OJ 287. We compose a list of requirements a model has to fulfil to explain the mysterious behaviour observed in OJ 287. The list includes not only characteristics of the light curve but also other properties of OJ 287, such as the black hole mass and restrictions on accretion flow properties. We rate all existing models using this list and conclude that none of the models is able to explain all observations. We discuss possible new explanations and propose a new approach to understanding OJ 287. We suggest that both the double-peaked bursts and the evolution of the optical polarization position angle could be explained as a sign of resonant accretion of magnetic field lines, a ‘magnetic breathing’ of the disc.
We present ∼2000 polarimetric and ∼3000 photometric observations of 15 γ-ray bright blazars over a period of 936 days (2008-10-11 to 2012-10-26) using data from the Tuorla blazar monitoring program ...(KVA DIPOL) and Liverpool Telescope (LT) RINGO2 polarimeters (supplemented with data from SkyCamZ (LT) and Fermi-LAT γ-ray data). In 11 out of 15 sources we identify a total of 19 electric vector position angle (EVPA) rotations and 95 flaring episodes. We group the sources into subclasses based on their broad-band spectral characteristics and compare their observed optical and γ-ray properties. We find that (1) the optical magnitude and γ-ray flux are positively correlated, (2) EVPA rotations can occur in any blazar subclass, four sources show rotations that go in one direction and immediately rotate back, (3) we see no difference in the γ-ray flaring rates in the sample; flares can occur during and outside of rotations with no preference for this behaviour, (4) the average degree of polarization (DoP), optical magnitude and γ-ray flux are lower during an EVPA rotation compared with during non-rotation and the distribution of the DoP during EVPA rotations is not drawn from the same parent sample as the distribution outside rotations, (5) the number of observed flaring events and optical polarization rotations are correlated, however we find no strong evidence for a temporal association between individual flares and rotations and (6) the maximum observed DoP increases from ∼10 per cent to ∼30 per cent to ∼40 per cent for subclasses with synchrotron peaks at high, intermediate and low frequencies, respectively.
In an attempt to constrain and understand the emission mechanism of γ-rays, we perform a cross-correlation analysis of 15 blazars using light curves in millimetre, optical and γ-rays. We use discrete ...correlation function and consider only correlations significant at the 99 per cent level. A strong correlation was found between 37 and 95 GHz with a near-zero time delay in most of the sources, and ∼1 month or longer in the rest. A similar result was obtained between the optical and γ-ray bands. Of the 15 sources, less than 50 per cent showed a strong correlation between the millimetre and γ-ray or millimetre and optical bands. The primary reason for the lack of statistically significant correlation is the absence of a major outburst in the millimetre bands of most of the sources during the 2.5 yr time period investigated in our study. This may indicate that only the long-term variations or large flares are correlated between these bands. The variability of the sources at every waveband was also inspected using fractional rms variability (F
var ). The F
var displays an increase with frequency reaching its maximum in the γ-rays.
In 1995, a binary black hole model was proposed for the quasar OJ287, where the smaller secondary black hole impacts the accretion disk of the primary black hole twice during its 12 yr orbit and ...causes a double peak of optical outbursts. The model predicted four major outbursts and one minor outburst during the period 1996-2010. All five have now been observed. In this paper, we ask how accurate the predictions were. We use the latest optical observations from Tuorla Observatory and the KVA telescope at La Palma together with previously published data to construct a light curve for this period. We average the data in 0.04 yr bins, and subtract the observed flux from the 1995 model flux at each bin. We find that the residuals are small: they are well described by random noise of amplitude 1.4 mJy. This level is small compared with the amplitudes of the major outbursts, 5-7 mJy. Ignoring the noise, the binary model explains the optical data remarkably well.
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