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Observation Date (UT) Observation Lat

Canonical Name:TXS 0506+056
TeVCat Name:TeV J0509+056
Other Names:EHE 170922A
3FGL J0509.4+0541
3FHL J0509.4+0542
VER J0509+057
Source Type:Blazar
R.A.:05 09 25 (hh mm ss)
Dec.:+05 42 09 (dd mm ss)
Gal Long: 195.39 (deg)
Gal Lat: -19.63 (deg)
Distance: z=0.3365
Flux:0.016 (Crab Units)
Energy Threshold:110 GeV
Spectral Index:4.8
Discovery Date:2017-10
Discovered By: MAGIC
TeVCat SubCat:Default Catalog

Source Notes:

This source was moved from the "Newly Announced" to the "Default"
catalogue on 180719.

A neutrino emitter:

The blazar TXS 0506+056 lies within the error circle of
IceCube-170922A, the IceCube high-energy neutrion candidate event
whose detection was reported in GCN circular #21916.
Follow-up observations were performed by a number of GeV-TeV
instruments with both Fermi-LAT and MAGIC reporting evidence for
gamma-ray emission from positions consistent with the IceCube neutrino
error circle which they thus associate with the blazar TXS 0506+056.
Upper limits on the gamma-ray emission from the region were
reported by H.E.S.S, HAWC and VERITAS. Detailed papers on all of
these observations can be found in the reference section below.

Detections of gamma-ray emission:

Fermi-LAT report the detection of increased gamma-ray activity from
TXS 0506+056.
From Tanaka et al. (2017):
- "We searched for Fermi-LAT sources inside the extremely high-energy
(EHE) IceCube-170922A neutrino event error region with all-sky survey
data from the Large Area Telescope (LAT), on board the Fermi Gamma-ray
Space Telescope."
- "We found that one Fermi-LAT source, TXS 0506+056 (3FGL J0509.4+0541
and also included in the 3FHL catalog, as 3FHL J0509.4+0542), is
located inside the IceCube error region. The FAVA (Fermi All-sky
Variability Analysis) light curve at energies above 800 MeV shows a
flaring state recently. Indeed, the LAT 0.1--300 GeV flux during 2018
September 15 to 27 was (3.6 +/- 0.5)e-7 photons cm-2 s-1 (errors are
statistical only), increased by a factor of approx. 6 compared to the
3FGL flux, with nearly the same power-law index of 2.0 +/- 0.1."

MAGIC report the detection of VHE gamma rays from a direction
consistent with the recent EHE neutrino event IceCube-170922A.
From Mirzoyan et al. (2017):
- "After the IceCube neutrino event EHE 170922A detected on 22/09/2017
(GCN circular #21916), Fermi-LAT measured enhanced gamma-ray emission
from the blazar TXS 0506+056 (05 09 25.96370, +05 41 35.3279 (J2000),
[Lanyi et al. (2010)]), located 6 arcmin from the EHE 170922A estimated
direction (ATel #10791)."
- "MAGIC observed this source under good weather conditions and a 5
sigma detection above 100 GeV was achieved after 12 h of observations
from September 28th till October 3rd."

Upper limits on gamma-ray emission:

H.E.S.S. performed follow-up observations of the high-energy neutrino
event detected by the IceCube collaboration on September 22nd, 2017
20:54:30 UTC.
From de Naurois et al. (2017):
- "H.E.S.S. observed the region around the IceCube best fit position
(RA=77.43 deg, Dec=5.72 deg; GCN circular #21916) in two consecutive
nights for about 1h each. First observations started 23 September 2017
at 01:05 UTC (about 4h after the neutrino detection). A second set of
observations were obtained the following night (24 September 2017 at
03:10 UTC). A preliminary on-site calibration and analysis searching
for a point-like gamma-ray source from within the 90% uncertainty
region of the neutrino event IceCube-170922A revealed no significant

HAWC reported on their observations just prior and after the neutrino
candidate time, since this sky location was not in the field of view
at the time of the neutrino event. They also performed an analysis of
archival data from this source position from November 2014 - June 2017:
From Martinez et al. (2017a):
Search for a steady source:
- "Assuming a spectral index of -2.5 we searched in a 1.3deg circle
around the IceCube reported location.The maximum significance is 3.5
sigma at RA=76.29deg and Dec=5.04deg. We estimate the number of trials
to be ~130."
An upper limit on the TeV emission was set.
Search for a transient source:
- "This analysis was performed using data corresponding to the two
nearest transits (MJD 58018.35-59018.60 and 58019.35-59019.60). Using
the same spectral index and search window, the maximum significance is
2.4 sigma at RA=76.45deg and Dec=5.58deg."
An upper limit on the TeV emission was set.
From Martinez et al. (2017b):
- "We have now also examined the time period that most closely matches
Fermi's observation (from 2017-09-15 09:04:47 UT to 2017-09-19
14:41:33 UT and from 2017-09-21 08:41:11 UT to 2017-09-27 14:10:06 UT).
We find no evidence for gamma ray emission during this 12 day time
period. We have placed a limit on the spectrum at 1 TeV"

VERITAS performed follow-up observations of the high-energy neutrino
event detected by the IceCube collaboration on September 22nd, 2017
20:54:30 UTC.
From Mukherjee et al. (2017):
- "VERITAS observed the location around the initial position reported by IceCube
in the GCN/AMON Notice dated Fri 22 Sep 17 20:55:13 UTC (RA = 77.29 deg,
Dec = 5.75 deg in J2000 coordinates) under partial cloud coverage for one
hour. Observations started on September 23rd, 2017 09:06 UTC, 12.2 hours
after the IceCube detection. No gamma-ray source was detected at the neutrino
position or anywhere else in the 3.5-degree VERITAS field of view."
After the Fermi-LAT Collaboration reported evidence for a flare from
TXS 0506+056, which is located within the neutrino error region:
- "A total of five hours of additional observations centered on the
blazar location were collected between September 28th 08:57 UTC and
September 30th 11:04 UTC. A preliminary analysis of the data optimized
for soft-spectrum sources shows no evidence of gamma-ray emission at
the blazar location."

Source position:
The source position was updated from that of NED to the position from VERITAS (2018).
From VERITAS Collaboration: (2018):
- R.A. (J2000): 77.354 deg = 05h 09m 25s
- Dec. (J2000): +5.702 deg = +05d 42' 09''
- Statistical error: 0.01 deg
- Systematic error: 0.007 deg

On 180208 the distance was updated from unknown to z = 0.3365 +/- 0.0010
due to the paper of Paiano et al. (2018).
From Paiano et al. (2018):
- "We present high signal-to-noise optical spectroscopy, in the range
4100-9000 Ang, obtained at the 10.4m Gran Telescopio Canarias. The
spectrum is characterized by a power law continuum and is marked by
faint interstellar features. In the regions unaffected by these
features, we found three very weak (equivalent width approx. 01 Ang)
emission lines that we identify with [O II] 3727 Ang, [O III] 5007
Ang, and [N II] 6583 Ang, yielding the redshift z = 0.3365 +/- 0.0010."

Spectral Information:
From VERITAS Collaboration: (2018):
- spectral index: 4.8 +/- 1.3
- E_0: 0.15 TeV
- N_0: 6.4 +/- 1.6 e-11 cm-2 s-1 TeV-1
From MAGIC Collaboration: (2018):
- the differential photon spectrum was measured between 80 GeV and 400 GeV
- spectral index ranges from 4.0 +/- 0.3 to 3.5 +/- 0.4
- "The systematic uncertainties of MAGIC spectral measurements can be
divided into: < 15% in energy scale, 11-18% in flux normalization and
+/- 0.15 for the energy spectrum power-law slope"
- The spectral properties were calculated for three different data selections:
- For E_0: 146 GeV:
- MJD 58029-03: spectral index: 3.86 +/- 0.32; flux normalisation: 2.91 +/- 0.62 e-10 TeV-1 cm-2 s-1
- MJD 58057: spectral index: 4.00 +/- 0.27; flux normalisation: 3.22 +/- 0.59 e-10 TeV-1 cm-2 s-1
- Low state: spectral index: 3.52 +/- 0.39; flux normalisation: 0.54 +/- 0.13 e-10 TeV-1 cm-2 s-1

From VERITAS Collaboration: (2018):
- integral flux (>110 GeV): 8.9 +/- 1.6 e-12 cm-2 s-1
- the flux corresponds to 1.6% that of the Crab above 110 GeV
From MAGIC Collaboration: (2018):
- "The probability of a constant flux is less than 0.3%. Two periods
of enhanced VHE gamma-ray emission can be clearly distinguished: one
on MJD 58029-58030 (Oct 3-4, 2017, already reported in Aartsen et al.
(2018)) and a second one on MJD 58057 (Oct 31, 2017)"

Seen by: MAGIC
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