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Observation Date (UT) Observation Lat

Canonical Name:PG 1553+113
TeVCat Name:TeV J1555+111
Other Names:1ES 1553+113, HESS J1555+111
Source Type:HBL
R.A.:15 55 44.7 (hh mm ss)
Dec.:+11 11 41 (dd mm ss)
Gal Long: 21.92 (deg)
Gal Lat: 43.96 (deg)
Distance: z=
Flux:0.034 (Crab Units)
Energy Threshold:300 GeV
Spectral Index:4.5
Discovery Date:2006-03
Discovered By: H.E.S.S.
TeVCat SubCat:Default Catalog

Source Notes:

Source position and its uncertainty:

From Aharonian et al. (2008):
- RA (J2000): 15 55 44.7 +/- 3.0s(stat) +/- 1.3s(syst)
- Dec (J2000): +11 11 41 +/- 53"(stat) +/- 20"(syst)


The redshift of this source is uncertain.

From Dorigo Jones et al. (2021):
- "Here, we significantly improve the precision of indirect redshift
constraints for blazars based on the edge of the HI-Ly-alpha forest
observed in their UV spectra. We develop a robust technique to
constrain the redshift of a z<0.5 AGN or blazar with a 1 sigma
uncertainty of approx. 0.01 using only the position of its
highest-redshift Ly-alpha absorber with log N_HI/cm^-2 > 12.6."
- "We present new COS NUV data for 1ES1553+113 and confirm its
redshift of z=0.433 using our technique."

From Zahoor et al. (2021):
- In this work, the effect of the absorption of the extragalactic
galactic background light was used to place constraints on the
redshifts of gamma-ray sources.
- "The very high energy (VHE) gamma-ray spectral indices of blazars
show strong correlation with the source redshift. Absence of any such
correlation in low energy gamma rays and X-rays indicate the presence
of Extragalactic Background Light (EBL) induced absorption of VHE
gamma rays. By employing a linear regression analysis, this
observational feature of blazars is used to constrain the redshift of
BL Lac objects which was unknown/uncertain earlier."
- Two different regression relations were used to constrain the source
redshift and the following values were found for PG 1553+113:
- z = 0.38 +/- 0.10
- z = 0.33 +/- 0.08

From Nicastro (2018):
- "In this contribution we first summarize the fidings from our recent
discovery of two intervening OVII-bearing absorption systems along the
line of sight to the blazar 1ES 1553+113, at redshifts z = 0.3551 and
z = 0.4339"

From Abramowski et al. (2015):
- "The most probable redshift found with this method is z = 0.49 +/- 0.04,
in good agreement with the independent measure of Danforth
et al. (2010)."

From Aliu et al. (2014):
- "When accounting for EBL evolution using the approach described (in
reference above), the 95% confidence level upper limit on the redshift
of PG 1553+115 is z<=0.62"

From Landoni et al. (2014):
- "We secured optical and near-infrared medium-resolution spectra of
four bright BL Lac objects of unknown redshift using the spectrograph
X-Shooter at the ESO-VLT."
- "In spite of the high quality of the spectra and the extended
spectral range of the observations, we have not detected intrisic
spectral features for these sources. However, we are able to provide
strigent lower limits to their redshift." (For two TeV sources: PG 1553+113
and H 1722+119)
- For PG 1553+113 they inferred z > 0.30

From Danforth et al. (2010):
- This distance estimate constrains the redshift to z > 0.400,
specifically putting it in the range: 0.43 - 0.58.

From Yang & Wang (2010):
- an upper limit of 0.78 is placed on the redshift of this object.

Spectral Properties:

From Abramowski et al. (2015):
- Spectral index: 4.8 +/- 0.2 (stat) +/- 0.2 (syst) (power law; pre-flare)
... a log-parabola model was also fit to the pre-flare data
- Spectral index: 4.9 +/- 0.3 (stat) +/- 0.2 (syst) (power law; flare)

From Aliu et al. (2014):
- Spectral index: 4.33 +/- 0.09

From Benbow et al. (2011):
- Spectral index: 4.41 +/- 0.14

From Aharonian et al. (2008):
- Spectral index: 4.5 +/- 0.3(stat) +/- 0.1(syst)

From Albert et al. (2008):
- Spectral index: 4.1 +/- 0.3


From Aliu et al. (2014):
- Fluxes of between 5.5% and 10.3% of the Crab are reported

From Benbow et al. (2011):
- a flux of 10% that of the Crab is reported

From Aharonian et al. (2008):
- a flux of 3.4% that of the Crab is reported

Information on the jet:

From Lister et al. (2019):
- "We were unable to identify any robust jet features in this BL Lac object."

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