Optical-Infrared Follow-Up Observations
For IceCube High-Energy Neutrino Sources

Tomoki Morokuma
Planetary Exploration Research Center (PERC)
Chiba Institute of Technology

IceCube ICRC2023 Satellite Workshop University

Chiba University

2023/08/07-08

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Contents
Crowded & Variable/Transient optical universe
What roles are expected? What are “optical”’s unique roles?
Previous successes: 170922A/TXS 0506+056, TDEs
What’s next?
better telescopes/instruments
better strategy
better neutrinos
Summary

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IceCube-170922A/TXS 0506+056: Fermi/LAT

0.25 deg

AMON Workshop 2019 @ Chiba University

IceCube 2019/05/21,22
Collaboration+2018

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IceCube-170922A/TXS 0506+056: Kiso/KWFC (optical)very crowded in optical
Subaru/HSC

0.25 deg

TM+2021
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2019/05/21,22

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IceCube-170922A/TXS 0506+056: PS1 (optical, 3pi)

0.25 degvery crowded in optical

0.25 deg
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Publications of the Astronomical Society of Japan (2019), Vol. 71, No. 474-4

Very Variable/Transient optical universe
1,824 supernova candidates over ~7 deg2
Subaru (8.2m) + Hyper Suprime-Cam

0.25 deg

non-transient: O(105-6) deg-2 (Furusawa+2008)2) deg-2 (TM+2008, Yoshida+2018)transient:O(10IceCube ICRC2023 Satellite Workshop University
Chiba University

Yasuda+2019
2023/08/07-08

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Roles/Uniquenessof optical -Mayobservations
SN candidates
2015Most of the transient phenomena are luminous in restframe UV to optical.
supernovae, TDEs, blazars/AGN, …
“Wide-field”:
narrower than other (gamma etc.) wavelengths, but..
almost no confusion limit (well spatially resolved)“Wide-field”, “deep”, and “spatially resolved” aresimultaneously achieved.
Host galaxies are bright(stellar light, nebular gas).
relatively easier redshiftdetermination (spectroscopyof transients/host galaxies)

Tominaga+2015, ATel, 7565
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IceCube-170922A : EM Counterpart Identification
Publications of the Astronomical Society of Japan (2021), Vol. 73, No. 1

IceCubenear-infrared variability detected

Fermi/LAT“The FAVA light curve at energies above 800 MeV shows aflaring state recently.” (Y. Tanaka+2017, ATel, #10791, Posted8Kanata(1.5m) / 2023/08/07-08
HONIR (TM+2021)
at 10:10 on 28
Sep. ICRC2023
2017 in UT)
IceCubeSatellite Workshop UniversityChiba University

Publications of the Astronomical Society of Japan (2021), Vol.

IceCube-170922A : EM Counterpart Discovery

TM+2021
We noticed the infrared variability for TXS 0506+056
meline of the follow-up observations from the IceCube alert. Kanata/HONIR, Kiso/KWFC, and Subaru/HSC observations are sho
w/ Kanata/HONIR
data.
magenta, respectively. The IceCube
alert and the first GCNby (Tanaka et al. 2017) are indicated by solid black and green lines, reline)Yasuyuki Tanaka (Hiroshima U.) and I conducted Subaru/HSC (remote) observation. We discussed

the Kanata/HONIR data and checked the FAVA light curve. We started thinking that TXS

may variability
be the neutrino
and reduced
integrationof a detection;
part of the
any 0506+056
significant flux
of TXSsource
0506+056.
levelthethanaround thetimes
neutrino
however,
HSC data to avoid CCD saturation so that the TXS source would be accurately measured. Inmate the median and 1 σ confidence level with atrino detection is reported in this period.
parallel, Yasu started examining Fermi/LAT data and quickly issued the ATel.
ap method.
The optical SF amplitudes of TXS 0506+056 areASAS-SN data are useful for evaluating variability
rable with or slightly larger than those of AGNs in9
ChibaatUniversity
2023/08/07-08
or before and IceCube
aroundICRC2023
the alert,Satellite
while Workshop
our ownUniversity
datavious studies
time scales of#t > 101 d. TXS 05

IceCube-170922A : redshift determination (spectroscopy)
TM+2017, ATel, #10890

0.1-0.3 hours
integration
Subaru(8.2m)/FOCAS
S/N~100

1% (S/N=100)

Paiano+2018 (background)
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IceCube-170922A : redshift determination (spectroscopy)
TM+2017, ATel, #10890

0.1-0.3 hours
integration
Subaru(8.2m)/FOCAS
S/N~100
10-hour integration
GTC(10m)/OSIRIS
S/N~500
30 times longer
1.5 times larger==> ~7 times better
than Subaru/FOCAS
1% (S/N=100)

Paiano+2018

z=0.336 < 0.4
consistent w/ MAGIC high-energy gamma detection

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Improvements on Follow-Ups for Identification?
IceCube-170922A
TXS 0506+056@z=0.336(TM+2021, blazar)
variability detectedw/ Kanata (1.5m)

200 days

200 days
IceCube-191001A: AT2019dsg@z=0.051 (TDE)
discovered w/ ZTF (1.2m) (Stein+2021)

200 days
IceCube-200530A: AT2019fdr@z=0.267 (TDE)
discovered w/ ZTF (1.2m) (Reusch+2022)

FIG. 1. The bottom plot shows the lightcurve in the optical ZTF g-band, the infrared P200 Ks- and WISE W1-band asas the modeled dust echo (black line, dashdot), with the neutrino arrival time marked with a red dotted vertical line.
SRG/eROSITA X-ray measurements are also included. The shaded gray areas are averaged and their respective SEDs
shown in the top panels, including a fitted blue and a red blackbody (blue dashed and red dotted curve; lab frame), as wethe combined spectrum (black solid curve). The left axes all show ⌫F⌫ , where F⌫ is the spectral flux density at frequenc
while the right axes show ⌫L⌫ , where L⌫ is the luminosity at frequency ⌫. The second epoch (middle plot on top) encompaseveral months to include both WISE and P200 infrared data points. The global values for line-of-sight dust extinction
AV = 0.45+0.14
0.14 mag, assuming RV = 3.1 and the Calzetti attenuation law-2[54]. Note that the X-ray measurements wereincluded in the blackbody fits. The luminosities are given in the source rest frame.

Previous identifications
lower-z than expectation (<z>~0.5-1)
other origin?: supernovae
suffering from many contaminations“How can we overcome the difficultyin identifying the counterparts?”
better telescopes/instruments
better strategy
betterneutrinos
IceCubeICRC2023 Satellite Workshop University

sourceSNe# [deg ] (z<1)~10trino arrival (corrected for absorption).
~10 if classified“interacting”)
The position of AT2019fdr was also visited bytions for SLSNe. Frederick et al. [45] 2-3
already( disfavored1
the SLSN hypothesis based on the long-lived U-band andthe UV emission, the flare’s longevity, emission at theblue end of the Balmer line profiles as well as its proximity to the nucleus of the galaxy. Here we add a late-time
X-ray detection and the detection of a strong infraredecho, rendering a SLSN interpretation less likely (see below).
After discovery, AT2019fdr was also observed by theUltraviolet and Optical Telescope (UVOT) [60] aboard
the Neil Gehrels Swift Observatory (Swift) [45, 61]. Additional observations continued up to 2020, June 7, including one epoch shortly after the neutrino detection.
By that point, the transient had faded by 84% in theUVW1-band from its peak luminosity of 2.1 ⇥ 1044 erg
s 1 . AT2019fdr was not detected in any of the simultaneous X-ray observations by the Swift X-ray Telescope(XRT) [62], yielding a combined 3 flux upper limit ofBlazars
TDEs

AGNs

Chiba UniversityeROSITA telescope [63] aboard the Spectrum-Roentg
Gamma (SRG) mission [64] four times. The first
0-1visits did not detect an excess, with a mean flux up
limit of 2.7 ⇥ 10 14 erg s 1 cm 2 at the 95% confide
level. However, at the third visit on 2021, March 10–
it detected late time X-ray emission from the trans14with an energy flux of 6.2+2.7
erg s 1 cm
2.1 ⇥ 10
the 0.3–2.0 keV band, thus showing temporal evolu
in the X-ray flux (see Fig. 1). The detection displa
3a very soft thermal spectrum with a best fit blackb
temperature of 56+32
26 eV.
The softness of the spectrum provides further evide
for AT2019fdr being a TDE rather than regular A
variability, where soft spectra are rare [65]. Tho
NLSy1 galaxies generally exhibit softer X-ray spec
the temperature of AT2019fdr is atypically low even~10<1~10

contamination rate

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Better Telescopes/Instruments: Wide-Field Deep Imaging
How Wide?
“a few deg2” is wide enough. (Subaru/HSC, Rubin/LSST, other smaller tels)
unpredictable location ==> wide-field/all-sky monitoring is favorable.
How Deep?
~25 mag for z~1 sources ==> 8m-class telescopes are necessary.
Subaru/HSC (8.2m, 1.8 deg2 FoV), Rubin/LSST (6.5m, 9.6 deg2 FoV)How to “identify”?
Most(?) of the targets at z~1 are too far (too apparently faint) for 8m-classtelescopes.
SN Ia@z~130m-class telescopes are needed.
Subaru/FOCAS, 6.3 hours

Subaruhttp://subarutelescope.org

Rubin/LSST
https://www.lsst.org

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larger spheres corresponding
to larger Wide-Field
signals. The total Deep Spectroscopy
Better Telescopes/Instruments:
charge recorded is ~5800 photoelectrons. Inset is an overhead

þ0:50
Prime Focus Spectrographonangle8.2m
Telescopeconsistent with (PFS)a zenith
5:7Subaru
degrees
below the horizo"0:30~2,400 science fibers over 1.3 deg diameter FoVMost of possible origins can be spectroscopically observed.
“Transients” from Rubin/LSST.
Blind spectroscopy for Fermi
sources, blazars, post-starburst
galaxies(?) etc.
Operation starts from 2024.
Equator (~< 1,000 deg2) will beobserved in the “SSP” survey.

Decsource# [deg-2] (z<1)

SNe~102-3 (~101 if classified “interacting”)

Blazars~100-1TDEs<1AGNs~103

Subaru/PFS

contamination rate
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Better Strategy (w/ current observing facilities): faint blazars?

optical brightness (g)Blazar Radio and Optical Survey (BROS; Itoh+2020)
flat-spectrum@radio: NVSS (1.4 GHz) + TGSS (151 MHz)Pan-STARSS(PS1)@optical~40% not detected in PS1 (r>23)
88,211 sources at Dec.>-40 deg
largest blazar candidate catalog
BROS catalog
3,516 (ROMA-BZCAT), ~11,000 (CRATES), ~1,500 (3FGL)
a new faint blazar population in early-type galaxies?
7 BROS sourcesin catalog
170922A region (TM+2021)
BROS
7Itoh+2020

ation of all the 56314
BROS
sources inGalacticcoordinates.
The large ellipticalregion
opticalcolor(g-r)
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where no galaxy is registered within 100 Mpc is set to07 Le Mpc−3 for the conservative estimate determined by the catalog
eness.

candidate following a neutrino multiplet detection.

Better Neutrinos: “Multiplet”

Yoshida+2022

IceCube Collaboration
2017
dentiﬁcationof a Transient
Associatedwith a Neutrino
z=0.1
z=1
Multiplet

described in Section 3.1, the multiplet source is mostlocated at a relatively small redshift (z < 0.15, with anprobability). Given that most astrophysical neutrino
lets will therefore originate from objects with small
ts, we can construct a follow-up search strategy for
z=0.023
ying the optical counterpart.
Figure 7 shows a ﬂow chart
trate our proposed procedure to ﬁnd a source candidate.
z=0.06st step in this procedure for the follow-up
observations is
if the optical transient counterparts are close enough. We
e z = 0.15 as the threshold for the ﬁrst level selection.
detect emission from transients at z  0.15, the required
vity for optical follow-up observations is about 23 mag.
8 shows thetypicalpeak
opticalmagnitudeof different(stronglydependingon neutrino
spectralshape)
nts as a function of redshift. The sensitivity of 23 mag is
ent to detect hypernovae and broad-lined, energeticc SNe at z = 0.15. To perform such an optical survey for
Figure 8. Observed magnitude as a function of redshift for target objects afterwide-ﬁeld
opticalwith a diameter of
g2 area,Muchbetterthantelescopes“singlet”
expected time delays (green: hypernovae
at peak; red: TDEs at 100 days after-2] (z<1)
such as DECam on the 4 m Blonco telescope (Flaugher source#[degmultipletthe event; blue: SNe Ibc at peak).
closerorigin,localization2015), HSC
on the8.2 m better
Subaru telescope(Miyazaki018), and==>the much
8.4 m Rubin
observatory
telescope (Ivezić~102-3 (~101 if classified “interacting”) ~101 (<1)
SNelower
contamination
The light curves of normal SNe are generated with the2019), are required.
==> jet/interacting SNe can besncosmo package (Barbaryet al. 2016) by using templates~100-1~<1 ofh a deep survey can identify not only the true multiplet Blazars
as a neutrino
sourcethe available spectral energy distributions and their time
but also claimed
unrelated transient
objects (i.e.,contaminants).
TDEs<1 the SALT2 model (Guy
<1 et al.
evolutions. For Type Ia SNe,following
we ﬁrst estimate the number ofw/sections,high confidence.
2007) is used as a template. The parameters of the SALT2
mination sources that may be found with such a survey,
3 color parameters, are ~101
Identified
w/ smaller
telescopes
AGNs
model, such as the stretch~10andrandomly
en discuss
the strategy
to identify(~4m)
the neutrino
multipletselected according to the measured distribution (Scolnic &(better “singlet” localization: ~10-1 deg-2).
Kessler 2016). The peak luminosity of Type Ia SNe is assigned
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Summary & Future Prospects
Optical universe is so crowded & variable/transient.
One blazar (TXS 0506+056@z=0.336) was identified as a counterpartof IceCube-170922A (w/ much contribution from Japanese group).
Current difficulties in identifying the neutrino origin (if it is really ofan astrophysical origin) may be overcome w/
better telescopes/instruments
wide-field imaging (~8m; Subaru/HSC, Rubin/LSST)+ 8-30m spectrographs
blind spectroscopy w/ Subaru/PFS for multiplet
better strategy
faint blazar population
better neutrinos
multiplet w/ lower contaminations w/ smaller telescopes
w/ as good as ~0.1 deg2 localization
near-future ultimate?
= Rubin/LSST (imaging) + Subaru/PFS (spectroscopy) for multiplet
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