9 research outputs found
Multifrequency variability of the blazar AO 0235+164. The WEBT campaign in 2004-2005 and long-term SED analysis
A huge multiwavelength campaign targeting the blazar AO 0235+164 was
organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the
variability properties of the source. Monitoring observations were carried out
at cm and mm wavelengths, and in the near-IR and optical bands, while three
pointings by the XMM-Newton satellite provided information on the X-ray and UV
emission. We present the data acquired during the second observing season,
2004-2005, by 27 radio-to-optical telescopes. They reveal an increased near-IR
and optical activity with respect to the previous season. Increased variability
is also found at the higher radio frequencies, down to 15 GHz, but not at the
lower ones. The radio (and optical) outburst predicted to peak around
February-March 2004 on the basis of the previously observed 5-6 yr
quasi-periodicity did not occur. The analysis of the optical light curves
reveals now a longer characteristic time scale of 8 yr, which is also present
in the radio data. The spectral energy distributions corresponding to the
XMM-Newton observations performed during the WEBT campaign are compared with
those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray
spectra can be described in terms of an extra component, which appears also
when the source is faint through a hard UV spectrum and a curvature of the
X-ray spectrum. Finally, there might be a correlation between the X-ray and
optical bright states with a long time delay of about 5 yr, which would require
a geometrical interpretation.Comment: 14 pages, 10 figures (8 included in the text and 2 PNG files), in
press for A&
Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71. I. Simultaneous broadband observations during November 2003
Some intra-day variable, compact extra-galactic radio sources show brightness
temperatures severely exceeding 10^{12} K, the limit set by catastrophic
inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation.
The violation of the IC limit, possible under non-stationary conditions, would
lead to IC avalanches in the soft-gamma-ray energy band during transient
periods. For the first time, broadband signatures of possible IC catastrophes
were searched for in S5 0716+71. A multifrequency observing campaign targetting
S5 0716+71 was carried out in November 2003 under the framework of the European
Network for the Investigation of Galactic nuclei through Multifrequency
Analysis (ENIGMA) together with a campaign by the Whole Earth Blazar Telescope
(WEBT), involving a pointing by the soft-gamma-ray satellite INTEGRAL, optical,
near-infrared, sub-millimeter, millimeter, radio, and Very Long Baseline Array
(VLBA) monitoring. S5 0716+71 was very bright at radio frequencies and in a
rather faint optical state during the INTEGRAL pointing; significant inter-day
and low intra-day variability was recorded in the radio regime, while typical
fast variability features were observed in the optical band. No correlation was
found between the radio and optical emission. The source was not detected by
INTEGRAL, neither by the X-ray monitor JEM-X nor by the gamma-ray imager ISGRI,
but upper limits to the source emission in the 3-200 keV energy band were
estimated. A brightness temperature Tb>2.1x10^{14} K was inferred from the
radio variability, but no corresponding signatures of IC avalanches were
recorded at higher energies. The absence of IC-catastrophe signatures provides
either a lower limit delta>8 to the Doppler factor affecting the radio emission
or strong constraints for modelling of the Compton catastrophes in S5 0716+71.Comment: 15 pages, 3 EPS figures, 3 tables, to appear in A&
Multifrequency variability of the blazar AO 0235+164 the WEBT campaign in 2004-2005 and long-term SED analysis
A huge multiwavelength campaign targeting the blazar AO 0235+164 was
organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the
variability properties of the source. Monitoring observations were carried out
at cm and mm wavelengths, and in the near-IR and optical bands, while three
pointings by the XMM-Newton satellite provided information on the X-ray and UV
emission. We present the data acquired during the second observing season,
2004-2005, by 27 radio-to-optical telescopes. They reveal an increased near-IR
and optical activity with respect to the previous season. Increased variability
is also found at the higher radio frequencies, down to 15 GHz, but not at the
lower ones. The radio (and optical) outburst predicted to peak around
February-March 2004 on the basis of the previously observed 5-6 yr
quasi-periodicity did not occur. The analysis of the optical light curves
reveals now a longer characteristic time scale of 8 yr, which is also present
in the radio data. The spectral energy distributions corresponding to the
XMM-Newton observations performed during the WEBT campaign are compared with
those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray
spectra can be described in terms of an extra component, which appears also
when the source is faint through a hard UV spectrum and a curvature of the
X-ray spectrum. Finally, there might be a correlation between the X-ray and
optical bright states with a long time delay of about 5 yr, which would require
a geometrical interpretation
Evolution of BL Lacertae host galaxies
We present and discuss deep, high-resolution I-band images of 24 BL Lac
objects between and 1.3 taken with the Nordic Optical
Telescope (NOT) and the ESO-NTT and VLT telescopes.
In addition, new redshifts for the BL Lac objects PKS 0406+121, PKS 0426–380
and PKS 1519–273 are reported.
In 17/24 (71%) of the BL Lac objects, we detected an
underlying nebulosity, in 11/17 for the first time. We assigned the underlying
nebulosity to the BL Lac host galaxy in 11 cases spanning the redshift range . The remaining 6 BL Lac objects have either intervening galaxies (S4
0218+35, PKS 0426–380), no redshift (MH 2133–449) or are probably
misidentified (Q 0230+3429, B2 0937+26, MS 2347.4+1924).
Restricting ourselves to the 11 BL Lac objects ( ),
where a core and host galaxy was
detected, we find that their host galaxies are luminous ()
and large ( kpc). They are on average about 0.6 mag brighter
than BL Lac host galaxies at indicative of evolution,
whereas their half-light radii are similar.
By combining our data with literature data at low-redshift and applying
evolutionary models to them, we show that the properties of the host galaxies
of BL Lac objects up to are compatible with passively evolving
elliptical galaxies formed at a redshift of
(13 Gyrs ago in our adopted cosmology).
Our results, however, are affected by an unavoidable luminosity bias and need
to be confirmed. Future prospects are described.
If they could be confirmed, host galaxies of low-luminosity radio-loud
AGN (BL Lac/FR I) have very similar properties
to the hosts of radio-quiet QSOs and high-luminosity radio-loud AGN
(radio-loud QSO/FR II) over a wide redshift range.
This supports the picture of the “Grand Unification”
in which AGN activity is a transient phenomenon in galaxy evolution
[Accepted Article Manuscript Version (Postprint)] Testing the Inverse-Compton Catastrophe Scenario in the Intra-Day Variable Blazar s5 0716+71. I. Simultaneous Broadband Observations During November 2003
Context. Some intra-day variable, compact extra-galactic radio sources show brightness temperatures severely exceeding 1012 K, the limit set by catastrophic inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation. The violation of the IC limit, actually possible under non-stationary conditions, would lead to IC avalanches in the soft-γ-ray energy band during transient periods.Aims. For the first time, broadband signatures of possible IC catastrophes were searched for in a prototypical source, S5 0716+71.Methods. A multifrequency observing campaign targetting S5 0716+71 was carried out during November 06-20, 2003. The observations, organized under the framework of the European Network for the Investigation of Galactic nuclei through Multifrequency Analysis (ENIGMA) together with a campaign by the Whole Earth Blazar Telescope (WEBT), involved a pointing by the soft-γ-ray satellite INTEGRAL, optical, near-infrared, sub-millimeter, millimeter, radio, as well as Very Long Baseline Array (VLBA) monitoring.Results. S5 0716+71 was very bright at radio frequencies and in a rather faint optical state () during the INTEGRAL pointing; significant inter-day and low intra-day variability was recorded in the radio regime, while typical fast variability features were observed in the optical band. No obvious correlation was found between the radio and optical emission. The source was not detected by INTEGRAL, neither by the X-ray monitor JEM-X nor by the γ-ray imager ISGRI, but upper limits to the source emission in the 3-200 keV energy band were estimated. A brightness temperature K (violating the IC limit) was inferred from the variability observed in the radio regime, but no corresponding signatures of IC avalanches were recorded at higher energies.Conclusions. In the most plausible scenario of negligible contribution of the interstellar scintillation to the observed radio variability, the absence of the signatures of IC catastrophes provides either a lower limit to the Doppler factor affecting the radio emission or strong constraints for modelling of the Compton-catastrophe scenario in S5 0716+71