TYPE Ia SUPERNOVA RATE IN THE GALACTIC CENTER REGION Stéphane Schanne (1,*) , Michel Cassé (1,2), Patrick Sizun (1) , Bertrand Cordier (1) , Jacques Paul (1) (1) CEA-Saclay, DAPNIA/Service d'Astrophysique, F-91191 Gif sur Yvette, France (2) Institut d’Astrophysique de Paris, 98 bis Bd Arago, F-75014, Paris, France (*) corresponding author: schanne{at}hep.saclay.cea.fr ABSTRACT 2. SN Ia AS GALACTIC POSITRON SOURCE? According to recent analyses of the type-Ia supernova Among all astrophysical source candidates, up to now, rate as a function of redshift, delayed and prompt type- supernovae of type Ia (SN Ia) have been proposed as Ia supernovae (SN Ia) should explode respectively in the main source of positrons in the Galactic bulge [8]. the Galactic bulge and in the nuclear bulge, a gas rich Indeed, in the Galactic bulge, formed of very old stars structure with ongoing star formation, located in the (~10 Gyr), core collapse SN are not expected to take central region of the Milky Way. We estimate the rate place; but those stars, if located in a binary system, of type-Ia supernovae in the Galactic bulge and nuclear could very well explode via SN Ia. Furthermore, during bulge. We show that this rate is insufficient by an order their explosion, SN Ia produce a copious amount of of magnitude to explain by positron escape from type- radioactive 56 Ni nuclei – about 0.6 solar masses (M ☼ ) – Ia supernovae envelopes alone the large positron which decay to 56 Co (after 6 days half-life), and injection rate into the Galactic central region, as re- subsequently to 56 Fe (after 77 days) releasing a observed recently by the Spectrometer on INTEGRAL, positron in 19% of the cases. The expanding envelope which amounts to 1.25×10 43 e+ s-1 and would require of a SN Ia is thinner than for a core-collapse SN and 0.5 SN Ia explosions per century. positrons therefore have a chance to escape from it and to be released in the interstellar medium, where they 1. INTRODUCTION can form the observed positronium and annihilate into The spectrometer SPI [1] on ESA's gamma-ray satellite 511 keV γ-rays. INTEGRAL has recently presented refined measurements of the 511 keV γ-ray line emission resulting from e +-e− annihilation in the Galactic center region. The 511 keV flux from this region measured by +0.21 -3 2 SPI to Φ511 =0.96 -0.14 10 ph/cm /s [2] is located in a narrow (~2.7 keV FWHM), non-shifted line at 511.02 Milne, The, Leising ±0.09 keV. Additionally, SPI has provided constraints Late light curves of 22 SN Ia on the morphology of the emission region, which can Trapped e + be adequately described by a spherical distribution with a radial Gaussian profile and a FWHM of ~8°, while ruling out a single point source [3,4]. The + extension of the annihilation region appears compatible Released e DD23C (3.3%) in size and shape with the Galactic bulge. Furthermore, by measuring with SPI the orth-positronium continuum located at E<511 keV, [5] have confirmed results of Fig. 1. The model DD23D by Milne, The and Leising [9] of earlier measurements by OSSE [6], that the dominant 22 SN Ia shows that a release of 3.3% of positrons is needed fraction of positrons (f Ps =0.94±0.06) form a to describe their late light curves. positronium intermediate state before annihilation. In their model DD23C, Milne et al. [9] (Fig. 1) have + From those results, the rate of e annihilation from the predicted that 3.3% of the positrons escape a typical region around the Galactic center, located at R=8.0±4 SN Ia envelope, reaching the total amount of 8 10 52 e+ 2 -1 56 kpc [7], can be computed as L e+=Φ511 (4 πR ) (2 −3 released. Firstly, in a simple calculation using the Ni -1 +0.35 fPs /2) , which is the enormous amount of 1.25 -0.29 decay law, we verify that, in order to release 3.3% of 43 + 10 e /s. Assuming a steady-state production- the initially produced positrons, the envelope must annihilation, the same amount of positrons must be become transparent at latest 390 days after the injected each second into the Galactic bulge region. explosion (in case of no mixing of the 56 Ni with the Consequently the question of the nature of this ejecta), which is consistent with the timescale in Fig. 1. powerful positron source is raised. Since the optical light curve of a SN is powered by _____________________________________________________________________ Proc. 6 th INTEGRAL Workshop ‘The Obscured Universe’, Moscow, 3-7 July 2006 (ESA SP-622) radioactivity (among others the produced positrons), infrared luminosity and to the stellar mass of the parent the late light curve of typical SN Ia should have a galaxies, using the new complete catalog of near- steeper decline than by radioactive decay alone, due to infrared galaxy magnitudes obtained by 2MASS. This the release of the positrons. To refine the model more result seems more relevant because the K-band observations of late SN Ia light curves are needed. luminosity is more directly related to the stellar Secondly, we deduce that, under the assumption of a population giving rise to SN Ia, namely low mass stars steady-state production-annihilation of positrons, in in binary systems. Mannucci et al. conclude that the order to explain the observed positron annihilation rate SN Ia rate shows a sharp dependence on both the Le+ by SN Ia events in the Galactic bulge alone, a mean morphology and the B-K color magnitude of the parent +0.14 SN Ia explosion rate of 0.50 -0.11 per century is galaxies (and therefore on the star formation activity). required (where errors of the SN Ia model are not taken In particular the SN Ia rate in irregular (late type) into account). But is this rate consistent with the galaxies is a factor ~20 higher than in elliptical (E/S0) observations? galaxies. Similarly, for galaxies bluer than the color magnitude B-K=2.6 the SN Ia rate is about 30 times 3. GALACTIC BULGE SN Ia RATE ESTIMATE larger than in galaxies with B-K>4.1. Mannucci et al. Our approach to get an estimate of the SN Ia rate in the show in particular (Fig 2), as a function of the B-K Galactic bulge is based on recent SN Ia rate color of the parent galaxy, the variation of the SN Ia measurements, obtained from studies of statistical rate per mass unit (SNuM, expressed in numbers of SN properties of large galaxy samples, in which the Ia per century and per 10 10 M☼ of the parent galaxy) as galaxies are characterized by a few global parameters. well as the SN Ia rate per luminosity unit (SNuK, If we can show that the Galactic bulge corresponds to a expressed in numbers of SN Ia per century and per K- certain class of galaxies in this parameter space, the SN band luminosity of the parent galaxy (LGAL,K ), Ia rate measurement for this class of galaxies gives an measured in units of 10 10 K-band solar luminosity estimate of the SN Ia rate in the Galactic bulge. (L☼ ,K ). If we determine the B-K color of the Galactic bulge, we can therefore get two estimates of the SN Ia Mannucci rate in the Galactic bulge, one using the mass of the Galactic bulge, and the second using its luminosity, both of which should of course be consistent. Launhardt K-band ~1.4 10 14 Hz B-band ~ 6.8 10 14 Hz Fig. 3. The spectral energy distribution of the Galactic bulge, from Launhardt et al. [14] is used to derive the B-K color magnitude of the Galactic bulge. Fig. 2. (Mannucci et al. [13]) B-K=4.8 Launhardt et al. [14] present a measurement of the spectral energy distribution of the Galactic bulge (see Fig. 3), which is best fit by an effective black-body spectrum, whose temperature is TGB ~4400 K. The B- band spectral window is defined around the frequency ν=6.8 10 14 Hz and the K-band around ν=1.4 10 14 Hz [15]. From [14] the integrated flux density Fν of the Our first estimate [10,11] was based on rates derived Galactic bulge in the B-band is measured to be -7 2 by Cappellaro et al. [12], relying on the blue integrated νFν(B)=3.6 10 W/m , and in the K-band νFν(K)=9.2 luminosity. In a recent paper, Mannucci et al. [13] have 10 -7 W/m2. Therefore we can compute the difference of computed the SN Ia rate normalized to the near- the magnitudes in the B- and K-band of the Galactic bulge, using the definition of the magnitude m ν=2.5 Matteucci et al. [21] (Fig 6 in their paper) based on log 10 (F ν(0)/Fν(m ν)) as a function of the absolute flux their model of the chemical evolution of the Galaxy. density Fν(m ν); for which the values at magnitude m=0 -8 2 This SN Ia rate is however incompatible with the are νFν(0)=2.9 10 W/m in the B-band, and hypothesis that SN Ia be the dominant Galactic -10 2 νFν(0)=9.2 10 W/m in the K-band [15]. As a positron injectors.