On the Burst Activity of the Crab Nebula and Pulsar at High and Ultra-High Energies A. S. Lidvansky Institute for Nuclear Research, Russian Academy of Sciences, Moscow

“Astronomy consists of two parts: the Crab Nebula and all the rest.” I.S. Shklovsky “They don't understand the results, and nobody else does either.” Correspondent of New Scientist after a talk with radio astronomers about observations of the Crab Pulsar

According to modern terminology the Crab Nebula is PWN (pulsar wind nebula)

The Crab Nebula as a standard candle X-ray astronomy: 1 Crab = 1060  Jy in the range 2-11 keV 1 milliCrab = 2.4  10  14 W cm keV Gamma-ray astronomy: Integral 1 Crab = 1.75  cm -2 s -1 at energy of 1 TeV Differential 1 Crab = (2.79 )   (E/1TeV) (-2.59) m -2 s -1 TeV -1 conversion according to the spectrum

CN standard spectrum (Crab unit)

At the moment Crab stopped being a standard candle In X-ray astronomy the flux intensity from Crab dropped down by 7% In a short time

In gamma-ray astronomy the status of a standard candle was lost due to burst activity: AGILE detection of strong gamma-ray flare in October 2007

First public announcement Sept. 22, 2010: AGILE issues the Astronomer’s Telegram n Tavani et al. Science Express, Jan. 6, 2011 Abdo et al., Science Express, Jan. 6, 2011 AGILE light curve (2-day binning) (PSR + Nebula)

Fermi-LAT light curve (4-day binning) (Nebula only)

Fermi LAT AGILE

Gamma-ray flares in the Crab Nebula (satellite telescopes) DateDurationPeak flux Instrument October 2007~ 15 days~ cm -2 s -1 AGILE February 2009~ 15 days~ cm -2 s -1 Fermi-LAT September 2010~ 4 days~ cm -2 s -1 AGILE, Fermi April 2011~ 5 days~ cm -2 s -1 AGILE, Fermi

Sky map provided by the Fermi-LAT telescope

During this flare the Crab was 30 times brighter than its usual luminosity (gamma > 100 MeV)

The first announcement about ultra-high energy burst in the Crab Nebula on February 23, 1989 V.V. Alexeenko, A.S. Lidvansky, V.A. Tizengauzen, A Search for > eV Gamma Rays from Point Sources at Baksan Air Shower Array, Proceed. of Intern. Workshop on Very High Energy Gamma Ray Astronomy, Crimea, 1989, ed. by A.A. Stepanyan, D.J. Fegan, and M.F. Cawley, p. 137

The Crab Nebula burst recorded by the Baksan air shower array on February 23, 1989

Phase analysis with pulsar PSR0531 period (without absolute timing) Time profile of the counting rate of the Crab Nebula cell with a radius of 2.5 

Publications about simultaneous detection of the Crab burst by several AS arrays Acharya B.S., Rao M.V.S., Sivaprasad K., Sreekantan B.V., and Vishwanath P.R., First simultaneous detection of PeV energy burst from the Crab Nebula, Nature, 347 (1990), V.V. Alexeenko, Yu.M. Andreyev, A.E. Chudakov, Ya.S. Elensky, A.S. Lidvansky, S.Kh. Ozrokov, Yu.V. Stenkin, V.A. Tizengauzen, L.J. Graham, J.L. Osborne, and A.W. Wolfendale, The ultra-high energy gamma ray burst from the Crab Nebula observed by the Baksan EAS array, J. Phys. G: Nucl. Part. Phys. 18 (1992) L83-L88.

Table 1. Summary of observations of the 'Crab burst'. The standard deviations of the excess counts are calculated according to the formula of Li and Ma. Array Observation time Counts ON Counts OFF Excess (SD) KGF Tien Shan Baksan EAS TOP (Gran Sasso)

Main result: The burst was observed by all EAS arrays within longitude range from India to Italy (with different significance) To the east (arrays Akeno SPICA and Ohyo Mine in Japan, active at that time) and to the west (HEGRA on Canary Islands) from this range no enhancements were observed) Combined probability of this event (taking strong non- uniformity of phase curves into account) was estimated as 1.25  The peak flux during the burst was estimated as (8  2)  10  12 cm  2 s  1 for energy eV

Energy spectrum of the Crab Nebula

Energy spectra of the pulsar

Spectrum of April 2011 flare is similar to the pulsar spectrum, but no pulsations observed

Light curve of Crab pulsar in radio, optical, and X-ray energy ranges Moffett, D. A., & Hankins, T. H. 1996, ApJ, 468, 779

Profiles of PSR0531 pulses uin the range from 0.1 keV to 10 GeV (From Kuiper et al, 2001.) Comptel Egret

MAGIC: pulsations at 500 GeV

At 100 MeV (Fermi) the main pulse still dominates, but at 120 GeV (Veritas) it becomes less than the interpulse

This old detection of a burst on February 23, 1989 in the light of modern data  Energies are not so incredibly high. Now Cherenkov telescopes have measured nebula spectrum almost up to eV, existence of electrons with energy eV is very probable (some people believe this fact to be firmly established).  Duration of this burst is less than that of Fermi and AGILE flares, but not drastically.  The only pulse on the pulsar light curve can be either P1 or P2 (both are very variable with energy), or P3 whose existence is supposed.  Radiation of pulsar frequency is observed practically up to energy of 500 GeV, though no flare activity has been detected in this range.  The latter fact can be due to small duty cycle of Cherenkov telescopes.

Conclusions  The gamma-ray burst detected more than 20 years ago reasonably agrees with modern pattern of flare activity of the Crab Nebula  Up to now this burst is the source detection at the highest energies  Some periodical structure of this burst is probably a very important fact that should be taken into account when constructing the source model

Spectra of two preceding flares

No flares in other ranges (with a possible exception of ARGO-YBJ in TeV region)

Crab Nebula flare at 100 TeV ? 23 February 1989  KGF (India) 3.4    Baksan (USSR) 3-4    EASTOP (Italy) 2.3  chance probability  KGF flux (> 100 TeV) = 1.3 ± ev cm -2 s -1   Flare duration  7 h

Enhanced TeV gamma ray flux from the Crab Nebula observed ATel #2921; G. Aielli, et al.) on 11 Oct 2010; 11:42 UT Credential Certification: Silvia Vernetto Aielli, et al.) Subjects: TeV, Neutron Star, Supernova Remnant, Pulsar Referred to by ATel #: 2967, Following the discovery by AGILE (ATEL #2855) and confirmation by Fermi/Lat (ATEL #2861) of enhanced gamma ray emission from the Crab Nebula region in the time interval from September 18 to September , we report on the observation performed during the same days by ARGO-YBJ. ARGO-YBJ is a full coverage air shower detector located in the Yangbajing Cosmic Ray Laboratory in Tibet (China) at 4300 m a.s.l. working with an energy threshold of a few hundred GeV. The field of view of about 2 sr and the duty cycle of about 90%, allow a continuous monitoring of a large part of the sky. The Crab Nebula region is observed every day for about 5.5 hours. A preliminary analysis of the data shows an excess of events with statistical significance of about 4 standard deviations from a direction consistent with the Crab Nebula in the time interval from September 17 to September 22, corresponding to a flux about 3-4 times higher than usual, at the median energy of about 1 TeV. Moreover, the ARGO-YBJ analysis in a time window extended up to September 27, suggests that at TeV energies the enhanced emission lasts more than that observed by lower energy experiments

Crab light curve - 2 days bin September 19th - flare onset Average rate in 8 days: 61  13 ev/h 4.8 times larger Average rate in 3 years: 12.8  1.3 ev/h N hit > 40

Crab Nebula September Chance probability: p = N hit > 40 8 days 46 observation hours Significance 4.8  Expected 1.0  from steady flux

September In 10 days: 61  11 ev/h Significance 5.4  (expected 1.1  Chance probability p = N hit > 40 Crab light curve - 10 days bin