SN 1987A as a Possible Source of Cosmic Rays with E 0 < 10 18 eV by Yakutsk EAS Array Data

1. SN 1987A as a Possible Source of Cosmic Rays with E0 < 1018 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 23rd European Cosmic Ray Symposium (and 32nd Russian Cosmic Ray Conference) Moscow, Russia,  July, 3 – 7, 2012

2. Yakutsk EAS Array Yakutsk extensive air shower array is a ground based experiment for the detection of cosmic rays with energies between 1015 and 1020 eV. It is located near Yakutsk, Russia 61.661°N, 129.367°E. In 1974 the first stage of array from 35 similar stations on area more than 17 km2 put into operation. http://eas.ysn.ru

3. At present the total area of array is ~10 km2. In the central part of the array there is a denser domain (shown in a square) with 100-250 m detector spacing.

4. Exposures of experiments (Bergman, Belz, 2007) Yakutsk array is the only one which is in operation since 1974, it has the best exposure around 1018eV On the whole, approximately 106 showers of the primary energy above 30 PeV are detected.

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

6. The integral intensity of CRs The measured excess of the number of particles with E0 ≥ 1017 eV during 12 years (1998-2011) is about 0.05 particle per m2.

7. The yearly mean ratio of muons number ρm(300) to total number of charged particles ρs(300) at 300 m from EAS axis d = log<ρm(300)/ρs(300)> <lnA> = ln(56) [(dexp - dp)/(dFe - dp)] <lnA>≈1.87 and 3.34

8. KASCADE experiment first measured the increase of <ln A> from 1.5 to 3.4 in the energy range 1015-1.6×1017 eV. These measurements were carried out from May 1998 to December 1999. Before this no experiment observe such increase. This is in agreement with our result

9. Supernova Remnants Origin of Cosmic Rays It is believed that the balk of cosmic rays up to ~1017eV 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 1015 eV. A supernova remnant expanding into a uniform circumstellar medium may accelerate protons to 1017 eV and heavy ions, with charge Ze, to Z x 1017 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.

10. 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.

12. Within the first 1200 days the remnant's expansion velocity was extremely high, about 35000 km s-1. During the subsequent period of 1500-3000 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 ~104 years before the explosion (Gaensler, et al. 1997; Berezhko, Ksenofontov 2000)

13. The maximum energy of accelerated nuclei is proportional to square of shock velocityand 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 in the energy range 1017-1018 eV CRs contain 1/10 of energy and the total number of released particles is 1042 at Earth distance for isotropic approximation gives about 0.03 particle per m2 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) ESN=1.5×1051 erg

14. 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 E0=3×1017eV 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.

15. Summary • We found that before 1996 the intensity of cosmic rays in energy range 1017-1018eV 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 1018 eV . • This could be a crucial evidence of CR acceleration in supernova remnants.