Hubble Space Telescope observations of an extraordinary flare in the M87 jet

TL;DR

This paper reports on the dramatic ultraviolet flare observed in the HST-1 knot of the M87 jet, showing a 90-fold increase in brightness and confirming synchrotron emission through multi-wavelength correlation.

Contribution

It provides detailed HST UV imaging analysis of the M87 jet flare, revealing unprecedented brightness increase and challenging typical AGN variability models.

Findings

NUV intensity of HST-1 increased 90 times

NUV and X-ray light curves are correlated

The flare confirms synchrotron origin of X-ray emission

Abstract

HST-1, a knot along the M87 jet located 0.85 arcsec from the nucleus of the galaxy has experienced dramatic and unexpected flaring activity since early 2000. We present analysis of Hubble Space Telescope Near-Ultraviolet (NUV) imaging of the M87 jet from 1999 May to 2006 December that reveals that the NUV intensity of HST-1 has increased 90 times over its quiescent level and outshines the core of the galaxy. The NUV light curve that we derive is synchronous with the light curves derived in other wavebands. The correlation of X-ray and NUV light curves during the HST-1 flare confirms the synchrotron origin of the X-ray emission in the M87 jet. The outburst observed in HST-1 is at odds with the common definition of AGN variability usually linked to blazars and originating in close proximity of the central black hole. In fact, the M87 jet is not aligned with our line of sight and HST-1 is located at one million Schwarzchild radii from the super-massive black hole in the core of the galaxy.