The First Gamma-Ray Outburst Of PMN J0948+0022
Relativistic jets are the most extreme expression of the power that can be generated by supermassive black holes at the centres of many active galaxies. Active galaxies are those with unusually bright centers that show evidence of particle acceleration to speeds approaching that of light itself. In 1943, astronomer Carl Seyfert described the first two types of active galaxy based on the width of spectral lines, a tell-tale sign of rapid gas motion in their cores.
To date, two classes of active galactic nuclei (AGNs) are known to generate these structures: blazars and radio galaxies, both hosted in giant elliptical galaxies (Blandford & Rees 1978). The first class is composed of AGNs with the jet viewed at small angles and, therefore, the effects of the special relativity play a dominant role in shaping the emission of the electromagnetic radiation, hence the more connotatively violent name: blazar. In the second class: radio galaxies, the jets are viewed at larger angles and therefore the Doppler boosting is less intense.
The recent discovery of variable γ-ray emission from the AGN ‘4NLS1s’ confirmed the presence of a third class of AGNs with relativistic jets (Abdo et al. 2009a,b,c; Foschini et al. 2010a). This finding poses intriguing questions about jet systems and how these structures are generated.
One of these sources, PMN J0948+0022 (with a measured redshift of z = 0.5846) is classified as a typical “NLS1”, with a FWHM(Hβ)< 2000 kms-1, [OIII]/Hβ<3, with the presence of the characteristic NLS1 FeII bump. However, PMN J0948+0022 also displays strong, compact and variable radio emission with inverted spectrum, suggesting the presence of a relativistic jet (Zhou et al. 2003).
The confirmation came with the detection of high-energy variable γ-rays by Fermi/LAT (Abdo et al. 2009b; Foschini et al. 2010a). A multi-wavelength (MW) campaign performed in 2009 from March to July displayed coordinated variability at all frequencies, thus confirming that the source detected by Fermi/LAT is indeed the high-energy counterpart of PMN J0948+0022 (Abdo et al. 2009c). The comparison of the spectral energy distribution of the NLS1 PMN J0948+0022 with that of a typical blazar – like 3C 273 – shows that the power emitted at γ-rays is extreme.
- Abdo, A. A. et al. (2009) Fermi/Large Area Telescope Discovery Of Gamma-Ray Emission From A Relativistic Jet: Quasar PMN J0948+0022. The Astrophysical Journal, 699 (2) pp. 976-984.
- Giroletti, M. et al. (2011) Global e-VLBI Observations Of The Gamma-Ray Narrow Line Seyfert 1 PMN J0948+0022. Astronomy & Astrophysics, 528 Article I.D.: L11.
Suggested Further Reading:
- Kembhavi, A.K.; Narlikar, J.V. (1999) Quasars & Active Galactic Nuclei: An Introduction. Cambridge University Press, Cambridge.