SN 1987A as a Possible Source of Cosmic Rays with E 0 < eV by Yakutsk EAS Array Data A.V. Glushkov, L.T. Ksenofontov, M.I. Pravdin Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy, Yakutsk, Russia 23 rd European Cosmic Ray Symposium (and 32 nd Russian Cosmic Ray Conference) Moscow, Russia, July, 3 – 7, 2012

Yakutsk EAS Array Yakutsk extensive air shower array is a ground based experiment for the detection of cosmic rays with energies between and eV. It is located near Yakutsk, Russia °N, °E. In 1974 the first stage of array from 35 similar stations on area more than 17 km 2 put into operation.

At present the total area of array is ~10 km 2. In the central part of the array there is a denser domain (shown in a square) with m detector spacing.

Exposures of experiments (Bergman, Belz, 2007) Yakutsk array is the only one which is in operation since 1974, it has the best exposure around eV On the whole, approximately 10 6 showers of the primary energy above 30 PeV are detected.

The increase of intensity reached 45±5%. The integral intensity of CRs

The measured excess of the number of particles with E 0 ≥ eV during 12 years ( ) is about 0.05 particle per m 2.

d = log = ln(56) [(d exp - d p )/(d Fe - d p )] ≈1.87 and 3.34 The yearly mean ratio of muons number ρ m (300) to total number of charged particles ρ s (300) at 300 m from EAS axis

KASCADE experiment first measured the increase of from 1.5 to 3.4 in the energy range ×10 17 eV. These measurements were carried out from May 1998 to December Before this no experiment observe such increase. This is in agreement with our result

Supernova Remnants Origin of Cosmic Rays It is believed that the balk of cosmic rays up to ~10 17 eV originated in supernova remnants (Berezhko,Völk, 2007) by diffusive shock acceleration (Krymsky, 1977) “Magnetic field can be amplified non-linearly by the cosmic rays to many times the pre-shock value, thus increasing the acceleration rate and facilitating acceleration to energies well above eV. A supernova remnant expanding into a uniform circumstellar medium may accelerate protons to eV and heavy ions, with charge Ze, to Z x eV: Expansion into a pre-existing stellar wind may increase the maximum cosmic ray energy by a further factor of 10.” (Bell, Lucek, 2001) However, there is a lack of an undoubted direct observational evidence for such idea.

SN 1987A SN 1987A was a supernova in the Large Magellanic Cloud, a nearby dwarf galaxy. The distance 50 kpc. The direction to supernova is 30° below Galactic plane. It could be seen only from the Southern hemisphere.

Within the first 1200 days the remnant's expansion velocity was extremely high, about km s -1. During the subsequent period of days the shock velocity dropped by an order of magnitude, to 3000 km s -1, explained by the shock entering a region of dense wind from a red giant which the SN progenitor have been ~10 4 years before the explosion (Gaensler, et al. 1997; Berezhko, Ksenofontov 2000)

E SN =1.5×10 51 erg in the energy range eV CRs contain 1/10 of energy and the total number of released particles is at Earth distance for isotropic approximation gives about 0.03 particle per m 2 The maximum energy of accelerated nuclei is proportional to square of shock velocity and can be attained during several tens of days (Bell, Lucek, 2001 ; Berezhko, Ksenofontov 2000 ). At 4-8 years after explosion the highest energy tail of already accelerated particles can’t retained anymore by the remnant and freely escaped from the remnant If CR acceleration occur at a supernova shock expanding into a circumstellar wind with a Parker spiral magnetic field, they drift to the poles producing a large cosmic ray pressure which can generate polar density cavities through which CR escape upstream of the shock. (Bell, 2008)

Since SN 1987A is out of Yakutsk array field of observation, to be observable particles should deviate in the Galactic magnetic field at least on 86°. The iron nuclei with E 0 =3×10 17 eV in magnetic field 3 µG have gyroradius 4.2 pc and the time to deviate on 86° is about 2 years; protons with the same energy in the same magnetic field have gyroradius 108 pc and corresponding time 52 years. In other words, some fraction of iron nuclei flux from SN 1987A could reach Yakutsk array field of observation, but protons can’t yet.

Summary  We found that before 1996 the intensity of cosmic rays in energy range eV fluctuate near some average value, then it start to increase. The increase of intensity reached 45±5%.  It is accompanied by significant changes in cosmic ray composition; it became heavier – iron dominated.  In the last 3-4 years it is a trend to return to the initial state.  This is in agreement with the scenario that SN 1987A is a source of CRs up to eV.  This could be a crucial evidence of CR acceleration in supernova remnants.