TY - JOUR

T1 - Search for electron-positron annihilation radiation from the jet in 3C 120

AU - Marscher, Alan P.

AU - Jorstad, Svetlana G.

AU - Gómez, José L.

AU - McHardy, Ian M.

AU - Krichbaum, Thomas P.

AU - Agudo, Iván

PY - 2007/8/10

Y1 - 2007/8/10

N2 - We report an attempt to detect the electron-positron annihilation line from the radio galaxy 3C 120, in which the jet interacts strongly with interstellar clouds. Such interactions should cause most of the jet plasma to mix with the gas in the clouds. This will thermalize the majority of any positrons in the jet, leading to continuous annihilation with ambient electrons. We derive the number density of the combined electron-positron population in the core and compact knots in the jet of 3C 120 using ultrahigh-resolution observations with the Global millimeter-VLB1 Array at 86 GHz and the Very Long Baseline Array at 43 GHz, along with the millimeter-wave continuum spectrum and a computational code that maps the synchrotron intensity of a model jet. If the jet contains a pure pair plasma, the production rate of positrons required to produce this density plus the efficiency of eventual annihilation predict the emission of a narrow spectral line at a rest energy of 511 keV, or 495 keV in the observer's frame. Our spectral observations with the SPI instrument on INTEGRAL failed to detect the line. The upper limit, which is 30% lower than our rather uncertain prediction, does not significantly constrain the positron-to-proton ratio in the jet of 3C 120. However, our procedure provides a robust method for determining the flux of electrons and positrons in a jet that will be useful when more sensitive son γ-ray spectrometers and millimeter-wave VLBI arrays become available.

AB - We report an attempt to detect the electron-positron annihilation line from the radio galaxy 3C 120, in which the jet interacts strongly with interstellar clouds. Such interactions should cause most of the jet plasma to mix with the gas in the clouds. This will thermalize the majority of any positrons in the jet, leading to continuous annihilation with ambient electrons. We derive the number density of the combined electron-positron population in the core and compact knots in the jet of 3C 120 using ultrahigh-resolution observations with the Global millimeter-VLB1 Array at 86 GHz and the Very Long Baseline Array at 43 GHz, along with the millimeter-wave continuum spectrum and a computational code that maps the synchrotron intensity of a model jet. If the jet contains a pure pair plasma, the production rate of positrons required to produce this density plus the efficiency of eventual annihilation predict the emission of a narrow spectral line at a rest energy of 511 keV, or 495 keV in the observer's frame. Our spectral observations with the SPI instrument on INTEGRAL failed to detect the line. The upper limit, which is 30% lower than our rather uncertain prediction, does not significantly constrain the positron-to-proton ratio in the jet of 3C 120. However, our procedure provides a robust method for determining the flux of electrons and positrons in a jet that will be useful when more sensitive son γ-ray spectrometers and millimeter-wave VLBI arrays become available.

KW - Galaxies: active

KW - Galaxies: individual (3C 120)

KW - Galaxies: jets

KW - Gamma rays: observations

KW - Radio continuum: galaxies

UR - http://www.scopus.com/inward/record.url?scp=34548319500&partnerID=8YFLogxK

U2 - 10.1086/519481

DO - 10.1086/519481

M3 - Article

AN - SCOPUS:34548319500

VL - 665

SP - 232

EP - 236

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 1 PART 1

ER -