Multi-Wavelength Observations of PKS 2142-75 during Active and Quiescent Gamma-Ray States

PKS2142-75 (a.k.a. 2FGL J2147.4-7534) is a flat-spectrum radio quasar that was observed quasi-simultaneously by a suite of instruments across the elec- tromagnetic spectrum during two flaring states in April 2010 and August 2011 as well as a quiescent state in December 2011 through January 2012. The results of these campaigns and model spectral energy distributions (SEDs) from the active and quiescent states are presented. The SED model parameters of PKS 2142-75 indicate that the two flares of the source are created by unique physical conditions. SED studies of flat spectrum radio quasars are beginning to indicate that there might be two types of flares, those that can be described purely by changes in the electron distribution and those that require changes in other parameters, such as the magnetic field strength or the size of the emitting region.


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Active galactic nuclei (AGN) are the brightest persistent sources of electromagnetic 53 radiation in the Universe. They radiate intensely from low-energy radio waves to very-high-54 energy γ-rays and are known to be highly variable at all frequencies. Blazars are the most 55 luminous and violently variable subclass of AGN. High and highly variable polarization has 56 been detected in the radio and optical emission of blazars, and they are the most common 57 type of AGN detected at GeV energies (Abdo et al. 2010a). Blazars have extended plasma 58 jets, which can be detected at radio and, in a growing number of cases, at optical and X-ray 59 frequencies as well 1 . Observational data suggest that blazars are those radio-loud AGN 60 whose jets are aligned at small angles with respect to the line of sight. In this scenario, the 61 bulk relativistic motion of the emitting material results in "relativistic beaming", i.e. both Lac) objects and flat spectrum radio quasars (FSRQs). BL Lac objects exhibit weak or no 68 emission lines, while FSRQs show strong, broad emission lines (Urry & Padovani 1995). 69 The SEDs of blazars show a characteristic "double-bumped" structure, with the lower 70 frequency bump almost certainly originating from beamed electron synchrotron emission 71 from the jet (Urry & Padovani 1995). Aside from their classification based on their optical 72 spectra, blazars are also classified based on their frequencies of peak synchrotron emission 73 1 Lists of X-ray detected and optically-detected jets can be found at http://hea-www.harvard.edu/XJET/ and http://astro.fit.edu/jets/, respectively. The origin of the higher frequency bump, peaking in γ rays, is less well understood. 78 Two families of models attempt to describe them: leptonic and hadronic models (Böttcher 79 2007). Leptonic models invoke a source of soft seed photons being Compton up-scattered  ) have all been considered as seed photon sources for EC processes. 86 Hadronic models assume that a significant fraction of the jet power is converted into the 87 acceleration of protons, so that the protons reach the threshold for pion production. The 88 pions and their secondaries can thus create the γ-ray emission (Mannheim & Biermann 89 1992). Protons at these high energies can also emit γ rays through synchrotron emission 90 (e.g., Mücke et al. 2003).

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Simultaneous flux density measurements of blazars at multiple wavelengths across the 92 electromagnetic spectrum can allow us to distinguish among the many different high-energy 93 emission models proposed (Böttcher 2007 (Atwood et al. 2009). The LAT has put better constraints on blazar variability within its 98 -7observing window and has shown that it is not unusual for blazars to exhibit fast variability 99 on timescales of days or even hours at GeV energies (e.g., Saito et al. 2013). The LAT 100 observes the entire sky every three hours, which makes it an ideal instrument on which to 101 support multi-instrument observing campaigns on sources in response to their changing 102 γ-ray state.  During the latest flaring period in 2011, a multi-wavelength observing campaign 114 was carried out using the Ceduna radio telescope, the Australia Telescope Compact 115 Array (ATCA), Swift, the Rapid Eye Mount Telescope (REM), and LAT. These quasi-116 simultaneous data were complemented with simultaneous data from the ongoing SMARTS 117 blazar monitoring program (Bonning et al. 2012). The data were used to construct a 118 broadband SED of this object in its active state. We organized another quasi-simultaneous 119 2 At this redshift, using a cosmology with H 0 = 71 km s −1 Mpc −3 , Ω m = 0.27, and Ω Λ = 0.73, PKS 2142−75 has a luminosity distance of d L = 7.7 Gpc. We use this d L for the source in this paper.  The LAT, on the Fermi spacecraft is a high-energy γ-ray detector with a wide field of 137 view (∼ 20% of the sky) covering the energy band from 20 MeV to greater than 300 GeV.

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It primarily operates in an all sky scanning mode, observing the entire sky every three 139 hours. All LAT results presented here were obtained using the Fermi science tools (version 140 09-27-00) and the P7SOURCE V6 instrument response functions (IRFs).

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The light curve in the top panel of Figure.    For the W 1, M2 and W 2 filters a higher uncertainty of 25% is used because they overlap 220 with a UV "bump" in the extinction curve, which is highly dependent on the line of sight 221 and is therefore difficult to model.   The results from each of our three data sets are plotted on the SEDs shown in Figure 6.  Table 1. This 306 -21model assumes that the lower-frequency bump is non-thermal synchrotron radiation emitted 307 by electrons that are isotropically oriented in the co-moving frame of the jet, in a tangled, 308 randomly oriented magnetic field in the jet plasma. Radio emission from the core is thought 309 to be a superposition of multiple jet components (Konigl 1981) and is therefore treated  Figure 4 and Section 2 above).

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The electron distribution within the emitting region is described as a broken power law 353 with indices p 1 below and p 2 above a break at γ ′ brk and a low and high energy cutoff at a