Abstract:
The
fl
at-spectrum radio quasar CTA
102 experienced a prolonged state of enhanced activity across the entire
observed electromagnetic spectrum during 2016
–
2017, most pronounced during a major outburst between 2016
December and 2017 May.
Fermi
-LAT observed a
fl
ux of
(
2.2
±
0.2
)
×
10
−
5
photons
cm
−
2
s
−
1
at energies above
100
MeV on 2017 April 19 during a single orbit. We report here the detection of signi
fi
cant
(
4.7
σ
)
fl
ux variations
down to timescales of
∼
5 minutes during this orbit. The measured variability timescale is much shorter than the
light-travel time across the central black hole
(
∼
70 minutes
)
indicating a very compact emission region within the
jet, similar to that seen in IC
310, Mrk 501, or PKS 1222
+
21 from MAGIC observations. This short-timescale
variability is unexpected since the
γ
-ray spectrum shows no sign of attenuation due to pair creation in interactions
with photons from the broad emission line region, and therefore must be assumed to originate far from the black
hole. The observed fast variability could either indicate the dissipation of magnetic islands or protons in a
collimated beam from the base of the jet encountering the turbulent plasma at the end of the magnetic nozzle.
