The jet of the BL Lac object PKS 0521 -365 in the near-IR : MAD adaptive optics observations
R. Falomo, E. Pian, A. Treves, G. Giovannini, T. Venturi, A. Moretti, C. Arcidiacono, J. Farinato, R. Ragazzoni, E. Diolaiti, M. Lombini, F. Tavecchio, R. Brast… 展开作者
R. Falomo, E. Pian, A. Treves, G. Giovannini, T. Venturi, A. Moretti, C. Arcidiacono, J. Farinato, R. Ragazzoni, E. Diolaiti, M. Lombini, F. Tavecchio, R. Brast, E. Marchetti, S. Tordo
备注：
accepted for publication in Astronomy and Astrophysics 9 pages. A & A 2009, in press
BL Lac objects are low--power active nuclei exhibiting a variety of peculiar properties that are caused by the presence of a relativistic jet and orientation effects.
We present here adaptive optics near-IR images at high spatial resolution of the nearby BL Lac object PKS 0521-365, which is known to display a prominent jet both at radio and optical frequencies. The observations were obtained in Ks--band using the ESO multi-conjugated adaptive optics demonstrator at the Very Large Telescope. This allowed us to obtain images with 0.1 arcsec effective resolution. We performed a detailed analysis of the jet and its related features from the near-IR images, and combined them with images previously obtained with HST in the R band and by a re-analysis of VLA radio maps. We find a remarkable similarity in the structure of the jet at radio, near-IR, and optical wavelengths. The broad--band emission of the jet knots is dominated by synchrotron radiation, while the nucleus also exhibits a significant inverse Compton component. We discovered the near-IR counterpart of the radio hotspot and found that the near-IR flux is consistent with being a synchrotron emission from radio to X-ray. The bright red object (red-tip), detached but well aligned with the jet, is well resolved in the near-IR and has a linear light profile. Since it has no radio counterpart, we propose that it is a background galaxy not associated with the jet. The new adaptive optics near-IR images and previous observations at other frequencies allow us to study the complex environment around the remarkable BL Lac object PKS 0521-365. These data exemplify the capabilities of multi conjugate adaptive optics observations of extragalactic extended sources.
We investigate the properties of the three BL Lac objects, PKS 2201+044, 3C 371 and PKS 0521-365, that exhibit prominent optical jets. We present high resolution near-IR images of the jet of the first two, obtained with an innovative adaptive-optics system (MAD) at ESO VLT telescope. Comparison of the jet in the optical, radio, NIR and X-ray bands reveals strong similarities in the morphology. A common property of these sources is the presence of broad emission lines in their optical spectra at variance with the typical featureless spectrum of the nearby BL Lac objects. Despite some resemblanc…
We report the discovery of a Broad Absorption Line (BAL) of \sim 10^4 km s-1 in width in the previously known BL Lac object PKS 0138-097, which we tentatively identified as a Mg II BAL. This is the first detection of a BAL, which is sometimes seen in powerful quasars with high accretion rates, in a BL Lac object. The BAL was clearly detected in its spectra of two epochs at a high luminosity state taken in the Sloan Digital Sky Survey (SDSS), while it disappeared in three SDSS spectra taken at a low luminosity state. The BAL and its variability pattern was also found in its historical multi-epo…
PKS 2155-304 is one of the brightest extragalactic source in the X-ray and EUV bands, and is a prototype for the BL Lac class of objects. In this paper we investigate the large-scale environment of this source using new multi-object as well as long-slit spectroscopy, together with archival spectra and optical images. We find clear evidence of a modest overdensity of galaxies at z=0.11610, consistent with previous determinations of the BL Lac redshift. The galaxy group has a radial velocity dispersion of 250km/s and a virial radius of 0.22Mpc, yielding a role-of-thumb estimate of the virial mas…
Aims: The determination of elusive redshifts of bright BL Lac objects Methods: We use the capabilities of newly available spectrograph X-Shooter at European Southern Observatory (ESO) Very Large Telescope, that combines high resolution and a large wavelength range, to obtain UVB to near-IR spectra of BL Lacs. Results: Our observations of PKS 0048-097 detect three emission lines that permit to derive a redshift z = 0.635. Moreover, a Mg II absorption system at z = 0.154 that is associated with a foreground spiral galaxy at 50 Kpc projected distance is found. Conclusions: The obtained redshift a…
Relativistic jets are a common feature of radio loud active galactic nuclei. Multifrequency observations are a unique tool to constrain their physics. We report on a detailed study of the properties of the jet of the nearby BL Lac object PKS 2201+044, one of the rare cases where the jet is detected from radio to X-rays. We use new adaptive optics near-IR observations of the source, obtained with the ESO multi-conjugated adaptive optics demonstrator (MAD) at the Very Large Telescope. These observations acquired in Ground-Layer Adaptive Optics mode are combined with images previously achieved by…
We present the result of our extensive intranight optical monitoring of the well known low-energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of $R$-band monitoring for a minimum duration of $\sim 4$ hours, on 17 nights spanning 11 years (1998-2008). Using the CCD as an N-star photometer, a detection limit of around 1% was attained for the intra-night optical variability (INOV). Remarkably, an INOV amplitude of $\geq 3%$ on hour-like time scale was not observed on any of the 17 nights, even though the likelihood of a typical LBL showing such INOV levels in a sing…
In this paper we present a fully selfconsistent SSC model with particle acceleration due to shock and stochastic acceleration (Fermi-I and Fermi-II-Processes respectively) to model the quiescent spectral energy distribution (SED) observed from PKS 2155. The simultaneous August/September 2008 multiwavelength data of H.E.S.S., Fermi, RXTE, SWIFT and ATOM give new constraints to the high-energy peak in the SED concerning its curvature. We find that, in our model, a monoenergetic injection of electrons at $γ_0 = 910$ into the model region, which are accelerated by Fermi-I- and Fermi-II-processes w…
Context. We present evidence that parsec-scale jets in BL Lac objects may be significantly distinct in kinematics from their counterparts in quasars. We argued this previously for the BL lac sources 1803+784 and 0716+714, report here a similar pattern for another well-known BL Lac object, PKS 0735+178, whose nuclear jet is found to exhibit kinematics atypical of quasars. Aims. A detailed study of the jet components' motion reveals that the standard AGN paradigm of apparent superluminal motion does not always describe the kinematics in BL Lac objects. We study 0735+178 here to augment and impro…
The BL-Lac blazar PKS 0447-439 was detected at very high energy gamma-rays by HESS following the discovery by Fermi-LAT. The lack of both emission and absorption lines in BL-Lacs make the estimation of their redshifts very difficult. Modeling the drop in gamma-ray spectra it was possible to have an estimation of redshift for PKS 0447-439 of z approx 0.2, which is compatible with the value z=0.205 reported by the identification of Ca II absorption lines in optical spectra. By the identification of a weak line of Mg II using spectra with average signal-to-noise S/N approx 80, it has been recentl…
PKS 1749+096 is a BL Lac object showing weak extended jet emission to the northeast of the compact VLBI core on parsec scales. We aim at better understanding the jet kinematics and variability of this source and finding clues that may applicable to other BL Lac objects. The jet was studied with multi-epoch multi-frequency high-resolution VLBI observations. The jet is characterized by a one-sided curved morphology at all epochs and all frequencies. The VLBI core, located at the southern end of the jet, was identified based on its spectral properties. The equipartition magnetic field of the core…
The spectral energy distribution (SED) of high-frequency peaked BL Lac objects (HBL) is characterized by two peaks: one in the UV-X-ray and one in the GeV-TeV regime. An interesting object for analyzing these broadband characteristics is PKS 2005-489, which in 2004 showed the softest TeV spectrum ever measured. In 2009, a multi-wavelength campaign has been conducted with, for the first time, simultaneous observations by H.E.S.S. (TeV), Fermi/LAT (GeV), RXTE (keV), Swift (keV, UV, optical) and ATOM (optical) to cover the two peaks of the SED. During this campaign PKS 2005-489 underwent a high s…
PKS 2155-304 is one of the brightest BL Lac object in the sky and a very well studied target from radio to TeV bands. We report on high-resolution (~ 0.12 arcsec) direct imaging of the field of PKS 2155-304 using adaptive optics near-IR observations in J and Ks bands obtained with the ESO multi-conjugate adaptive optic demonstrator (MAD) at the Very Large Telescope. These data are complemented with archival VLA images at various frequencies to investigate the properties of the close environment of the source. We characterized the faint galaxies that form the poor group associated to the target…
We report on the first simultaneous observations that cover the optical, X-ray, and high energy gamma-ray bands of the BL Lac object PKS 2155-304. The gamma-ray bands were observed for 11 days, between 25 August and 6 September 2008, jointly with the Fermi Gamma-ray Space Telescope and the H.E.S.S. atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectral energy distribution with the new generation of gamma-ray telescopes. The ATOM telescope and the RXTE and Swift observatories provided optical and X-ray coverage of the low-energy component over the same time period. The o…
VHE observations of PKS 2005-489 were made with HESS from 2004 through 2007, together with three simultaneous multi-wavelength campaigns performed with XMM-Newton and RXTE in 2004 and 2005. A strong VHE signal, ~17sigma total, is detected during the four years of HESS observations (90.3 hrs live time). The integral flux above the average analysis threshold of 400 GeV is ~3% of the Crab and varies weakly on time-scales from days to years. At X-ray energies the flux is observed to vary by more than an order of magnitude between 2004 and 2005. Strong changes in the X-ray spectrum (DeltaGamma~0.7)…
Context. We present the light curves of six BL Lac objects, PKS 0537-441, PKS 0735+17, OJ 287, PKS 2005-489, PKS 2155-304, W Comae, and of the flat spectrum radio quasar PKS 1510-089, as a part of a photometric monitoring program in the near-infrared/optical bands started in 2004. All sources are Fermi blazars. Aims. Our purpose is to investigate flux and spectral variability on short and long time scales. Systematic monitoring, independent of the activity of the source, guarantees large sample size statistics, and allows an unbiased view of different activity states on weekly or daily time sc…
One of the defining characteristics of BL Lacertae objects is their strong variability across the electromagnetic spectrum. PKS 0537-441 is one such object and is one of the most luminous blazars from radio to gamma-ray wavelengths. It was detected as a strong and highly variable source by EGRET and has been reported several times to be in an active state by Fermi . It is one of the brightest gamma-ray blazars detected in the southern sky so far. The TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) program is monitoring PKS 0537-441 at VLBI resolutions. We pr…
The high frequency peaked BL Lac PKS 2155-304 with a redshift z=0.116 was discovered 1997 in the VHE range by the University of Durham Mark 6 gamma-ray telescope in Australia with a flux corresponding to approx. 0.2 times the Crab Nebula flux. It was later observed and detected with high significance by the Southern observatories CANGAROO and H.E.S.S. establishing this source as the best studied Southern TeV blazar. Detection from the Northern hemisphere was very difficult due to challenging observation conditions under large zenith angles. In July 2006, the H.E.S.S. collaboration reported an …
In this paper we present multiband optical polarimetric observations of the VHE blazar PKS 2155-304 made simultaneously with a H.E.S.S./Fermi high-energy campaign in 2008, when the source was found to be in a low state. The intense daily coverage of the dataset allowed us to study in detail the temporal evolution of the emission and we found that the particle acceleration timescales are decoupled from the changes in the polarimetric properties of the source. We present a model in which the optical polarimetric emission originates at the polarised mm-wave core and propose an explanation for the…
Simultaneous HESS/CHANDRA/optical observations were performed on the BL Lac object PKS 2155-304 in the night of July 29-30 2006, when the source underwent its second major gamma-ray outburst in Summer 2006. This event took place about 44 hours after the July 28 outburst, known for its ultrafast variability. An unprecedented 6 to 8 hours of uninterrupted coverage was achieved, with spectra and light curves measured down to 7 and 2-minute timescales, respectively. The gamma-ray flux reached a maximum of 11x the Crab flux (>400 GeV), with rise/decay timescales of ~1 hour, plus a few smaller-ampli…
We report radio imaging and monitoring observations in the frequency range 0.235 - 2.7 GHz during the flaring mode of PKS 2155-304, one of the brightest BL Lac objects. The high sensitivity GMRT observations not only reveal extended kpc-scale jet and FRI type lobe morphology in this erstwhile `extended-core' blazar but also delineate the morphological details, thanks to its arcsec scale resolution. The radio light curve during the end phase of the outburst measured in 2008 shows high variability (8.5%) in the jet emission in the GHz range, compared to the lower core variability (3.2%) seen at …
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 ha…
Radio-bright BL Lacertae objects (BLOs) are typically variable and exhibit prominent flaring. We use a sample of 24 BLOs to get a clear idea of their flaring behavior and to find possible commonalities in their variability patterns. Our goal was to compare the results given by computational time scales and the observed variability parameters determined directly from the flux curves. Also, we wanted to find out if the BLO flares adhere to the generalized shock model. We use long-term monitoring data from 4.8, 8, 14.5, 22, 37, 90 and 230 GHz. The structure function, discrete correlation function…
We report on multiwavelength observations of the blazar PKS 0537-441 (z = 0.896) obtained from microwaves through gamma rays by SMA, REM, ATOM, Swift and Fermi during 2008 August-2010 April. Strong variability has been observed in gamma rays, with two major flaring episodes (2009 July and 2010 March) and a harder-when-brighter behaviour, quite common for FSRQs and low-synchrotron-peaked BL Lacs, in 2010 March. In the same way the SED of the source cannot be modelled by a simple synchrotron self-Compton model, as opposed to many BL Lacs, but the addition of an external Compton component of seed…
The active galactic nucleus PKS 0301-243 (z=0.266) is a high-synchrotron-peaked BL Lac object that is detected at high energies (HE, 100 MeV < E < 100 GeV) by Fermi/LAT. This paper reports on the discovery of PKS 0301-243 at very high energies (E>100 GeV) by the High Energy Stereoscopic System (H.E.S.S.) from observations between September 2009 and December 2011 for a total live time of 34.9 hours. Gamma rays above 200 GeV are detected at a significance of 9.4σ. A hint of variability at the 2.5σ level is found. An integral flux I(E > 200 GeV) = (3.3 +/- 1.1_stat +/- 0.7_syst)e-12 ph cm^-2s^-1 …
