A new insight into the innermost jet regions: probing extreme jet variability with LOFT

I Donnarumma,E Bozzo,A De Rosa,R Campana,S Turriziani,A Tramacere,L Costamante

doi:10.1051/epjconf/20136104015

Abstract

Blazars are highly variable sources over timescales that can be as low as minutes. This is the case of the High Energy Peaked BL Lac (HBL) objects showing strong variability in X-rays, which highly correlate with that of the TeV emission. The degree of this correlation is still debated, particularly when the flaring activity is followed down to very short time scales. This correlation could challenge the synchrotron-self-Compton scenario in which one relativistic electron population dominates the entire radiative output. We argue that the LOFT Large Area Detector (10 m$^2$, LAD), thanks to its unprecedented timing capability, will allow us to detect the X-ray counterpart (2-50 keV) of the very fast variability observed at TeV energies, sheding light on the nature of X-TeV connection. We will discuss the test case of PKS 2155-304, showing as it would be possible to look for any X-ray variability occurring at very short timescales, never explored so far. This will put strong constraints on the size and the location of any additional electron population in the multi-zone scenario. Under this perspective, LOFT and the CTA observatories, planned to operate in the same time frame, will allow us to investigate in depth the connection between X-ray and TeV emissions. We also discuss the potentialities of LOFT in measuring the change in spectral curvature of the synchrotron spectra in HBLs which will make possible to directly study the mechanism of acceleration of highly energetic electrons. LOFT timing capability will be also promising in the study of Flat Spectrum Radio Quasars (FSRQs) with flux $\ge 1$ mCrab. Constraints to the location of the high energy emission will be given by temporal investigation on second timescale and spectral trend analysis on minute timescales. This represents a further link with CTA because of the rapid (unexpected) TeV emission recently detected in some FSRQs.

Highlights

The Large Observatory For X-ray Timing (LOFT) is one of the ESA M3 mission candidates competing for a launch planned in 2022
We argue that the LOFT Large Area Detector (10 m2, LAD), thanks to its unprecedented timing capability, will allow us to detect the X-ray counterpart (2-50 keV) of the very fast variability observed at TeV energies, sheding light on the nature of X-TeV connection
On the basis of the portion of the spectral energy distribution accessible to LOFT (2-50 keV), we argue that the High Energy Peaked BL Lac objects (HBLs) objects are the best candidates for pointed observations with LAD

Summary

Introduction

LAD background is dominated (> 70%) by high energy photons of the Cosmic X-ray Background (CXB) and Earth albedo leaking through the collimator. LOFT is planned in the same timeframe of other observatories which are opening up the AGN time domain on short timescales, e.g. CTA at TeV energies and SKA and ALMA in the radio band In this context, LOFT/LAD will be able to detect a large number (> 100) of AGNs, providing weekly monitoring (a few days for the brightest) thanks to the WFM sky coverage. In the case of the High Energy Peaked BL Lac objects (HBLs), the X-ray band samples the synchrotron emission by the freshly accelerated and rapidly cooling high-.

High Energy Peaked BL Lac objects

The acceleration mechanism

FSRQs: X-Gamma connection and bulk-Comptonization

Findings

Conclusions