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Register a new userFirst NuSTAR observations of the BL Lac - type blazar PKS~2155-304: constraints on the jet content and distribution of radiating particles
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We report the first hard X-ray observations with NuSTAR of the BL Lac type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and gamma-ray data from the Fermi Large Area Telescope, obtained in April 2013 when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5 - 60 keV, is best described by a model consisting of a log-parabola component with curvature beta = 0.3(+0.2,-0.1) and a (local) photon index 3.04 +/- 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 +/- 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous gamma-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to the minimum electron Lorentz factor gamma_min = 1 and that there is one proton per electron, an unrealistically high total jet power L_p of roughly 10^47 erg/s is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron-positron pairs with lepton-to-proton ratio of at least 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor gamma_br1 of roughly 100. In either case, the jet composition is expected to be strongly matter-dominated.
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The Infrared Space Observatory (ISO) observed the BL Lac object PKS 2155-304 16 times from 1996, May 7 to June 8, with both the ISOCAM camera and the ISOPHOT photometer, as part of a more general multiwavelength campaign. Two additional observations were performed on 1996, November 23 and 1997, May 15. This is the first time that there are simultaneous mid and far infrared data in a multiwavelength monitoring of a BL Lac object. We obtained four light curves at 4.0, 14.3, 60 and 90 microns, and a broad-band filter spectrum from 2.8 to 170 micron. No variability was detected in the infrared, although the source was varying at shorter wavelengths. The IR spectrum can be fitted by a single power law with an energy spectral index alfa = 0.40 +/- 0.06 and it can be explained as due to synchrotron emission only, with no noticeable contributions from thermal sources. Using the simultaneous data, we constructed the SED of PKS 2155-304.
Supermassive black holes can launch powerful jets which can be some of the most luminous multi-wavelength sources; decades after their discovery their physics and energetics are still poorly understood. The past decade has seen a dramatic improvement in the quality of available data, but despite this improvement the semi-analytical modelling of jets has advanced slowly: simple one-zone models are still the most commonly employed method of interpreting data, in particular for AGN jets. These models can roughly constrain the properties of jets but they cannot unambiguously couple their emission to the launching regions and internal dynamics, which can be probed with simulations. However, simulations are not easily comparable to observations because they cannot yet self-consistently predict spectra. We present an advanced semi-analytical model which accounts for the dynamics of the whole jet, starting from a simplified parametrization of Relativistic Magnetohydrodynamics in which the magnetic flux is converted into bulk kinetic energy. To benchmark the model we fit six quasisimultaneous, multi-wavelength spectral energy distributions of the BL Lac PKS 2155-304 obtained by the TANAMI program, and we address the degeneracies inherent to such a complex model by employing a state-of-the-art exploration of parameter space, which so far has been mostly neglected in the study of AGN jets. We find that this new approach is much more effective than a single-epoch fit in providing meaningful constraints on model parameters.
Axion-like particles are hypothetical new light (sub-eV) bosons predicted in some extensions of the Standard Model of particle physics. In astrophysical environments comprising high-energy gamma rays and turbulent magnetic fields, the existence of axion-like particles can modify the energy spectrum of the gamma rays. This modification would take the form of an irregular behavior of the energy spectrum in a limited energy range. Data from the H.E.S.S. observations of the distant BL Lac PKS 2155-304 are used to derive conservative upper limits on the strength of the axion-like particle coupling to photons. This study gives rise to the first exclusions on axion-like particles from gamma-ray astronomy. The derived constraints apply to both light pseudo-scalar and scalar bosons that couple to the electromagnetic field.
Time variability of the photon flux is a known feature of active galactic nuclei (AGN) and in particular of blazars. The high frequency peaked BL Lac (HBL) object PKS 2155-304 is one of the brightest sources in the TeV band and has been monitored regularly with different instruments and in particular with the H.E.S.S. experiment above 200 GeV for more than 11 years. These data together with the observations of other instruments and monitoring programs like SMARTS (optical), Swift-XRT/RXTE/XMM-Newton (X-ray) and Fermi-LAT (100 MeV < E < 300 GeV) are used to characterize the variability of this object in the quiescent state over a wide energy range. Variability studies are made by looking at the lognormality of the light curves and at the fractional root mean square (rms) variability Fvar in several energy bands. Lognormality is found in every energy range and the evolution of Fvar with the energy shows a similar increase both in X-rays and in TeV bands.
Axionlike particles (ALPs) are hypothetical light (sub-eV) bosons predicted in some extensions of the Standard Model of particle physics. In astrophysical environments comprising high-energy gamma rays and turbulent magnetic fields, the existence of ALPs can modify the energy spectrum of the gamma rays for a sufficiently large coupling between ALPs and photons. This modification would take the form of an irregular behavior of the energy spectrum in a limited energy range. Data from the H.E.S.S. observations of the distant BL Lac object PKS 2155-304 (z = 0.116) are used to derive upper limits at the 95% C.L. on the strength of the ALP coupling to photons, $g_{gamma a} < 2.1times 10^{-11}$ GeV$^{-1}$ for an ALP mass between 15 neV and 60 neV. The results depend on assumptions on the magnetic field around the source, which are chosen conservatively. The derived constraints apply to both light pseudoscalar and scalar bosons that couple to the electromagnetic field.
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