Observation of Hard Radiation from the Region of the Galactic Center

Haymes, R. C.; Ellis, Darwin V.; Fishman, G.; Glenn, S. W.; Kurfess, J. D.

doi:10.1086/150164

Cited by 17 publications

(10 citation statements)

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“…Emission at 511 keV, the characteristic signature of positron annihilation, has been observed in the direction of the Galactic Centre (GC), since the 1970s. Apparently diffuse gammaray emission at approximately this energy was first detected in 1970 by the balloon-borne experiments of the Rice group (Johnson, Harnden, & Haymes 1972;Johnson & Haymes 1973;Haymes et al 1975; preliminary indications were also reported in Haymes et al 1969) and was confirmed as positron annihilation emission in 1978 by the balloon-borne experiments of the Bell-Sandia group (Leventhal, MacCallum, & Stang 1978;Leventhal et al 1980). High-energy balloon experiments and space observatories through the early 1990s continued to detect the 511 keV emission; however the relatively low spatial resolution of these detectors prevented determination of the true location and distribution of the emission (see Purcell et al 1997, Jean et al 2003, or Teegarden et al 2005 for a summary of these early observations).…”

Section: Introductionmentioning

confidence: 85%

On the origin of the 511-keV emission in the Galactic Centre

Bandyopadhyay¹,

Silk

Taylor

et al. 2009

Monthly Notices of the Royal Astronomical Society

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Diffuse 511‐keV line emission, from the annihilation of cold positrons, has been observed in the direction of the Galactic Centre for more than 30 yr. The latest high‐resolution maps of this emission produced by the SPI instrument on INTEGRAL suggest at least one component of the emission is spatially coincident with the distribution of ∼70 luminous, low‐mass X‐ray binaries detected in the soft gamma‐ray band. The X‐ray band, however, is generally a more sensitive probe of X‐ray binary populations. Recent X‐ray surveys of the Galactic Centre have discovered a much larger population (>4000) of faint, hard X‐ray point sources. We investigate the possibility that the positrons observed in the direction of the Galactic Centre originate in pair‐dominated jets generated by this population of fainter accretion‐powered X‐ray binaries. We also consider briefly whether such sources could account for unexplained diffuse emission associated with the Galactic Centre in the microwave (the Wilkinson Microwave Anisotropy Probe‘haze’) and at other wavelengths. Finally, we point out several unresolved problems in associating Galactic Centre 511‐keV emission with the brightest X‐ray binaries.

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Section: Introductionmentioning

confidence: 85%

On the origin of the 511-keV emission in the Galactic Centre

Bandyopadhyay¹,

Silk

Taylor

et al. 2009

Monthly Notices of the Royal Astronomical Society

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“…The interpretation of the morphology of γ-ray emission from positron annihilation in the Milky Way has remained in contention since the discovery of the galactic 511 keV line in the late 1960's (Haymes et al 1969;Johnson et al 1972). Unlike at any other wavelength, the bulge region dominates the signal, with a flux ratio between bulge and disk of 0.6 (Siegert et al 2016) (see also Milne & Leising 1997;Knödlseder et al 2005;Bouchet et al 2010a).…”

Section: Introductionmentioning

confidence: 99%

Constraints on positron annihilation kinematics in the inner Galaxy

Siegert

Crocker²,

Diehl

et al. 2019

A&A

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Context. The annihilation of cosmic positrons with electrons in the interstellar medium results in the strongest persistent γ-ray line signal in the sky. For the past 50 years, this 511 keV emission -predominantly from the galactic bulge region and from a low surfacebrightness disk -has puzzled observers and theoreticians. A key issue for understanding positron astrophysics is found in cosmic-ray propagation, especially at low kinetic energies ( 10 MeV). Aims. We want to shed light on how positrons propagate and the resulting morphology of the annihilation emission. We approach this "positron puzzle" by inferring kinematic information of the 511 keV line in the inner radian of the Galaxy. This constrains propagation scenarios and positron source populations in the Milky Way. Methods. By dissecting the positron annihilation emission as measured with INTEGRAL/SPI, we derive spectra for individual and independent regions in the sky. The centroid energies of these spectra around the 511 keV line are converted into Doppler-shifts, representing the line-of-sight velocity along different galactic longitudes. This results in a longitude-velocity diagram of positron annihilation. From high-resolution spectra, we also determine Doppler-broadening from γ-ray line shape parameters to study annihilation conditions as they vary with galactic longitude. Results. We find line-of-sight velocities in the 511 keV line that are consistent with zero, as well as with galactic rotation from CO measurements (2-3 km s −1 deg −1 ), and measurements of radioactive 26 Al (7.5-9.5 km s −1 deg −1 ). The velocity gradient in the inner ±30 • is determined to be 4 ± 6 km s −1 deg −1 . The width of the 511 keV line is constant as a function of longitude at 2.43 ± 0.14 keV, with possibly different values towards the disk. The positronium fraction is found to be 1.0 along the galactic plane. Conclusions. The weak signals in the disk leave open the question whether positron annihilation is associated with the high velocities seen in 26 Al or rather with ordinarily rotating components of the Milky Way's interstellar medium. We confirm previous results that positrons are slowed down to the 10 eV energy scale before annihilation, and constrain bulk Doppler-broadening contributions to 1.25 keV in the inner radian. Consequently, the true annihilation conditions remain unclear.

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“…For more than 40 years, gamma-ray astronomers measure a strong and extended signal from the centre of the Milky Way which they cannot explain thoroughly: the electron-positron annihilation (e − , e + ) signal at photon energies around 511 keV [1,2,3]. Our current knowledge in this context is restricted to where the positrons annihilate (gamma-ray morphology [4,5,6,7,8]), and how they annihilate (gamma-ray spectral shape [3,9,10,11,8]).…”

Section: Contextmentioning

confidence: 99%

“…The 511 keV morphology in the Galaxy is dominated by largely-extended diffuse emission from the direction of the galactic centre, called the bulge 1 . This bulge can be characterised by at least two 2D-Gaussian-shaped components with radial extents of ∼ 6 • (FWHM, offset to l ≈ −1 • ), and ∼ 20 • (FWHM, centred on l = 0 • ).…”

Section: Contextmentioning

confidence: 99%

Positron annihilation in the Milky Way and beyond

Siegert

2017

Proceedings of 11th INTEGRAL Conference Gamma-Ray Astrophysics in Multi-Wavelength Perspective — PoS(INTEGRAL2016)

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The electron-positron annihilation gamma-ray signal at 511 keV in the Milky Way is investigated towards a possible dark matter interpretation. If all bulge positrons were created by dark matter particle annihilation, the satellite galaxies of the Milky Way, apparently being dominated by dark matter, should also show measurable 511 keV signals. Using INTEGRAL/SPI, we test for emission in 39 neighbouring dwarf satellite galaxies, and found a consistent trend against a dark matter scenario. One galaxy, Reticulum II, shows up as a strong source of annihilation emission, which we interpret as the presence of a microquasar, ejecting pair-plasma into the galaxy's interstellar medium.

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Observation of Hard Radiation from the Region of the Galactic Center

Cited by 17 publications

References 0 publications

On the origin of the 511-keV emission in the Galactic Centre

On the origin of the 511-keV emission in the Galactic Centre

Constraints on positron annihilation kinematics in the inner Galaxy

Positron annihilation in the Milky Way and beyond

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