BL Lacertae

Observation Date (UT)	Observation Lat
	ARGO-YBJ (30.11) ASTRI Mini-Array (28.3) CANGAROO (-31.10 ) CAT (42.50) Crimea (45) CTA pSCT (31.68) CTA South (-24.6272) Durham (-30.48) FACT (28.76) H.E.S.S. (-23.16) HAGAR (30) HAWC (19.0) HEGRA (28.76) LHAASO (29.358) LST-1 (28.76) MACE (32.78) MAGIC (28.76) Milagro (35.88 ) Potchefstroom (-26.9) TACTIC (24.6) TAIGA (103.07) Telescope Array (39.3) Tibet (30.11) VERITAS (31.68) Whipple (31.68)

Canonical Name:	BL Lacertae
TeVCat Name:	TeV J2202+422
Other Names:	1ES 2200+420,
Source Type:	IBL
R.A.:	22 02 43.3 (hh mm ss)
Dec.:	+42 16 40 (dd mm ss)
Gal Long:	92.59 (deg)
Gal Lat:	-10.44 (deg)
Distance:	z=0.069
Flux:	0.03 (Crab Units)
Energy Threshold:	200 GeV
Spectral Index:	3.6
Extended:	No
Discovery Date:	2001-04
Discovered By:	Crimea
TeVCat SubCat:	Default Catalog

Source Notes:

Source position and its uncertainty:
From Arlen et al. (2013):
- R.A. (J2000): 22h 02m 37s
- Dec. (J2000): +42d 15' 25''
- statistical uncertainty of approx. 0.01 deg in each direction
From Albert et al. (2007):
- This is the position that is reported in TeVCat.
- R.A. (J2000): 22h 02m 43.3s
- Dec. (J2000): 42d 16' 40"
- "The small offset and the extension of the image are comparable
to the telescope PSF (0.1deg) and the telescope pointing error (1.5')"

Spectral Properties:
From Abeysekara et al. (2018):
- This is the spectrum during a flare that reached 180% of the Crab at its peak and averaged
0.95 Crab Units over its whole duration (above 200 GeV).
- A log-parabola model was found to fit the VERITAS data better than a power law.
- dN/dE = F0 (E/E0) ^(-alpha - b log10(E/E0))
- spectral index (alpha): 2.4 +/- 0.1
- b: 1.8 +/- 0.3
- F0: 2.22 +/- 0.07 x10e-5 m-2 s-1 TeV-1
- E0: 200 GeV
From Arlen et al. (2013):
- This is the spectrum during a flare that reached 125% of the Crab flux at these energies:
- dN/dE = F0 (E/E0)^-alpha
- spectral index (alpha): 3.6 +/- 0.4
- F0: 0.58 +/- 0.07 x10e-9 cm-2 s-1 TeV-1
- E0: 300 GeV
From Albert et al. (2007):
- This is the spectrum during a low-flux state corresponding to 3% of the Crab flux at these energies:
- dN/dE = F0 (E/E0)^-alpha
- spectral index (alpha): 3.6 +/- 0.5
- F0: 1.9 +/- 0.5 x10e-11 cm-2 s-1 TeV-1
- EO: 300 GeV

Classification:
Initially, this source was classified as an LBL in TeVCat. It has
been updated (121113) to an IBL based on its classification in
the Fermi 2nd AGN Catalog (Ackermann et al. 2012)


Seen by: Crimea, MAGIC, VERITAS, LST-1

• Do multifrequency polarimetric observations of BL Lac exclude a hadronic origin of its X-ray emission?
  Tavecchio, F. et al., arXiv e-prints parXiv:2505.19784 (2025) [LINK]
  
• BL Lacertae under the Flare of 2024: Probing Temporal and Spectral Dynamics
  Majumdar, Joysankar et al., arXiv e-prints parXiv:2505.18666 (2025) [LINK]
  
• Origin of the X-ray emission in blazars through multiwavelength polarization
  Liodakis, Ioannis et al., arXiv e-prints parXiv:2505.13603 (2025) [LINK]
  
• High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet
  Agudo, Ivan et al., arXiv e-prints parXiv:2505.01832 (2025) [LINK]
  
• Fermi-LAT detection of renewed gamma-ray activity from the blazar BL Lacertae
  Bartolini, C., Bronzini, E. & Monti-Guarnieri on behalf of the Fermi Large Area Telescope Collaboration, The Astronomer's Telegram 17039 p1 (2025) [LINK]
  
• Spectral Energy Distribution Modeling of BL Lacertae during a Large Submillimeter Outburst and Low X-Ray Polarization State
  Mondal, Ayon et al., ApJ 978 p43 (2025) [LINK]
  
• LAST optical observations of the blazar BL Lacertae in bright optical state
  Garrappa, S. et al. and on behalf of the LAST Collaboration, The Astronomer's Telegram 16865 p1 (2024) [LINK]
  
• VERITAS Detection of Gamma-ray Flaring Activity from BL Lacertae
  Furniss, A. for the VERITAS Collaboration, The Astronomer's Telegram 16854 p1 (2024) [LINK]
  
• LHAASO detects rapid variability of the TeV Gamma-ray Activity of BL Lacertae
  Xiang, G. et al. on behalf of the LHAASO Collaboration, The Astronomer's Telegram 16850 p1 (2024) [LINK]
  
• Swift follow-up observations of BL Lacertae
  Nozaki, S. and Sanchez, D., The Astronomer's Telegram 16855 p1 (2024) [LINK]
  
• The blazar BL Lacertae is brightening in Optical bands
  Kishore, S. and Gupta, A., The Astronomer's Telegram 16856 p1 (2024) [LINK]
  
• Detection of flaring very-high-energy gamma-ray emission from BL Lacertae with the MAGIC telescopes
  Paneque, D. et al. on behalf of the MAGIC Collaboration, The Astronomer's Telegram 16861 p1 (2024) [LINK]
  
• Exceptional X-ray activity in BL Lacertae
  Wierzcholska, Alicja and Wagner, Stefan, arXiv e-prints parXiv:2412.18680 (2024) [LINK]
  
• Minute time scale variability in gamma-ray flare of BL Lacertae
  Majumdar, Joysankar et al., arXiv e-prints parXiv:2410.23778 (2024) [LINK]
  
• X-Ray Polarization of BL Lacertae in Outburst
  Peirson, Abel L. et al., ApJ 948 pL25 (2023) [LINK]
  
• Intra-night optical flux and polarization variability of BL Lacertae during its 2020-2021 high state
  Bachev, Rumen et al., MNRAS 522 p3018-3035 (2023) [LINK]
  
• Analysis of the intra-night variability of BL Lacertae during its August 2020 flare
  Agarwal, Aditi et al., arXiv e-prints parXiv:2302.10177 (2023) [LINK]
  
• The optical behaviour of BL Lacertae at its maximum brightness levels: a blend of geometry and energetics
  Raiteri, C.M. et al., arXiv e-prints parXiv:2302.10555 (2023) [LINK]
  
• X-ray Polarization Observations of BL Lacertae
  Middei, Riccardo et al., arXiv e-prints parXiv:2211.13764 (2022) [LINK]
  
• Detection of minute-timescale gamma-ray variability in BL Lacertae by Fermi-LAT
  Pandey, Ashwani and Stalin, C.S., arXiv e-prints parXiv:2210.00799 (2022) [LINK]
  
• Multiwavelength periodicity search in a sample of gamma-ray bright blazars
  Otero-Santos, J. et al., arXiv e-prints parXiv:2210.16327 (2022) [LINK]
  
• The microvariability and wavelength dependence of polarization degree/angle of BL Lacertae in the outburst 2020 to 2021
  Imazawa, Ryo et al., arXiv e-prints parXiv:2211.10300 (2022) [LINK]
  
• Broadband study of BL Lac during flare of 2020: Spectral evolution and emergence of HBL component
  Prince, Raj, arXiv e-prints parXiv:2105.00221 (2021) [LINK]
  
• The jet collimation profile at high resolution in BL Lacertae
  Casadio, C. et al., arXiv e-prints parXiv:2102.08952 (2021) [LINK]
  
• Fermi-LAT detection of enhanced gamma-ray activity from BL Lac
  La Mura, G. on behalf of the Fermi-LAT Collaboration, The Astronomer's Telegram 14583 p1 (2021) [LINK]
  
• Intra-day variability of BL Lacertae from 2016 to 2018
  Li, Tian et al., arXiv e-prints parXiv:2108.04699 (2021) [LINK]
  
• A thirteen-year-long broadband view of BL Lac
  Sahakyan, N. and Giommi, P., arXiv e-prints parXiv:2108.12232 (2021) [LINK]
  
• Constraining Evolution of Magnetic Field Strength in Dissipation Region of Two BL Lac Objects
  Fan, Xu-Liang et al., arXiv e-prints parXiv:2109.11229 (2021) [LINK]
  
• Dual-high-frequency VLBI study of blazar-jet brightness-temperature gradients and collimation profiles
  Burd, P.R. et al., arXiv e-prints parXiv:2112.04403 (2021) [LINK]
  
• Detection of very-high-energy gamma-ray emission from BL Lac with the LST-1
  Cortina, J. for the CTA LST collaboration, The Astronomer's Telegram 14783 p1 (2021) [LINK]
  
• Acceleration of high energy protons in AGN relativistic jets
  Istomin, Ya. N. and Gunya, A.A., Phys. Rev. D 102 p043010 (2020) [LINK]
  
• Multi-Wavelength Variability of BL Lacertae Measured with High Time Resolution
  Weaver, Zachary R. et al., arXiv e-prints parXiv:2007.07999 (2020) [LINK]
  
• Studies of stationary features in jets: BL Lacertae. I. Dynamics and brightness asymmetry on sub-parsec scales
  Arshakian, T.G. et al., arXiv e-prints parXiv:2003.08181 (2020) [LINK]
  
• Active galactic nuclei imaging programs of the RadioAstron mission
  Bruni, G. et al., arXiv e-prints p (2019) [LINK]
  
• Optical Variability of TeV Blazars on long time-scales
  Gaur, H. et al., arXiv e-prints p (2019) [LINK]
  
• A Fast Very High Energy gamma-ray Flare from BL Lacertae during a Period of Multiwavelength activity in June 2015
  MAGIC Collaboration et al., arXiv e-prints p (2019) [LINK]
  
• Multiwavelength intraday variability: what do the studies tell us about blazar jets?
  Bhatta, G., ArXiv e-prints p (2018) [LINK]
  
• Gamma-ray quasi-periodicities of blazars. A cautious approach
  Covino, S. et al., ArXiv e-prints p (2018) [LINK]
  
• The Feasibility of Magnetic Reconnection Powered Blazar Flares from Synchrotron Self-Compton Emission
  Morris, P.J. et al., ArXiv e-prints p (2018) [LINK]
  
• Long-term optical monitoring of TeV emitting blazars. I. Data analysis
  Nilsson, K. et al., A&A 620 pA185 (2018) [LINK]
  
• Constraints on Lorentz Invariance and CPT Violation using Optical Photometry and Polarimetry of Active Galaxies BL Lacertae and S5 B0716+714
  Friedman, A.S. et al., ArXiv e-prints p (2018) [LINK]
  
• Multi-color optical monitoring of ten blazars from 2005 to 2011
  Meng, N. et al., ArXiv e-prints p (2018) [LINK]
  
• Revealing the Nature of Blazar Radio Cores through Multi-Frequency Polarization Observations with the Korean VLBI Network
  Park, J. et al., ArXiv e-prints p (2018) [LINK]
  
• Multiwavelength observations of the blazar BL Lacertae: a new fast TeV gamma-ray flare
  Abeysekara, A.U. et al., ArXiv e-prints p (2018) [LINK]
  
• Quasi-periodicities of BL Lac Objects and Their Origin
  Sandrinelli, A. et al., ArXiv e-prints p (2018) [LINK]
  
• Hard X-ray properties of NuSTAR blazars
  Bhatta, G. et al., ArXiv e-prints p (2017) [LINK]
  
• Gamma-ray and Optical Oscillations of PKS 0716+71, MRK 421, and BL Lac
  Sandrinelli, A. et al., ArXiv e-prints p (2017) [LINK]
  
• BL Lacertae: X-ray spectral evolution and a black-hole mass estimate
  Titarchuk, L. and Seifina, E., ArXiv e-prints p (2017) [LINK]
  
• Radio and optical intra-day variability observations of five blazars
  Liu, X. et al., ArXiv e-prints p (2017) [LINK]
  
• Core shift effect in blazars
  Agarwal, A. et al., ArXiv e-prints p (2017) [LINK]
  
• Intraday optical variability of BL Lacertae
  Meng, N. et al., MNRAS 469 p3588-3596 (2017) [LINK]
  
• A search for cyclical sources of gamma-ray emission on the period range from days to years in the Fermi-LAT sky
  Prokhorov, D.A. and Moraghan, A., ArXiv e-prints p (2017) [LINK]
  
• Highlights from the VERITAS AGN Observation Program
  Benbow, W. and the VERITAS Collaboration, ArXiv e-prints p (2017) [LINK]
  
• Multiwavelength observations of the blazar BL Lacertae in June 2015
  Tsujimoto, S. et al., ArXiv e-prints p (2017) [LINK]
  
• VERITAS long term monitoring of Gamma-Ray emission from the BL Lacertae object
  Udara Abeysekara, A., ArXiv e-prints p (2017) [LINK]
  
• Multiwavelength observations of the blazar BL Lacertae: a new fast TeV gamma-ray flare
  Feng, Q. et al., ArXiv e-prints p (2017) [LINK]
  
• Signature of Inverse Compton emission from blazars
  Gaur, H. et al., ArXiv e-prints p (2017) [LINK]
  
• The Millimeter-Radio Emission of BL Lacertae During Two gamma-ray Outbursts
  Kim, D.-W. et al., ArXiv e-prints p (2017) [LINK]
  
• Microvariability in BL Lac: Zooming into the Innermost Blazar Regions
  Bhatta, G. and Webb, J., Galaxies 6 p2 (2018) [LINK]
  
• Multi-Band Intra-Night Optical Variability of BL Lacertae
  Gaur, H. et al., ArXiv e-prints p (2017) [LINK]
  
• Probing Acceleration and Turbulence at Relativistic Shocks in Blazar Jets
  Baring, M.G. et al., ArXiv e-prints p (2016) [LINK]
  
• VERITAS Detection of an Exceptional VHE Gamma-Ray Flare from BL Lacertae
  Mukherjee, R., The Astronomer's Telegram 9599 p1 (2016) [LINK]
  
• Exploring the magnetic field configuration in BL Lac using GMVA
  Rani, B. et al., ArXiv e-prints p (2016) [LINK]
  
• An innovative blazar classification based on radio jet kinematics
  Hervet, O. et al., A&A 592 pA22 (2016) [LINK]
  
• Optical and X-ray Variability of Blazars
  Gupta, A.C., ArXiv e-prints p (2015) [LINK]
  
• Optical and Radio Variability of BL Lacertae
  Gaur, H. et al., ArXiv e-prints p (2015) [LINK]
  
• Nature of intranight optical variability of BL Lacertae
  Gaur, H. et al., MNRAS 452 p4263-4273 (2015) [LINK]
  
• MAGIC detects an increased activity from BL Lacertae at very high energy gamma-rays
  Mirzoyan, R., The Astronomer's Telegram 7660 p1 (2015) [LINK]
  
• Short Timescale Photometric and Polarimetric Behavior of two BL Lacertae Type Objects
  Covino, S. et al., ArXiv e-prints p (2015) [LINK]
  
• The awakening of BL Lacertae: observations by Fermi, Swift, and the GASP-WEBT
  Raiteri, C.M. et al., ArXiv e-prints p (2013) [LINK]
  
• The Stability of BVRI Comparison Stars Near Selected TeV Blazars
  Pace, C.J. et al., ArXiv e-prints p (2013) [LINK]
  
• Synchrotron and inverse-Compton emission from blazar jets I: a uniform conical jet model
  Potter, W.J. and Cotter, G., ArXiv e-prints p (2012) [LINK]
  
• Rapid TeV Gamma-Ray Flaring of BL Lacertae
  Arlen, T. et al., ArXiv e-prints p (2012) [LINK]
  
• Observation of selected IBLs and LBLs with VERITAS
  Majumdar, P. and for the VERITAS Collaboration, ArXiv e-prints p (2011) [LINK]
  
• Fermi LAT detection of increasing gamma-ray activity of BL Lac
  Cutini, S. for the Fermi LAT Collaboration, The Astronomer's Telegram 3368 p1-+ (2011) [LINK]
  
• The First Fermi Multifrequency Campaign on BL Lacertae: Characterizing the Low-activity State of the Eponymous Blazar
  Abdo, A.A. et al., ApJ 730 p101-+ (2011) [LINK]
  
• Is BL Lacertae an ``orphan'' AGN?. Multiband and spectroscopic constraints on the parent population
  Capetti, A. et al., A&A 516 pA59+ (2010) [LINK]
  
• The correlated optical and radio variability of BL Lacertae. WEBT data analysis 1994-2005
  Villata, M. et al., A&A 501 p455-460 (2009) [LINK]
  
• Lognormal variability in BL Lacertae
  Giebels, B. and Degrange, B., A&A 503 p797-799 (2009) [LINK]
  
• The inner jet of an active galactic nucleus as revealed by a radio-to-gamma-ray outburst
  Marscher, Alan P. et al., Nature 452 p966-969 (2008) [LINK]
  
• Results of observation of Cyg gamma-2, BL Lac, 3C66A, Mk 501, and the Crab nebula by the GT-48 gamma-ray telescope in 2006
  Neshpor, Y.I. et al., Bulletin Crimean Astrophysical Observatory 104 p141-144 (2008) [LINK]
  
• Discovery of Very High Energy Gamma-ray Emission from the Low-Frequency-peaked BL Lacertae Object BL Lacertae
  Albert, J. et al., ApJ 666 pL17-L20 (2007) [LINK]
  
• BL Lac: A New Ultrahigh-Energy Gamma-Ray Source
  Neshpor, Y.I. et al., Astronomy Reports 45 p249-254 (2001) [LINK]
  

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