Fermi-LAT Upper Limit for NGC 4151 and its Implications for Physics of Hot Accretion Flow

TL;DR

This study uses Fermi-LAT data to set upper limits on gamma-ray emission from NGC 4151, constraining hot accretion flow models and the physics of non-thermal processes near the supermassive black hole.

Contribution

It provides the first gamma-ray upper limit for NGC 4151, testing and constraining models of hot accretion flows and non-thermal particle acceleration.

Findings

Gamma-ray flux from NGC 4151 is below the detection threshold.

Upper limits constrain non-thermal proton energy content to a few percent.

Results favor rapid black hole rotation and rule out weak magnetic fields.

Abstract

We present preliminary results of our analysis of the {\it Fermi}-LAT data from the direction of NGC 4151. We find a new gamma-ray source with a statistical significance sigma > 5, shifted by 0.5degr from the position of NGC 4151. Apparently, the source was bright only during a 1.5-year period between December 2011 and June 2013 and it strongly contaminated the signal from NGC 4151. Therefore, we neglect this period in our analysis. We find two additional, persistent gamma-ray sources with high sigma, shifted from NGC 4151 by ~1.5degr and 5degr, whose presence has been recently confirmed in the Third Fermi Catalog. After subtracting the above sources, we still see a weak residual, with sigma ~< 3, at the position of NGC 4151. We derive an upper limit (UL) for the gamma-ray flux from NGC 4151 and we compare it with predictions of the ADAF model which can explain the X-ray observations of this object. We find that the Fermi UL strongly constrains non-thermal acceleration processes in hot flows as well as the values of some crucial parameters. Here we present the comparison with the hot flow models in which heating of electrons is dominated by Coulomb interactions with hot protons. In such a version of the model, the gamma-ray UL, combined with the X-ray data, constrains the energy content in the non-thermal component of proton distribution to at most a few per cent, rules out a weak (sub-equipartition) magnetic field and favors a rapid rotation of the supermassive black hole.