[Paper Review] SPI observations of positron annihilation radiation from the 4th galactic quadrant: sky distribution

G. Weidenspointner, V. Lonjou+15 more|arXiv (Cornell University)|Jun 7, 2004

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This study analyzes one year of INTEGRAL/SPI data to map the 511 keV positron annihilation line emission in the 4th Galactic quadrant, finding a Gaussian bulge component with a full width at half maximum of $8^\circ{}^{+3^\circ}_{-2^\circ}$, consistent with OSSE results, and placing upper limits on disk emission that constrain the bulge-to-disk flux ratio to at least 0.5–0.8, ruling out certain source distributions like DIRBE maps or Type Ia SNe.

ABSTRACT

During its first year in orbit the INTEGRAL observatory performed deep exposures of the Galactic Center region and scanning observations of the Galactic plane. We report on the status of our analysis of the positron annihilation radiation from the 4th Galactic quadrant with the spectrometer SPI, focusing on the sky distribution of the 511 keV line emission. The analysis methods are described; current constraints and limits on the Galactic bulge emission and the bulge-to-disk ratio are presented.

Motivation & Objective

• To determine the sky distribution of 511 keV positron annihilation radiation in the 4th Galactic quadrant using INTEGRAL/SPI data.
• To constrain the flux and spatial extent of the Galactic bulge and disk components of the 511 keV emission.
• To evaluate the bulge-to-disk flux ratio (B/D) and assess its implications for positron source models.
• To test for the presence of a positive latitude enhancement (PLE) or other extended components.
• To improve upon previous OSSE-based flux and spatial constraints with higher dynamic range and spectral resolution.

Proposed method

• Analysis of 1.8 and 2.0 × 10⁶ s of deep exposures (GCDE) and Galactic Plane Scans (GPS) from INTEGRAL's Core Program.
• Use of SPI's high-resolution spectrometer to detect and characterize the 511 keV gamma-ray line with energy resolution of ~6 keV FWHM.
• Modeling of the sky distribution using a Gaussian radial profile for the bulge and separate components for disk and halo emission.
• Fitting the observed flux distribution to determine the FWHM, centroid position, and fluxes of the bulge and disk components.
• Application of exposure maps and background modeling to correct for instrumental and sky-dependent effects.
• Use of DIRBE 35 and 240 μm maps to derive lower limits on the B/D ratio by comparing to observed flux upper limits.

Experimental results

Research questions

• RQ1 What is the spatial extent and distribution of the 511 keV positron annihilation line emission in the 4th Galactic quadrant?
• RQ2 Is there evidence for a significant offset of the emission centroid from the Galactic Center?
• RQ3 What are the flux limits for the Galactic disk component, and what do they imply for the bulge-to-disk flux ratio (B/D)?
• RQ4 Does the data support the existence of a positive latitude enhancement (PLE) component?
• RQ5 Which astrophysical positron source models are consistent with the observed flux and spatial distribution?

Key findings

• The 511 keV line emission in the 4th Galactic quadrant is best described by a Gaussian bulge component with a full width at half maximum of $8^\circ{}^{+3^\circ}_{-2^\circ}$, consistent with OSSE results at the 2σ level.
• The bulge flux is measured at $(0.96^{+0.21}_{-0.14}) \times 10^{-3}$ ph cm⁻² s⁻¹, with uncertainty dominated by the width of the Gaussian profile.
• No significant deviation from spherical symmetry is observed in the bulge distribution, though a slight elongation in longitude is hinted at, possibly indicating faint disk emission.
• The centroid of the Gaussian bulge model is consistent with the Galactic Center at the 2.1σ level and nearly coincides with the position reported by Tueller et al. (1996) using OSSE data.
• Upper limits on the disk flux yield a lower bound on the bulge-to-disk ratio (B/D) of 0.5 (using 35 μm map) and 0.8 (using 240 μm map), incompatible with DIRBE maps or Type Ia SNe as sole sources.
• No significant evidence is found for a positive latitude enhancement (PLE) component, contradicting earlier tentative OSSE detections.