Bounds to the gamma--ray flux emitted by SN 2011fe before the maximum of light as obtained by INTEGRAL/SPI

ATel #3683; J. Isern (ICE-CSIC/IEEC}, P. Jean (IRAP), E. Bravo (DFEN-UPC), J. Knodlseder (IRAP), R. Diehl (MPE), A. Hirschmann (ICE-CSIC/IEEC), N. Elias-Rosa (ICE-CSIC/IEEC), M. Hernanz (ICE-CSIC/IEEC), C. Badenes (U. Pittsburgh), I. Dominguez (DFMC-UGR), B. Kulebi (ICE-CSIC/IEEC), D. Garcia (DFEN-UPC), C. Jordi (ICC-UB/IEEC), G. Lichti (MPE), G. Vedrenne (CESR, UPS-CNRS), P. Von Ballmoos (IRAP)
on 14 Oct 2011; 12:56 UT
Credential Certification: Eduardo Bravo (eduardo.bravo@upc.edu)

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SN2011fe (RA = 14:03:05.81, Dec = +54:16:25.4; J2000) is a Type Ia supernovae that was discovered in M101 on August 24th 2011, about one day after the explosion by the Palomar Transient Factory (Nugent et al, ATEL #3581). The optical maximum was reached around September 10th-11th. The prospected gamma-ray emission maximum in radioactive decay lines should be around 70-100 days after the explosion.

The source was observed with all the instruments on board of INTEGRAL (SPI, IBIS/ISGRI,JEM-X and OMC). The observations with INTEGRAL span from 4258.8733 IJD (August 29th 20:59) to 4272.1197 IJD (September 12th 2:52). The total observation time was 975419 s.

This schedule was essentially determined by constraints imposed by the Sun, according to the TVP tool of INTEGRAL, that might prevent the observation just beyond the expected maximum of the 56Ni lines. The early observations can be used to constrain any prospected early gamma-ray emission as may be expected from some variants of sub-Chandrasekhar models.

At this epoch, the expected behavior of the spectrum (see the cross-checked models of Milne et al ApJ 613, 1101, 2004) should be as follows:
i) The spectrum is dominated by the 56Ni 158 and 812 keV lines.
ii) Because of the rapid expansion the lines are blue-shifted but their peak quickly evolves to the red. Because of the Doppler effect the 812 line blends with the quickly growing 56Co 847 keV line, forming a broad feature. The emergent lines are broad, typically from  3%  to 5%.
iii) The intensity of the lines rises very quickly with time (Gomez-Gomar, Isern, Jean MNRAS 295, 1,1998), being very weak at the beginning, even in the case of Sub-Chandrasekhar models.

Taking into account these properties, the bounds (2 sigma upper limits) obtained with SPI for the 158 keV line and the 812 and 847 keV complex are:
Energy band        Flux (ph/s/cm2)
160-166 keV        < 7.5e-5
140-175 keV        < 2.3e-4
814-846 keV        < 2.3e-4
800-900 keV        < 3.5e-4

For comparison, the time-averaged predicted fluxes from model W7 at a distance of 6.4 Mpc are:
Energy band        Flux (ph/s/cm2)
160-166 keV        7.2e-7
140-175 keV        3.9e-6
814-846 keV        2.2e-6
800-900 keV        4.0e-6

The bounds obtained with the other instruments will be published very soon. Further INTEGRAL observations are ongoing, and are planned to cover the gamma-ray emission maximum.

We acknowledge Chris Winkler (ESA, ESTEC) and the INTEGRAL personnel for their help and advise.