The all-sky distribution of 511 keV electron-positron annihilation emission

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The all-sky distribution of 511 keV
electron-positron annihilation emission

• Knödlseder, J., Jean, P., Lonjou, V., et al.
  2005, AA, 441, 513

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Outline

• Introduction
• Data
• Background modelling
• Results
• 1. Imaging
• 2. Morphological characterisation
• 3. Correlation with tracer maps
• 4. Point-source search
• Conclusions

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Candidates for sources of positrons
Introduction

• cosmic-ray interactions with the interstellar
  medium (Ramaty et al. 1970)
• Pulsars (Sturrock 1971)
• compact objects housing either neutron stars or
  black holes (Ramaty Lingenfelter 1979)
• Gamma-ray bursts (Lingenfelter Hueter 1984)
• (light) dark matter (Rudaz Stecker 1988 Boehm
  et al. 2004)
• stars expelling radioactive nuclei produced by
  nucleosynthesis
• supernovae (Clayton 1973)
• hypernovae (Cassé et al. 2004)
• novae (Clayton Hoyle 1974)
• red giants (Norgaard 1980)
• Wolf-Rayet stars (Dearborn Blake 1985)

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INTEGRAL Mission Summary

• Launch date 17 October 2002
• Dimensions 5x3.7x3.7 metres.
• Launch dry mass 3600 kg
• Science instruments mass 2087 kg
• Nominal mission lifetime 2 years
• Design lifetime 5 years
• Highly eccentric orbit
• Scientific Payload
• Main instruments
• Spectrometer on Integral (SPI)
• Imager on Board the Integral Satellite (IBIS)
• Monitors
• Joint European X-Ray Monitor (JEM-X)
• Optical Monitoring Camera (OMC)

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Orbit Highly eccentric orbit Period
72h Inclination 51.6 deg Time above 40 000 km
90
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IMAGER IBIS Accurate point source imaging Broad
line spectroscopy continuum
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SPECTROMETER SPI Fine spectroscopy of narrow
lines Study of diffuse emission on large scales
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X-RAY MONITOR JEM-X Source identification Monitori
ng _at_ X-rays
OPTICAL MONITOR OMC Optical monitoring of high
energy sources
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INTEGRAL payload key parameters
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data

• December 10, 2004 public INTEGRAL data release
  (i.e. orbital revolutions 19-76, 79-80, 89-122)
• the INTEGRAL Science Working Team data of the
  Vela region observed during revolutions 81-88.
• covers approximately 95 of the celestial sphere
• 6821 pointed observations
• Exposure time 15.3 Ms.
• instrumental energy resolution 2.12 keV _at_ 511
  keV
• Energy interval 507.5514.5keV
• The data have been analysed by sorting the events
  in a 3-dimensional data-space, spanned by the
  (calibrated) event energy, the detector number,
  and the SPI pointing number.

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Map of the effective SPI exposure at 511 keV
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SPI narrow line 3s point-source sensitivity at
511 keV
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Background modelling

• DETE
• p pointing number
• d detector number
• e energy bin

The background model for a given data-space bin
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the continuum component of the instrumental
background
the line component of the instrumental background
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Residual count rate

• ORBIT-DETE

adjust the model parameters ß(2)d,e
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Imaging

• Richardson-Lucy algorithm

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Richardson-Lucy image of 511 keV gamma-ray line
emission
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Morphological characterisation
Method

• use a maximum likelihood multi-component model
  fitting algorithm.

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(Likelihood function)
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log likelihood
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Maximum log likelihood-ratio
Reduced maximum log likelihood-ratio
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2d angular Gaussian surface brightness
distribution
detection of the bulge emission 50s
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Galactic models
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detection of the disk emission 3-4s
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Correlation with tracer maps
None of the tracer maps is consistent with the
data.
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Point-source search

• They looked for evidence of 511 keV gamma-ray
  line emission from a list of potential candidate
  objects.
• None of the sources which were searched for
  showed a significant 511 keV flux.

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Summary of results

• 511 keV emission is significantly (50s) detected
  towards the galactic bulge region, and, at a very
  low level (4s), from the galactic disk
• There is no evidence for a point-like source in
  addition to the diffuse emission, down to a
  typical flux limit of 10-4 ph cm-2 s-1
• There is no evidence for the positive latitude
  enhancement that has been reported from OSSE
  measurements (3s upper flux limit 1.510-4 ph
  cm-2 s-1).
• The bulge emission is spherically symmetric and
  is centred on the galactic centre with an
  extension of 8? (FWHM) it is equally well
  described by models that represent the stellar
  bulge or the halo populations.
• the bulge annihilation rate (1.5 0.1) 1043
  s-1
• the disk annihilation rate (0.3 0.2) 1043
  s-1
• the bulge-to-disk luminosity ratio 3-9.

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Conclusions

• The bulge emission arises from a different source
  which has only a weak or no disk component.
• The disk emission can be attributed to the
  ß-decay of the radioactive species 26Al and
  44Ti.
• Type Ia supernovae and/or low-mass X-ray binaries
  are the prime candidates for the source of the
  galactic bulge positrons.
• Light dark matter annihilation could also explain
  the observed 511 keV bulge emission
  characteristics.

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Thank you