VLBI

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VERA observation of flaring blazar TXS 0506+056:
plausible counterpart of IceCube-170922A
K. Niinuma (Yamaguchi U.), H. Nagai (NAOJ/ALMA), Y. Y. Tanaka (Hiroshima U.), M. Hayashida (Konan U.),M. Honma (NAOJ/VLBI), Y. Inoue (RIKEN), K. Murase (Pennsylvania State U.), A. Doi (JAXA)

Introduction: high-energy neutrino from blazar TXS 0506+056

IceCube Collaboration et al. (2018a) recently reported the detection of a high-energy (HE) neutrino (IceCube-170922A) with the IceCube
Observatory, for the first time coinciding spatially and temporally with a gamma-ray flaring blazar TXS 0506+056 (also known as 3FGL
J0509.4+0541, Acero et al. 2015) at a confidence level of 3 sigma (Fig. 1 left). Following the alert of IceCube-170922A, intensive follow-up
observations were conducted for TXS 0506+056 at electro-magnetic (EM) wave (see, Fig. 1 right). Several theoretical works have predictedthat a flaring blazar is considered to be promising source as neutrinoemitter (e.g., Dermer et al. 2014, Petropoulou et al. 2016,
Gao et al. 2017). Therefore an EM observation is crucial to identify
true origin of astrophysical neutrinos.
Long-lasting active state in GeV
Among the EM observations, the very
long baseline interferometry (VLBI) is
one of the most powerful techniques
allowing us to identify the flaring
component in response to the neutrino
event with a sub-milli to milliarcsecond (mas) scale because of its
extremely high angular resolution.

VERA observation

Long-lasting active state in radio

In order to explore the unprecedented
radio signature in blazar object related
Fig. 1 Left: The 50% and 90% containment regions for the neutrino IceCube-170922A. Right: Time-dependent multiwavelength
to the latest huge γ-ray flare, which isobservations of TXS 0506+056 before and after IceCube-170922A (IceCube Collaboration 2018b)
associated with the HE neutrino event,
we performed four epochs of biweekly monitoring, which began eight days after the detection time of IceCube-170922A with VERA. Eachobservation was performed at 23GHz with a band width of 256MHz. The specific dates are 2017 Sep 30, Oct 13, Nov 1, and 13.

Resultn VLBI images

Fig. 2 VLBI images of TXS 0506+056. Contour image was obtained by VLBA on 2008 Dec 19, and color
images were obtained by VERA in 2017. All images are restored with a synthesized beam of 1 mas indiameter, shown in the bottom-left corner. Peak fluxes and image rms are indicated in each imagen Proper motion of jet components
In Fig. 3, we show the positional displacement of each jet component
obtained by not only VERA but also VLBA at 15 GHz provided byMOJAVE project (from 2015 Jan to 2017 Jun). JV is possibly the result of
the mixture of J2, J3, J4 components in VLBA image as shown in Fig 3(A) and 3-(B) because of not only insufficient angular resolution, but alsoless image dynamic range of VERA compared to VLBA observations.
As shown in Fig. 3-(C), all jet components seen in VLBA image show theproper motion within the positional error for recent two years, and JV
showed no systematic motion during follow-up observation. Also, no newborn jet associated with the HE neutrino event appeared by 2017 Nov 13.

Discussionn Neutrino event possibly occurred in the VLBI core of the flaring
blazar, because both γ-ray and radio fluxes in recent long-lasting
active state show gradual flux increase (i.e., γ-ray and radio activities
in 2017 originate in the same location = VLBI core).

5.0
(C)

(A)

VERA@23 GHz, 2017 Sep 30

C

4.0
JV

Sp = 436 mJy beam-1
f.c. = 5.26 mJy beam-1(B)

VLBA@15 GHz, 2016 Jan 23CJ4J3
J2

Position from the core [mas]In Fig. 2, we show VERA and VLBA images of TXS
0506+056. The southward elongated structure can beclearly seen in these images. VLBI fluxes in 2017 haveincreased 1.6 times compared to the one in 2008. Also,no significant flux variability can be seen during ourfour epochs of VERA observations. These flux behaviors
are consistent with the one seen in OVRO light curve at15 GHz (Fig. 1).

vapp ~ 4.00.4 c
VLBA J1

3.0

2.0

1.0

VLBA J2

vapp ~ 0 c
VERA JVVLBA J3

vapp ~ -2.11.1 c

J1
Sp = 211 mJy beam-1f.c. = 0.814 mJy beam-1

VLBA J40.0

2015.0vapp ~ 0 c2016.0

2017.02018.0

YearFig. 3 Left panels: jet structure of TXS 0506+056 deconvolved by the two-dimensional
circular Gaussian function. (A) shows 23 GHz VERA image reproduced by 2 components(C and JV) observed in 2017 Sep 30, and (B) shows 15 GHz VLBA image reproduced by
5 components (C, J1, J2, J3, and J4). (C): radial proper motion of each jet component
with respect to the position of the VLBI core under the assumption of linear motion.
Vertical dashed line indicates the time when IceCube-170922A was detected.

n If newly born jet is associated with the HE neutrino event, and to resolve it with VERA, the jet should move at least ~0.5 mas
downstream from the positional center of the VLBI core. Based on observational facts and the assumption that the distance between thecentral engine and the VLBI core is ~10 pc as discussed in previous works (e.g., Orienti et al. 2013), and viewing angle of jet is 1 deg, weconstrained its apparent velocity to less than ~60c.

Summary & Prospects
n We clarified the HE neutrino event possibly occurred in the VLBI core of flaring blzar, and no significant structural change related to theHE neutrino event was seen by 2017 Nov 13 (52 days after the detection of IceCube-170922A)
n Further VLBI observations at higher-frequency (i.e., millimeter wavelengths) and powerful interferometric observations at sub-millimeter
wavelength (e.g., ALMA) will allow us to investigate the more optically thin part possibly reflecting the behavior of an upstream end of theblazar jet.
n To obtain new information on time evolutions of not only mm-fluxes and detailed structure but also opacity in the VLBI core, we will starthigher quality VLBI imaging monitor with East Asia VLBI Network (EAVN: KaVA+Tianma+Nobeyama) at mm-wavelengths (both23GHz and 43GHz quasi-simultaneously).
References: IceCube Collaboration et al., 2018a, Science, 361, 147; IceCube Collaboration et al., 2018b, Science, 361, eaat1378; Acero, F., et al., 2015, ApJS, 218, 23; Dermer, C. D., et al.,2014, Journal of High Energy Astrophysics, 3, 29; Petropoulou, M., et al., 2016, Astroparticle Physics, 80, 115; Gao, S. et al., 2017, ApJ, 843, 109; Orienti, M., 2013, MNRAS, 428, 2418