IceCube-210516A

The IceCube Collaboration (http://icecube.wisc.edu/) reports:

On 16 May 2021 at 14:38:20.34 UT IceCube detected a track-like event with a moderate probability of being of astrophysical origin. The event was selected by the ICECUBE_Astrotrack_Bronze alert stream. The average astrophysical neutrino purity for Bronze alerts is 30% . This alert has an estimated false alarm rate of 3.1 events per year due to atmospheric backgrounds. The IceCube detector was in a normal operating state at the time of detection.

After the initial automated alert (https://gcn.gsfc.nasa.gov/notices_amon_g_b/135292_56063172.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 16 May 2021
Time:  14:38:20.34 UT
RA: 91.76 (+1.05  -0.97 deg  90% PSF containment) J2000
Dec: +9.52 ( +0.50 -0.58 deg 90% PSF containment) J2000

We encourage follow-up by ground and space-based instruments to help identify a possible astrophysical source for the candidate neutrino.

There are no Fermi 4FGL or 3FHL catalog sources in the 90% uncertainty region. The nearest gamma-ray source in either catalog is 4FGL J0608.6+1149 at RA: 92.17 deg, Dec: 11.83 deg J2000 (2.34 deg away from the best-fit event position).

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu

The IceCube Collaboration (http://icecube.wisc.edu/) reports:

IceCube has performed a search for additional track-like muon neutrino events arriving
from the direction of IceCube-210516A (https://gcn.gsfc.nasa.gov/gcn3/30026.gcn3
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 ) in a time
range of 2 days centered on the alert event time (2021-05-15 14:38:20.340 UTC to 2021-05-17 14:38:20.340 UTC) during which IceCube was collecting good quality data. Excluding the
event that prompted the alert, one additional track-like event is found in spatial coincidence
with the 90% containment region of IceCube-210516A. We find that these data are consistent with atmospheric background expectations, with a p-value of 1.0. We accordingly derive a time-integrated muon-neutrino flux upper limit at the alert position of E^2 dN/dE = 3.6 x 10^-5 TeV cm^-2 at 90% CL, under the assumption of an E^-2 power law. 90% of events IceCube would detect from a source at this declination with an E^-2 spectrum are approximately between 1 TeV and 2 PeV.

A subsequent search was performed to include the month of data prior to the alert event (2021-04-16 14:38:20.340 UTC to 2021-05-17 14:38:20.340 UTC). In this case, we report a p-value of 1.0, consistent with no significant excess of track-like events, and a corresponding time-integrated muon-neutrino flux upper limit assuming an E^-2 spectrum (E^2 dN/dE) of
4.7 x 10^-5 TeV cm^-2 at the 90% CL.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu<mailto:roc@icecube.wisc.edu>.

Hugo Ayala (Penn State) reports on behalf of the HAWC
collaboration (http://www.hawc-observatory.org/collaboration):

On 2021/05/16 14:38:20 UTC, the IceCube collaboration reported a
track-like very-high-energy event  that has a high probability of
being an astrophysical neutrino, IceCube-210516A. Location is at
RA: 91.76 (+1.05/-0.97 deg  90% PSF containment) J2000
Dec: 9.52 (+0.5/-0.58 deg 90% PSF containment) J2000
(GCN circular 30026
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 ).

We performed two types of analyses for the follow-up. The first is for
a steady source in archival data and the second is a search for a
transient source. We assume a power-law spectrum with an index of -2.3
for both analyses.

Search for a steady source in archival data:
The archival data spans from November 2014 to June 2019. We searched
inside the reported IceCube error region.
The most significant location, with p-value 3.0e-2 (2.9e-1 post-trials),
is at RA 92.81 deg, Dec +10.01 deg (��x.xx deg 68% containment) J2000.
We set a time-integrated 95% CL  upper limit on gamma rays at the
maximum position of:

E^2 dN/dE = 1.98e-13 (E/TeV)^-0.3 TeV.cm^-2.s^-1

Search for a transient source.

Since the event was not in our field of view at the time reported,
we report the combined result for the transits before and after the
IceCube event.

Data acquisition started on 2021/05/14 23:42:33 UTC and ended
2021/05/16 23:55:20 UTC.
The most significant location, with p-value 1.3e-2  (1.3e-1 post-trials),
is at RA 91.93 deg, Dec +1.01 deg (��0.20 deg 68% containment) J2000.
We set a time-integrated 95% CL upper limit at the position of
maximum significance of:

E^2 dN/dE = 4.78e-12 (E/TeV)^-0.3 TeV.cm^-2.s^-1

HAWC is a very-high-energy gamma-ray observatory operating in Central
Mexico at latitude 19 deg. north. Operating day and night with over
95% duty cycle, HAWC has an instantaneous field of view of 2 sr and
surveys 2/3 of the sky every day. It is sensitive to gamma rays from
300 GeV to 100 TeV.

S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and C. C. Cheung (Naval Research Laboratory) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the high-energy IC210516A neutrino event (GCN 30026
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 ) with all-sky survey data from the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope. The IceCube event was detected on 2021-05-16 at 14:38:20.34 UT (T0) with J2000 position RA = 91.76 (+1.05, -0.97) deg, Decl. = 9.52 (+0.50, -0.58) deg (90% PSF containment). No cataloged >100 MeV gamma-ray source is located within the 90% IC210516A localization region. 

We searched for intermediate (days to years) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (> 5 sigma) new excess emission (> 100 MeV) at the IC210516A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC210516A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 6.6e-10 ph cm^-2 s^-1 for ~12-years (2008-08-04 to 2021-05-16 UTC), and < 6.5e-9 (< 7.1e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.  

Within the 90% confidence localization of the neutrino, 0.8 deg offset from the best-fit IC210516A position, an excess of gamma rays, Fermi J0610.5+0946, was detected in an analysis of the ~12-years integrated LAT data (100 MeV - 1 TeV) prior to T0. This putative new source is detected at a statistical significance ~4 sigma (calculated following the prescription adopted in the 4FGL). Assuming a power-law spectrum, the excess has best-fit localization of RA = 92.644 deg, Decl. = 9.782 deg (7 arcmin 68% containment, 14 arcmin 99% containment) with best-fit spectral parameters, flux = (7.4 +/- 4.2)e-9 ph cm^-2 s^-1, index = 2.7 +/- 0.3. In a preliminary analysis of the LAT data over one month prior to T0, Fermi J0610.5+0946 is not significantly detected in the LAT data. All values include the statistical uncertainty only. 

Since Fermi normally operates in an all-sky scanning mode, regular monitoring of the IceCube event region will continue. For these observations the Fermi-LAT contact persons are S. Garrappa (simone.garrappa at desy.de) and S. Buson (sara.buson at uni-wuerzburg.de).

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland)
J. Rodi (IAPS-Roma, Italy)
A. Coleiro (APC, France)
S. Mereghetti (INAF IASF-Milano, Italy)

on behalf of the INTEGRAL multi-messenger collaboration:
https://www.astro.unige.ch/cdci/integral-multimessenger-collaboration

Using a combination of INTEGRAL all-sky detectors (following [1])
SPI/ACS, IBIS/Veto, and IBIS, we have performed a search for a prompt
gamma-ray counterpart of IceCube-210516A (GCN 30026
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 ).

At the time of the event (2021-05-16 14:38:20 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 130 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (4.5% of optimal) response of ISGRI, somewhat suppressed
(60% of optimal) response of IBIS/Veto, and strongly suppressed (30%
of optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was somewhat
unstable (excess variance 1.8).

We have performed a search for impulsive events in INTEGRAL SPI-
ACS (as described in [2]), IBIS, and IBIS/Veto data.

We do not detect any significant counterparts and estimate a 3-sigma
upper limit on the 75-2000 keV fluence of 5.7e-07 erg/cm^2 (within the
50% probability containment region of the source localization) for a
burst lasting less than 1 s with a characteristic short GRB spectrum
(an exponentially cut off power law with alpha=-0.5 and Ep=600 keV)
occurring at any time in the interval within 300 s around T0. For a
typical long GRB spectrum (Band function with alpha=-1, beta=-2.5, and
Ep=300 keV), the derived peak flux upper limit is ~5.4e-07 (1.3e-07)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

We report for completeness and in order of FAP, all excesses
identified in the search region. We find: 4 likely background
excesses:

T-T0 | scale | S/N | flux ( x 1e-05 erg/cm2/s) | FAP
-118 | 1.85 | 4.2 | 0.632 +/- 0.176 +/- 0.33 | 0.17
-189 | 4.3 | 3.4 | 0.351 +/- 0.115 +/- 0.183 | 0.374
52.2 | 0.4 | 3.8 | 1.26 +/- 0.38 +/- 0.656 | 0.515
93.7 | 0.55 | 3.8 | 1.06 +/- 0.324 +/- 0.555 | 0.753

Note that FAP estimates (especially at timescales above 2s) may be
possibly further affected by enhanced non-stationary local background
noise. This list excludes any excesses for which FAP is close to
unity.



All results quoted are preliminary.

This circular is an official product of the INTEGRAL Multi-Messenger
team.

[1] Savchenko et al. 2017, A&A 603, A46 [2] Savchenko et al. 2012, A&A
541A, 122S