PKS 0735+178 is a classical BL Lac object in the northern constellation of Gemini. This is one of the brightest objects of its type in the night sky.[5] It has a redshift of z = 0.424, with a luminosity distance of 7,380 million light-years (2,263 Mpc). PKS 0735+178 is a nearly point-like source with an angular size of a milliarcsecond.[3]
This object was identified as a radio source during the third part of a radio survey at the Parkes Observatory,[6] and became catalogued as PKS 0735+178.[4] An optical counterpart was found in 1970 which showed a continuous optical spectrum. In contrast, the radio emission is variable at frequencies greater than 6 GHz. The radio spectrum appears mostly flat above 1 GHz and the properties are similar to BL Lacertae. That is, it is an extragalactic object that resembles a quasar but lacks optical emission lines.[7] It has a very complex light curve which shows indications of periodicity.[8]
Gamma-ray emission was detected from this source in 1999. X-ray and gamma-ray emission was found to be steady from this source, whereas it displayed extreme variability in radio and optical bands.[9] Radio images of this object produced via very long baseline interferometry show a compact core with a jet that extends toward the northeast. The latter displays features of superluminal motion. Higher resolution observations show a pair of bends in the jet.[3]
In 2021, this target was found to be a candidate source for multiple neutrino events. Neutrino event 211208A was detected by the IceCube observatory and a best fit location was found within 2.2° of PKS 0735+178.[10] This observation occurred while PKS 0735+178 was undergoing a particularly strong flare event that was detected in the optical, ultraviolet, X-ray, and gamma-ray bands.[5]
^ abSahakyan, N.; et al. (February 2023), "A multimessenger study of the blazar PKS 0735+178: a new major neutrino source candidate", Monthly Notices of the Royal Astronomical Society, 519 (1): 1396–1408, arXiv:2204.05060, Bibcode:2023MNRAS.519.1396S, doi:10.1093/mnras/stac3607.
^Day, G. A.; et al. (February 1966), "The Parkes catalogue of radio sources, declination zone 0° to +20°", Australian Journal of Physics, 19: 35, Bibcode:1966AuJPh..19...35D.
^Ding, S. X.; et al. (2004), "The Periodicity Analysis of the Light Curve of PKS 0735+178 and Implications for its Central Structure", International Journal of Modern Physics D, 13 (4): 771–782, Bibcode:2004IJMPD..13..771D, doi:10.1142/S0218271804004694.
^Acharyya, A.; et al. (September 2023), "Multiwavelength Observations of the Blazar PKS 0735+178 in Spatial and Temporal Coincidence with an Astrophysical Neutrino Candidate IceCube-211208A", The Astrophysical Journal, 954 (1), id. 70, arXiv:2306.17819, Bibcode:2023ApJ...954...70A, doi:10.3847/1538-4357/ace327.
Bharathan, Athira M.; et al. (April 2024), "Multiwavelength spectral modelling of the candidate neutrino blazar PKS 0735+178", Monthly Notices of the Royal Astronomical Society, 529 (4): 3503–3510, arXiv:2401.12680, Bibcode:2024MNRAS.529.3503B, doi:10.1093/mnras/stae296.
McCall, Callum; et al. (March 2024), "Detection of an intranight optical hard lag with colour variability in blazar PKS 0735+178", Monthly Notices of the Royal Astronomical Society, 528 (3): 4702–4719, arXiv:2402.00633, Bibcode:2024MNRAS.528.4702M, doi:10.1093/mnras/stae310.
Prince, Raj; et al. (January 2024), "Dissecting the broad-band emission from γ-ray blazar PKS 0735+178 in search of neutrinos", Monthly Notices of the Royal Astronomical Society, 527 (3): 8746–8754, arXiv:2301.06565, Bibcode:2024MNRAS.527.8746P, doi:10.1093/mnras/stad3804.
Yuan, Y. H.; Fan, J. H. (July 2021), "Optical Monitoring and Intraday Variabilities of the BL Lac Object PKS 0735+178", Publications of the Astronomical Society of the Pacific, 133 (1025), id. 074101, Bibcode:2021PASP..133g4101Y, doi:10.1088/1538-3873/ac015f.
Goyal, Arti; et al. (March 2017), "Multiwavelength Variability Study of the Classical BL Lac Object PKS 0735+178 on Timescales Ranging from Decades to Minutes", The Astrophysical Journal, 837 (2), id. 127, arXiv:1702.02504, Bibcode:2017ApJ...837..127G, doi:10.3847/1538-4357/aa6000.
Bregman, J. N.; et al. (October 1981), "Detection of Lyman continuum absorption in the BL Lacertae object PKS0735+178", Astrophysical Journal, 249: 13–16, Bibcode:1981ApJ...249...13B, doi:10.1086/159253.
Marscher, A. P. (October 1977), "Structure of radio sources with remarkably flat spectra: PKS 0735+178.", Astronomical Journal, 82: 781–784, Bibcode:1977AJ.....82..781M, doi:10.1086/112125.
