IceCube-220624A

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

On 2022-06-24 at 16:13:16.41 UT IceCube detected a track-like event with a high probability of being of astrophysical origin. The event was selected by the ICECUBE_Astrotrack_GOLD alert stream. The average astrophysical neutrino purity for Gold alerts is 50%. This alert has an estimated false alarm rate of 0.173 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/136766_7637140.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 2022-06-24
Time:  16:13:16.41 UT
RA: 224.12 (+2.23, -1.95 deg  90% PSF containment) J2000
Dec: 41.31 (+1.56, -1.56 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 J1504.6+4343 at RA: 226.16 deg, Dec: 43.72 deg (2.84 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

Simeon Reusch, Jannis Necker (DESY), Robert Stein (Caltech), Sven Weimann (Ruhr University Bochum) and Anna Franckowiak (DESY/Ruhr University Bochum) report:

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations:

We observed the localization region of the neutrino event IceCube-220624A (Santander et. al, GCN 32260
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 ) with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2022-06-25 05:15 UTC, approximately 13.0 hours after event time. We covered 83.2% (8.1 sq deg) of the reported localization region. This estimate accounts for chip gaps. Both exposures were 30s long, with a typical depth of 20.5 mag.

The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019).

No candidate counterparts were detected. We will continue monitoring the localization region with ZTF in the g-band for the next days.

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA; WIS, Israel; OKC, Sweden; JSI/UMd, USA; DESY, Germany; TANGO, Taiwan; UW Milwaukee, USA; LANL, USA; TCD, Ireland; IN2P3, France.

GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949.
Alert distribution service provided by DIRAC@UW (Patterson et al. 2019).
Alert database searches are done by AMPEL (Nordin et al. 2019).
Alert filtering is performed with the AMPEL Follow-up Pipeline (Stein et al. 2021).

Woo-Hyeon Heo, Hugo Ayala (UOS, PSU) reports on behalf of the HAWC
collaboration (http://www.hawc-observatory.org/collaboration):

On 2022/06/24 16:13:16 UTC, the IceCube collaboration reported a
track-like very-high-energy event  that has a high probability of
being an astrophysical neutrino, IceCube-220624A. Location is at
RA: 224.12 (+2.23/-1.95 deg  90% PSF containment) J2000
Dec: 41.31 (+1.56/-1.56 deg 90% PSF containment) J2000
(GCN circular 32260
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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 1.81e-3 (1.13e-1 post-trials),
is at RA 222.32 deg, Dec +39.84 deg (��0.80 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 = 1.9e-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 2022/06/23 06:07:04 UTC and ended
2022/06/25 06:24:04 UTC.
The most significant location, with p-value 8.74e-4  (5.6e-2 post-trials),
is at RA 224.56 deg, Dec +40.18 deg (��0.19 deg 68% containment) J2000.
We set a time-integrated 95% CL upper limit at the position of
maximum significance of:

E^2 dN/dE = 1.0e-11 (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.

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

IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-220624A (https://gcn.gsfc.nasa.gov/gcn3/32260.gcn3
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 ) in a time range of 1000 seconds centered on the alert event time (2022-06-24 16:04:56.405 UTC to 2022-06-24 16:21:36.405 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-220624A. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-220624A is 1.5e-01 GeV cm^-2 in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 2e+02 GeV and 6e+04 GeV.

A subsequent search was performed including 2 days of data centered on the alert event time (2022-06-23 16:13:16.405 UTC to 2022-06-25 16:13:16.405 UTC). In this case, we report a p-value of 1.00, consistent with no significant excess of track events. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-220624A is 1.8e-01 GeV cm^-2 in a 2 day time window.

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>.

[1] IceCube Collaboration, R. Abbasi  et al., ApJ 910 4 (2021)

S. Garrappa (DESY-Zeuthen) and S. Buson (Univ. of Wuerzburg) on behalf 
of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the high-energy 
IC220624A neutrino event (GCN 32260
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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 2022-06-24 at 16:13:16.41�� 
UT (T0) with J2000 position RA = +224.12 (+2.23, -1.95) deg, Decl. = 
41.31 (+1.56, -1.56) deg (90% PSF containment). No cataloged gamma-ray 
(>100 MeV) sources are located within the 90% IC220624A localization 
region (4FGL-DR3; arXiv:2201.11184
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 ; The Fermi-LAT collaboration 2020, 
ApJS, 247, 33).

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 IC220624A 
best-fit position. Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC220624A best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 9.3e-11 ph cm^-2 s^-1 for 
~14-years (2008-08-04 to 2022-06-24 UTC), and < 3.9e-9 (< 5.4e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

Within the 90% confidence localization of the neutrino, 0.2 deg offset 
from the best-fit IC220624A position, an excess of gamma rays, Fermi 
J1458.0+4119, was detected in an analysis of the ~14-years integrated 
LAT data (100 MeV - 1 TeV) prior to T0. This putative new source is 
detected at a statistical significance ~4.5 sigma (calculated following 
the prescription adopted in the The Fourth Fermi-LAT catalog, The 
Fermi-LAT collaboration 2020, ApJS, 247, 33). Assuming a power-law 
spectrum, the excess has best-fit localization of RA = 224.52 deg, Decl. 
= 41.32 deg (5 arcmin 68% containment, 10 arcmin 99% containment) with 
best-fit spectral parameters, flux = (5 +/- 2)e-10 ph cm^-2 s^-1, index 
= 1.9 +/- 0.2. In a preliminary analysis of the LAT data over one month 
prior to T0, Fermi J1458.0+4119 is not significantly detected in the LAT 
data. All values include the statistical uncertainty only.

A probable counterpart of Fermi J1458.0+4119 is the high-synchrotron 
peaked blazar WISEA J145820.77+412101.9 (aka 3HSP J145820.8+412102) at 
RA=224.58658 deg, Dec=41.35028 deg, and redshift 0.176463 +/- 0.000027 
(Sloan Digital Sky Survey Data Release 13, 2016 SDSS). It is located 
about 4 arcmin from the Fermi J1458.0+4119 best-fit position, and within 
the gamma-ray 68% positional uncertainty. This source has been proposed 
as a promising very-high-energy candidate emitter (>100 TeV; Arsioli et 
al. 2015, A&A, 579, 34).

Since Fermi normally operates in an all-sky scanning mode, regular 
monitoring of this source 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.

P.A. Evans (U. Leicester) and J.A. Kennea (PSU) report on behalf of the 
Swift-XRT team:

Swift-XRT observed the blazar WISEA J145820.77+412101.9 for 4.9 ks, 
starting at 15:07 UT on 2022 June 28. This blazar was posited by the 
Fermi team (GCN Circ. 32285
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 , ATEL #15478) to be the potential 
counterpart to a new gamma-ray source, Fermi J1458.0+4119, itself a 
possible counterpart to the high energy neutrino IceCube 220624A (GCN 
Circ. 32260
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 ).

The Swift observations show an approximately constant count-rate of 
9+/-2- x 10^-3 ct/sec. A spectrum created from the 26 photons detected 
can be fitted with a power-law with a photon index of 3.0 (+1.8, -1.1) 
absorbed by a column NH = 1.0 (+2.8, -1.0) x 10^21 cm^-2; the large 
uncertainties arising due to the low number of counts. This corresponds 
to a 0.3-10 keV flux of 1.72 (+1.14, -0.60) x 10^-13 erg cm^-2 s^-1.

The HEASARC X-ray Master catalogue reports several ROSAT fluxes for this 
source, ranging from 3--10 x 10^-13 erg cm^2 s^-1. Converting our 
spectrum to the ROSAT PSPC energy band (0.1-2.4 keV), we find a flux of 
1.5 (+3.3, -0.4) x 10^-13 erg cm^2 s^-1. Thus, presuming that the 
catalogued fluxes correspond to quiscence (which is not guaranteed), we 
find no evidence for an X-ray outburst from WISEA J145820.77+412101.9.

Felicia McBride, Derek Fox, Doug Cowen, Hugo Ayala (PSU) and Alexis Coleiro
(APC) report for AMON (https://amon.psu.edu/):

The Neil Gehrels Swift Observatory X-ray Telescope (XRT) observed the
uncertainty region of the IceCube neutrino event IceCube-220622A (GCN Circ.
32260
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 ) between 2022-06-24 20:07 UTC and 2022-06-25 02:24 UTC.
A 7-point tiling observation centered on the neutrino position was taken
for a total of 6.8 ksec.

One possible X-ray candidate counterparts was detected with a flux of
~5x10^-13 erg/s/cm^2.
The counterpart is consistent with the active galaxy 1RXS J145552.7+414026
at an angular distance of 0.38 degrees from the neutrino position.

Multiwavelength follow-up of the source is encouraged.

Robert Stein (Caltech), Jannis Necker, Simeon Reusch (DESY), Shreya Anand, Kaustav Das (Caltech), Jesper Sollerman (Stockholm Uni), Viraj Karambelkar, Mansi Kasliwal (Caltech) and Anna Franckowiak (DESY/Ruhr University Bochum) report:

We observed neutrino IC220624A (Santander et. al, GCN 32260
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 ) with the Zwicky Transient Facility (Reusch et al., GCN 32270
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 ) as part of our ZTF neutrino follow-up program (Stein et al. 2022). Though no candidates were identified with our initial ToO observation, we continued our regular monitoring of the localisation with additional ToO and serendipitous observations. 

As part of these additional observations, we identified the transient ZTF22aaparxg/AT2022nit
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  as a possible optical counterpart. We undertook spectroscopic observations of AT2022nit
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  with the Low Resolution Imaging Spectrometer (LRIS, Oke et al. 95) at the Keck I Observatory.

Using SNID (Blondin et al. 2007), we classify AT2022nit
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  as a type Ia supernova at redshift z=0.16 (see https://www.wis-tns.org/object/2022nit for the full spectrum). Given that type Ia supernovae are not predicted to emit high-energy neutrinos, we therefore exclude AT2022nit
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  as a candidate counterpart to IC220624A.

We also identified ZTF19adgzidh/AT2021bei
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  as another optical counterpart candidate and observed it with ALFOSC on the 2.5m NOT telescope at Roque de los Muchachos Observatory.

Using SNID (Blondin et al. 2007), we classify AT2021bei
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  as flare in an AGN at redshift z=0.48 (see https://www.wis-tns.org/object/2021bei for the full spectrum). Because such flares occur frequently across the sky, we expect to find coincidences between AGN flares and neutrinos by random chance. Given the lack of any exceptional flaring activity in this source, we conclude that AT2021bei
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  is not a promising counterpart to IC220624A.

We have not identified any other candidate optical counterparts with ZTF.

Based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias.

The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.