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تسجيل مستخدم جديدThe multiwavelength variability of 3C 273
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We present an update of 3C 273s database hosted by the ISDC, completed with data from radio to gamma-ray observations over the last 10 years. We use this large data set to study the multiwavelength properties of this quasar,especially focussing on its variability behaviour. We study the amplitude of the variations and the maximum variability time scales across the broad-band spectrum and correlate the light curves in different bands, specifically with the X-rays, to search for possible connections between the emission at different energies. 3C 273 shows variability at all frequencies, with amplitudes and time scales strongly depending on the energy and being the signatures of the different emission mechanisms. The variability properties of the X-ray band imply the presence of either two separate components (possibly a Seyfert-like and a blazar-like) or at least two parameters with distinct timing properties to account for the X-ray emission below and above ~20 keV. The dominant hard X-ray emission is most probably not due to electrons accelerated by the shock waves in the jet as their variability does not correlate with the flaring millimeter emission, but seems to be associated to long-timescale variations in the optical. This optical component is consistent with being optically thin synchrotron radiation from the base of the jet and the hard X-rays would be produced through inverse Compton processes (SSC and/or EC) by the same electron population. We show evidence that this synchrotron component extends from the optical to the near-infrared domain, where it is blended by emission of heated dust that we find to be located within about 1 light-year from the ultraviolet source.
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We report the results of a 3-weeks multi-wavelength campaign on the flat spectrum radio quasar 3C 273 carried out with the AGILE gamma-ray mission, covering the 30 MeV -50 GeV and 18-60 keV, the REM observatory (covering the near-IR and optical), Swi
ft (near-UV/Optical, 0.2-10 keV and 15-50 keV), INTEGRAL (3 - 200 keV) and Rossi XTE (2-12 keV). This is the first observational campaign including gamma-ray data, after the last EGRET observations, more than 8 years ago. Our study was carried out using simultaneous light curves of the source flux from all the involved instruments, in the different energy ranges, in search for correlated variability. Then a time-resolved SED was used for a detailed physical modelling of the emission mechanisms. The source was detected in gamma-rays only in the second week of our campaign. We found indication of a possible anti-correlation between the emission at gamma-rays and at soft and hard X-rays, supported by the complete set of instruments. Instead, optical data do not show short term variability. Only in two EGRET observations (in 1993 and 1997) 3C 273 showed intra-observation variability in gamma-rays. The SED with almost simultaneous data, partially covers the regions of the synchrotron emission, the big blue bump, and the inverse-Compton. The soft X-ray emission is consistent with combined SSC and EC mechanisms, while hard X and gamma-ray emissions are compatible with EC from thermal photons of the disk. The time evolution of the SED is well interpreted and modelled in terms of an acceleration episode of the electrons population, leading to a shift in the IC peak towards higher energies.
Detailed investigation of broadband flux variability in the blazar 3C 273 allows us to probe the location and size of emission regions and their physical conditions. We report the results on correlation studies of the flaring activity observed betwee
n 2008 and 2012. The observed broadband variations were investigated using the structure function and the discrete correlation function, and power spectral density analysis (PSD) methods. The PSD analysis showed that the optical/IR light curve slopes are consistent with the slope of white noise processes, while, the PSD slopes at radio, X-ray and gamma-ray energies are consistent with red-noise processes. The flux variations at gamma-ray and mm-radio bands are found to be significantly correlated. Using the estimated time lag of (110pm27) days between gamma-ray and radio light curves, we constrained the location of the gamma-ray emission region at a de-projected distance of 1.2pm0.9 pc from the jet apex. Flux variations at X-ray bands were found to have a significant correlation with variations at both radio and gamma-rays energies. The correlation between X-rays and gamma-rays light curves suggests presence of two components responsible for the X-ray emission. A negative time lag of -(50pm20) days, where the X-rays are leading the emission, suggests X-rays are emitted closer to the jet apex from a compact region at a distance of ~(0.5pm0.4) pc from the jet apex. A positive time lag of (110pm20) days suggests jet-base origin of the other X-ray component at ~(4--5)~pc from the jet apex. The flux variations at radio frequencies were found to be well correlated with each other such that the variations at higher frequencies are leading the lower frequencies, which could be expected in the standard shock-in-jet model.
We have analysed the first 15 months of Fermi/LAT data of the radio loud quasar 3C 273. Intense gamma-ray activity has been detected, showing an average flux of F(> 100 MeV) = 1.4e-6 ph/cm^2/s, with a peak at F(> 100 MeV) = 5.6e-6 ph/cm^2/s detected
during a flare in September 2009. Together with the brightening of the source, a possible hardening of the gamma-ray spectrum is observed, pointing to a shift of the inverse Compton peak toward higher energies than the 1-10 MeV range in which 3C 273 inverse Compton emission is typically observed to peak. During the 15 months of observations the photon index is measured to vary between 2.4 and 3.3, with an average value of 2.78 +/- 0.03. When compared to the observations at other wavelengths, the gamma-rays show the largest flux variations and we discuss the possibility that two different components are responsible for the inverse Compton hump emission below and above the MeV peak.
Aims. The high energy spectrum of 3C 273 is usually understood in terms of inverse-Compton emission in a relativistic leptonic jet. This model predicts variability patterns and delays that could be tested with simultaneous observations from the radio
to the GeV range. Methods. The instruments IBIS, SPI, JEM-X on board INTEGRAL, PCA on board RXTE, and LAT on board Fermi have enough sensitivity to follow the spectral variability of 3C 273 from the keV to the GeV. We looked for correlations between the different energy bands, including radio data at 37 GHz collected at the Metsahovi Radio Observatory and built quasi-simultaneous multiwavelength spectra in the high energy domain when the source is flaring either in the X-rays or in the {gamma} rays. Results. Both temporal and spectral analysis suggest a two-component model to explain the complete high energy spectrum. X-ray emission is likely dominated by a Seyfert-like component while the {gamma}-ray emission is dominated by a blazar-like component produced by the relativistic jet. The variability of the blazar-like component is discussed, comparing the spectral parameters in the two different spectral states. Changes of the electron Lorentz factor are found to be the most likely source of the observed variability.
We present preliminary results on the low-redshift Lyman alpha forest as based on STIS spectra of 3C 273. A total of 121 intergalactic Lyman alpha-absorbing systems were detected, of which 60 are above the 3.5 sigma completness limit, log N(HI)~12.3.
The median Doppler parameter, b=27 km/s, is similar to that seen at high redshift. However the distribution of HI column densities (dN/dN(HI) propto N(HI)^-beta) has a steeper slope, beta = 2.02 +- 0.21, than is seen at high redshift. Overall, the observed N(HI)-b distribution is consistent with that derived from a Lambda CDM hydrodynamic simulation.
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