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

Knödlseder, J., Jean, P., Lonjou, V., et al. 2005, A&A, 441, 513

OutlineIntroductionDataBackground modellingResults1. Imaging2. Morphological characterisation3. Correlation with tracer maps4. Point-source search

Conclusions

Candidates for sources of positronscosmic-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 nucleosynthesissupernovae (Clayton 1973)

hypernovae (Cassé et al. 2004)

novae (Clayton & Hoyle 1974)

red giants (Norgaard 1980)

Wolf-Rayet stars (Dearborn & Blake 1985)

Introduction

INTEGRAL Mission SummaryLaunch date: 17 October 2002Dimensions: 5x3.7x3.7 metres.Launch dry mass: 3600 kgScience instruments mass: 2087 kgNominal mission lifetime: 2 yearsDesign lifetime: 5 yearsHighly eccentric orbitScientific PayloadMain instruments:

Spectrometer on Integral (SPI)Imager on Board the Integral Satellite (IBIS)

Monitors:Joint European X-Ray Monitor (JEM-X)

Optical Monitoring Camera (OMC)

OrbitHighly eccentric orbitPeriod: 72hInclination: 51.6 degTime above 40 000 km: ~90 %

SPI

IBIS detector

JEM-X coded mask

OMC

IBIS Coded Mask

JEM-X detectors

IMAGER IBISAccurate point source imagingBroad line spectroscopy & continuum

Coded Mask

Coolers

Veto

Veto

Ge detectors

SPECTROMETER SPIFine spectroscopy of narrow linesStudy of diffuse emission on large scales

X-RAY MONITOR JEM-XSource identificationMonitoring @ X-rays

OPTICAL MONITOR OMCOptical monitoring ofhigh energy sources

INTEGRAL payload: key parameters

dataDecember 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 sphere6821 pointed observationsExposure time: 15.3 Ms.instrumental energy resolution: 2.12 keV @ 511 keVEnergy interval: 507.5~514.5keVThe 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.

Map of the effective SPI exposure at 511 keV

SPI narrow line 3σ point-source sensitivity at 511 keV

Background modellingDETE

p: pointing number d: detector number e: energy bin

The background model for a given data-space bin

gp,d : the GEDSAT(saturated events in the germanium detectors) rateTp,d : the lifetime

∆e : the energy bin size for spectral bin eEadj : 523-545 keV

the continuum component of the instrumental background

the line component of the instrumental background

ORBIT-DETEadjust the model parameters β(2)

d,e

Residual count rate

ImagingRichardson-Lucy algorithm

Rij : the instrumental response matrixni : the measured number of countsi : data space binj : image space bink : iteration λk : acceleration factor

(the predicted number of counts)

Richardson-Lucy image of 511 keV gamma-ray line emission

Morphological characterisation

use a maximum likelihood multi-component model fitting algorithm.

Method

(Likelihood function)

log likelihood

fkj : sky intensity model

bi(β) : background model

αk and β are scaling factors

Maximum log likelihood-ratio

Reduced maximum log likelihood-ratio

detection of the bulge emission ~ 50σ

2d angular Gaussian surface brightness distribution

Galactic models

detection of the disk emission ~ 3−4σ

Correlation with tracer maps

None of the tracer maps is consistent with the data.

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.

Summary of results511 keV emission is significantly (∼50σ) detected towards the galactic bulge region, and, at a very low level (∼4σ), 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 (3σ upper flux limit ~1.5×10−4 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.

ConclusionsThe bulge emission arises from a different source which has only a weak or no disk component.

The disk emission can be attributed to the β+-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.

Thank you