Origin of X-rays in the low state of the FSRQ 3C 273: evidence of inverse Compton emission

Abstract

We analyze the 2.5-10 keV X-ray spectra of the luminous quasar 3C 273 and simultaneous observations in UV wavelengths from XMM-Newton between 2000 and 2015. The lowest flux level ever was observed in 2015. The continuum emission from 3C 273 is generally best described by an absorbed power-law but during extremely low states the addition of fluorescence from the K-shell iron line improves the fit. We study the spectral evolution of the source during its extended quiescent state and also examine connections between the X-ray and ultraviolet emissions, which have been seen in some, but not all, previous work. We detect a possible anti-correlation between these two bands during the low state that characterized 3C 273 for most of this period; however, this was not present during a flaring state. A harderwhen-brighter trend for the X-ray spectrum was observed in these long-term observations of 3C 273 for the first time. We suggest that the X-ray emission in 3C 273 is the result of inverse Compton scattering of soft UV seed photons (emitted from the local environment of the AGN), most likely in a thermal corona. We can explain the significant temporal variation of the spectral continuum as an outcome of changing optical depth of the comptonizing medium, along the lines of the wind-shock model proposed by Courvoisier and Camenzind (1989).

X-ray Intraday Variability of the TeV Blazar Mrk 421 with Chandra

“…1. We have searched for IDV in these LCs and visual inspection of them frequently show the presence of clear variability (Kalita et al 2017). We have computed IDV variability parameters in Table 2, which include excess variance, fractional variance (Fvar) and any timescales indicated by the ACFs.…”

X-ray Intraday Variability and Power Spectral Density Profiles of the Blazar 3C 273 with XMM-Newton during 2000-2021

“…Multi-band X-ray, UV and optical observations of 3C 273 including data from XMM-Newton were used to study flux, spectral and cross-correlated variabilities on diverse timescales. Long-term observations of 3C 273 found that IDV X-ray LCs showed small amplitude variability, along with large amplitude variability in optical to X-ray energies on longer timescales, an anti-correlation in UV and X-ray emission in a low flux state, and a harder-whenbrighter trend for the X-ray spectrum (Kalita et al 2015;Kalita et al 2017). There are 16 Obs IDs studied in the present paper that are also considered in (Kalita et al 2015;Kalita et al 2017).…”

“…Yaqoob & Serlemitsos (2000) detected strong Fe Kα line from a 1996 July observing campaign of 3C 273 with RXTE and ASCA, and another detection could be made 1 https://www.lsw.uni-heidelberg.de/projects/extragalactic/charts/1226+023.html from all the data on the blazar taken by ASCA from 1993 -2000. In XMM-Newton observation taken on 7 July 2003 (Obs ID 159960101, see Table 1) the FeKα line also was detected (Kalita et al 2017). The FeKα line is generally seen in Seyfert 1 galaxies from which it can be concluded that the X-ray emission from 3C 273 is a mixture of a thermal AGN component associated with the inner accretion flow emission from the disc's corona as well as non-thermal processes produced in the jet component (Magdziarz & Zdziarski 1995).…”

“…3C 273 is among the most extensively studied blazars across the complete EM spectrum on diverse timescales. Observations have been carried out in single bands as well as in MW campaigns (simultaneous and/or non-simultaneous) (e.g., Collmar et al 2000;Romero et al 2000;Sambruna et al 2001;Courvoisier et al 2003;Paltani & Türler 2005;Attridge et al 2005;Savolainen et al 2006;Uchiyama et al 2006;Soldi et al 2008;Fan et al 2009;Abdo et al 2010;Fan et al 2014;Kalita et al 2015;Chidiac et al 2016;Kalita et al 2017;Liu et al 2019;Carini et al 2020;Fernandes et al 2020, and references therein).…”

HILIGT, Upper Limit Servers I -- Overview

“…The light curve is quite stable, having maintained a mean 2-12 keV flux of 1.0 × 10 −10 erg s −1 cm −2 over the 50 year span (45 years in the source restframe; z=0.158) with a peak-to-peak variation of a factor 4. We note that the minimum flux of 5.5 × 10 −11 erg s −1 cm −2 was recorded on 2019-07-02 being ∼ 20% fainter than the previous minimum registered in July 2015 (Kalita et al, 2017). This long time series gives confidence that fluxes from old and new missions can sensibly be compared despite the widely differing beam sizes of the instrumentation.…”