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Observation Date (UT) Observation Lat

Canonical Name:GRB 180720B
TeVCat Name:TeV J0002-029
Other Names:
Source Type:GRB
R.A.:00 02 07.6 (hh mm ss)
Dec.:-02 56 06 (dd mm ss)
Gal Long: 94.83 (deg)
Gal Lat: -63.08 (deg)
Distance: z=0.654
Flux: (Crab Units)
Energy Threshold: GeV
Spectral Index:
Extended:No
Discovery Date:2019-05
Discovered By: H.E.S.S.
TeVCat SubCat:Default Catalog

Source Notes:

Overview of the Detection:
The detection of VHE emission from GRB 180720B was announced by Ruiz-Velasco at the
1st International CTA Symposium in May 2019.
This burst had a duration of approx. 150 sec (Siegel et al.) making it a long GRB.

From Wang et al. (2019):
- "HESS starts to observe GRB 180720B at about 10 hr after the burst
and detected 100-440 GeV photons at such late times (Ruiz-Velasco 2019)."
From Abdalla et al. (2019):
- "Here we report observations of very-high-energy emission in the
bright GRB 180720B deep in the GRB afterglow—ten hours after the end
of the prompt emission phase, when the X-ray flux had already decayed
by four orders of magnitude."
- "Two possible explanations exist for the observed radiation: inverse
Compton emission and synchrotron emission of ultrarelativistic
electrons."
- "This discovery places distinct constraints on the GRB environment
for both emission mechanisms, with the inverse Compton explanation
alleviating the particle energy requirements for the emission observed
at late times."
- "Observations with the High Energy Stereoscopic System (H.E.S.S.)
array began at T0 + 10.1 h and lasted for two hours. The data were
analysed using methods optimized for the detection of the
lowest-energy events, revealing a new gamma-ray source with an
excess of 119 gamma-ray events and a statistical significance of 5.3 sigma."
- "The gamma-ray excess is well fitted by a point-like source model
centred at a right ascension of 00h 02 min 7.6 s and a declination
of -02d 56' 06'' (J2000) with a statistical uncertainty of 1.31', con-
sistent with the measurements at other wavelengths."
- "To rule out the association of this source with an unknown steady
γ-ray emitter (such as an active galactic nucleus) or persistent
systematic effects, the GRB region was re-observed under similar
conditions 18 days after these observations. In total, 6.75 h of data
were analysed, resulting in a sky map consistent with background
events."

GRB Parameters:
- A long GRB: this GRB was detected by the Fermi Gamma-ray
Burst monitor (GBM; Roberts et al. 2019) and then, 5 seconds later, by the Neil Gehrels Swift
Burst Alert Telescope (BAT; Siegel et al. 2019).
Both reported initial durations placing this GRB in the long burst category.

- T0: 14:21:39.65 UT (GBM; Roberts et al. 2019)

- T90: 48.9 +/- 0.4 s (Abdalla et al. 2019)

- E_iso: the isotropic energy release was 6.0 +/- 0.1 x10e53 erg (50 - 300 keV; 1 erg = 10e-7) (Abdalla et al. 2019)

- "The prompt emission phase of GRB 180720B is extremely bright,
ranking seventh in brightness among the over 2,650 GRBs detected by
Fermi-GBM so far" (Abdalla et al. 2019)

Source Position:
On 191210 the GRB position in TeVCat was updated from that give in Martone et al. (2018) to that given in Abdalla et al. (2019).
From Abdalla et al. (2019):
- R.A. (J2000): 00h 02m 7.6s
- Dec. (J2000): -02d 56' 06''
- statistical uncertainty: 1.31'
From Martone et al. (2018):
- R.A. (J2000): 00h 02m 06.87s
- Dec. (J2000): -02d 55' 05.2''

Distance:
From Vreeswijk et al. (2018):
The European Southern Observatory's Very Large Telescope measured the redsift:
- "Several absorption features are detected, which can be identified
as due to Fe II, Mg II, Mg I, and Ca II, all at z = 0.654. We also
identify fine-structure transitions due to both Fe II* and Ni II**,
thus making the redshift association with the GRB secure."

Spectral Properties:
From Abdalla et al. (2019):
- "The flux spectrum detected by H.E.S.S. (100–440 GeV) was fitted with
a function of the form":
... F_obs(E) = F_int(E) x e-tau(E,z)
"where the exponential term accounts for the absorption of photons by the extragalactic background
light, tau is the optical depth and":
... F_int(E) = F0_int(E/E0_int)^-A_int
"is a powerlaw describing the intrinsic source emission."
- "The analysis resulted in a photon index of":
... A_int = 1.6 +/- 1.2 (stat) +/- 0.4 (syst)
"and a flux normalization of":
... F0_int = 7.52 +/- 2.03 (stat) -3.84(syst) x10e-10 TeV-1 cm-2 s-1
"evaluated at an energy of":
... E0_int = 0.154 TeV
"The spectrum measured by Fermi-LAT (100 MeV-10 GeV) from T0 + 55 s to
T0 + 700 s is well fitted by a power-law model with photon index:"
... A_LAT = 2.10 +/- 0.10.

Temporal Evolution:
From Abdalla et al. (2019):
- "Apart from the exceptionally high flux level, the light curves show
a typical power-law behaviour in the X-ray and optical afterglow with
a temporal flux decay of the form F(t) prop. to t^-a with:"
... a_XRT = 1.29 +/- 0.01 and
... a_optical = 1.24 +/- 0.02.
- "The light curve measured by Fermi-LAT (100 MeV-10 GeV) from T0 + 55 s
to T0 + 700 s is fitted by a power law with a temporal decay index of:"
... a_LAT = 1.83 ± 0.25.

Fermi-LAT Detection:
This GRB was detected by the Fermi Large Area Telescope: From Bissaldi et al. (2018):
- "At 14:21:44.55 UT on July 20, 2018, Fermi-LAT detected high-energy
emission from GRB 180720B"
- "The highest-energy photon is a 5 GeV event which is observed 137
seconds after the GBM trigger."
From Abdalla et al. (2019):
- "No further high-energy emission was detected in the successive
observation windows after 700 s."


Seen by: H.E.S.S.
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