Gamma-ray Flares in The Relativistic Jet of The
Quasar 3C 273 in 2015--2019
1Dae-Won Kim and Sascha Trippe
1

1Seoul National University (Korea); dwkim@astro.snu.ac.kr

ABSTRACT
3C 273 is an obvious candidate to reveal the nature of 𝛄-ray emission in the relativistic jet of blazars due to its prominent 𝛄-ray emission and powerful jet
activity. Since the historical huge 𝛄-ray outburst in 2009, 3C 273 became relatively weak in 𝛄-ray ﬂux density for a long time. However, recent Fermi-Large
Area Telescope observations indicate some ﬂaring periods in 3C 273. We aim to investigate the origin of the 𝛄-ray ﬂares in 3C 273 by utilizing light curves atradio and 𝛄-rays, plus the radio images obtained from a very long baseline interferometry (VLBI). In this study, we present radio/𝛄-ray light curves, correlation,
𝛄-ray photon properties, and VLBI jet images of 3C 273, and discuss its physical indications for the connection between the radio jet and 𝛄-ray ﬂares.

Introduction
It has been known that ~75 % of MeV 𝛄-ray sources in the sky areclassiﬁed as Blazar which is one of active galactic nuclei (AGN) types withits very small viewing angle (e.g., ~5 degrees). In general, the relativistic
jet is thought to be a main source of the 𝛄-rays due to its extreme
condition and radiative power. However, the details of the physicalprocesses are still an open issue. In this study, we explore 𝛄-rays and thejet of 3C 273 in 2015--2019 with the observational data: ALMA (~100GHz), Fermi-LAT (0.1--300 GeV), and VLBA (43 GHz; VLBA-BU-BLAZAR [1]).

<𝛄-ray Countmap><Radio jet image from Kovalev et al. 2016>

Power-law indices of the 𝛄-rays
Left: index vs. ﬂux plots forthe 𝛄-ray photons. The
dashed lines indicate theaverage values, and thevertical solid line points to
1.03 x 10-7 ph cm-2 s-1.
Right: Index vs. time plot.
The vertical gray areas are
the same as 7d & 30d 𝛄-ray
light curves. The dashed &dotted lines are the averagevalue for the 7d & 30d,respectively.

The average power-law indices of the 𝛄-ray photons for both 7dand 30d are -2.85 and -2.78, respectively. In the 30d indices, wesee a transition above 1.03 x 10-7 ph cm-2 s-1 (Pearson rp=0.48 at >0.99%). As binning interval gets shorter, small-scale 𝛄-rays seemdominating the 𝛄-ray emission. Although we have limited photon
statics, there seems to be a hump of the 30d index in each 𝛄-ray
ﬂaring period (i.e., ﬁrst on MJD 57338 and second on MJD 57878).

Parsec-scale jet near the core
Left: a public 𝛄-ray light curve of 3C
273 observed by the LAT. Full timecoverage of ~10 years with weekly
binning. The light purple area is thetime range of this study.

Radio/𝛄-ray light curves∙ A major mm ﬂare ~20 Jy on MJD57486.
∙ An extra peak of the major mm
ﬂare ~18 Jy on MJD 57408.
∙ Three minor mm ﬂares below 13Jy.
∙ Monthly 𝛄-ray variations: smootharound 2016 & complex around2017.
∙ In weekly 𝛄-rays, two prominent
ﬂares in early 2016 and mid-2017.
∙ The ﬁrst 𝛄-ray ﬂare: 8.7 x 10-7 phcm-2 s-1 at ~24 σ on MJD 57383.
∙ The second 𝛄-ray ﬂare: 7.0 x 10-7
ph cm-2 s-1 at ~15 σ on MJD 57880.

<3C 273 jet shortly after the ﬁrst 𝛄-ray ﬂare (left) and the second 𝛄-ray ﬂare (right) >The jet ﬂows toward the southwest fromthe core which is the upstream end of thejet. In our images, we ﬁnd the known
three stationary components (e.g., [2]):the core, S1, and S2 at ~0.16 and ~0.33
mas from the core (set to be map center).
The total intensity peak shows a spatial
displacement along the jet right after thetwo 𝛄-ray ﬂares. The apparent speed of
the ﬁrst one is ~7c (proper motion, 𝛍:0.69 mas/year), and of the second one is<Evolutions of the core, S1, and S2> ~3c (𝛍: 0.29 mas/year).

Summary & References
We present the recent radio/𝛄-ray activity of the 3C 273 jet. In
2015--2019, we identify the most signiﬁcant two 𝛄-ray events whichcoincides with a mm-wave ﬂare for each. Given the observational
evidences, we summarize our results as below.
∙ The radio/𝛄-ray emissions during 4 years are correlated each other.
<Daily 𝛄-rays for two 𝛄-ray ﬂares><DCF & Pearson between mm-wave & 𝛄-ray>

We ﬁnd two most signiﬁcant 𝛄-ray ﬂares in 2015--2019 thatoverlaps one mm-wave ﬂare for each. The weekly 𝛄-rays seem
more complicated than the monthly, and the second 𝛄-ray peak isisolated. A DCF curve and Gaussian ﬁt returns about -105 days (vs.
7d) and -112 days (vs. 30d) time-lags (𝛄-ray leading) with a good
coeﬃcient.

∙ Monthly binning better describes the systematic trends in the 3C 273
𝛄-rays.
∙ With shorter binning intervals, we deﬁne two primary 𝛄-ray outbursts.
∙ We suspect that the S1 component is the origin of the ﬁrst 𝛄-ray ﬂare.
∙ The second 𝛄-ray ﬂare seems more complicated, but the core is likely
to be its origin as it shows the onset of a mm-wave ﬂare.
-Ref. [1] Jorstad et al. 2017, ApJ, 846, 98[2] Lisakov et al. 2017, MNRAS, 468, 4478