High energy phenomena associated with gamma ray burst sources

Adv. Space Res. Vol. 22, No. 7, pp. 1101-l 104, 1998 0 1998 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/98 $19.00 + 0.00 PII: SO273-1177(98)00202-6

Pergamon

HIGH ENERGY PHENOMENA ASSOCIATED GAMMA RAY BURST SOURCES

WITH

A.R. Rao and M.N. Vahia

Space Physics Group, Tata Institute Mumbai 400 005, INDIA

of Fundamental

Research,

Homi Bhabha Road, Colaba,

ABSTRACT Gamma-ray burst (GRB) spectrum is known to extend upto GeV energies, indicating the occurrence of transient high energy phenomena near GRB sources. Here we present evidence of statistical association between the locations of ultra high energy cosmic rays and GRBs. We examine our earlier finding that bright GRBs are associated with high energy cosmic rays (Energy > 3 x 10ls eV). Additionally we find a significant positional association between faint GRBs and cosmic rays. If confirmed, these results have very far-reaching implications for GRB source models: GRB sources have to be nearby; GRBs should repeat with a time scale of about 10 years; GRBs We discuss these results in the light of our earlier are heterogeneous and not mono-luminous. suggestion that GRBs and cosmic rays originate from Magnetically Active Stellar Systems. 01998 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION The general similarities of the properties of gamma ray bursts (GRBs) and ultra high energy cosmic rays (UHECR) like the typical occurrence rate, homogeneity etc. have prompted several workers (Waxman, 1995; Vietri, 1995) to suggest that these two enigmatic astrophysical events may have the same origin. Though some evidence was presented for a direct one to one association between individual GRB and UHECR sources (Milgrom and Usov, 1995), an attempt to associate a larger population of sources did not yield any positive results (Stanev et al., 1996). For any reasonable source distance the Galactic magnetic field will deflect the UHECRs and smear out any positional or temporal association with GRBs. In Vahia and Rao (1991) a statistical approach was taken and GRBs and UHECRs were found to be associated at a confidence level of 99.5%. Cosmic rays of energy > 3 x 101’ eV were found to be associated with highly localized GRBs. This result was explained using the hypothesis that GRBs and UHECRs originate from the inter-binary regions of Magnetically Active Stellar Systems (MASS). In th is work we extend this correlation using the CGRO/BATSE 3B catalog of GRBs. MAGNETICALLY

ACTIVE

STELLAR

SYSTEMS

Solar-like flaring activity due to magnetic interaction occurs in most of the late type stars and these activities are in the most intense form in the Magnetically Active Stellar Systems (MASS). The stars classified in this category are flare stars, RS CVn binaries and Cataclysmic Variables (Rao and V&a, 1987, Vahia and Rao, 1988; 1990). Most of the Fast Transient X-ray (FTX)

1102

A. R. Raoand M. N. Vahia

sources detected by the Ariel V satellite were identified as the most intense form of stellar X-ray flares with energies in the range of 103” - 1O37 ergs (Pye and McHardy, 1975; Rao and Vahia, 1987). Though these identifications are of statistical nature, the conclusions are vindicated by the recent direct detection of soft X-ray and UV flares of similar energies in such stars (Ottmann and Schmitt, 1996; Pagan0 et al., 1996). Some of the FTX sources are identified with GRBs and there appears to be a causal relationship between MASS and GRBs. The energetics of such systems are discussed in Vahia and Rao (1988) and some GRBs are associated with MASS. In a later work (Vahia and Rao, 1990) the interbinary regions of MASS were shown to be an attractive site for particle acceleration and a direct association between UHECRs and GRBs was obtained at a confidence level of 99.5%. For this study 76 GRBs with positional error less than 1 square degrees were selected from the pre-GRO observations (Atteia et al., 1987; Golenetskii et al., 1987) and UHECR data was taken from the observations of SUGAR array (Winn et al., 1986), Volcano Ranch (Linsley 1980) and Haverah Park (Reid and Watson 1980). The sky around each GRB was divided into annular bins of equal angle and the number of UHECRs in each bin was counted. Data for all GRBs are co-added and a significant excess in the first bin was detected. ASSOCIATION

OF UHECRS WITH GRBS

Since positional association between UHECRs and GRBs are possible for only nearby sources (due to the deflection caused by the Galactic magnetic field) we have selected the brightest GRBs (fluence > 5 x 10s5 ergs s-l) from the CGRO/BATSE data base which, according to our hypothesis, originate from the nearby Cataclysmic Variables (CVs). We adopt the same procedure for comparing the GRB and UHECR catalogues that is discussed in detail in Vahia and Rae (1990). For 76 UHECRs and 43 GRBs we find an association at a significance level of about 80%. This significance is much lower compared to the earlier work of pre-GRO data. This can partly be explained by the lower number of GRBs in the data used for the association. Further, the UHECR data and the pre-GRO GRB data is almost contemporaneous and if GRBs are indicative of enhanced magnetic activity, one can expect cosmic ray acceleration during that time. This is not the case for the CGRO/BATSE o b servations which are about a decade later than the UHECR observations. Detailed analysis of the comparison of GRBs with MASS showed that while the most energetic of the GRBs would occur on CVs, the flare stars would contributed significantly to the lower fluency end of GRBs (Rao and Vahia, 1994). On the other hand, the GRBs on flare stars would be of much more local origin (distance of the order of a few parsecs). Hence while looking for GRBs we specifically look for weak GRBs in the catalogues for comparison with the cosmic ray catalogue. The results of the comparison are shown in Figure 1. The observed integrated number of UHECRs of energy greater than 10 l8 eV (a total of 943) in the vicinity of the 353 weak GRBs (fluence < 2 x lo-’ erg) are expressed per unit solid angle and plotted against the angle. A significant peak is seen in the first bin (corresponding to an angular deviation less than 5”). We estimate the significant level to be 99.8%. For the intermediate fluence GRBs (between 10e6 ergs/s and 5 x 10D5 ergs/s) we do not find any significant association with UHECRs. According to our hypothesis these intermediate energy GRBs originate from very distant CVs and RS CVn binaries and hence significant association with UHECRs is not expected.

1103

High Energy Phenomena Associated with GRBs

2.4~10~

0

20

40

Angle (degrees)

Fig. 1. The number of UHECRs per unit solid angle around faint GRBs plotted as a function of angle.

DISCUSSION Both the cosmic ray and GRB positions have a few degrees uncertainty. More than 65% of the sources in both the catalogues have < 5” uncertainty, justifying the choice of 5” as the bin size. For the weak GRBs, however, the positional uncertainties are larger (65% of the sources have < 8” uncertainty). Since the association presented here are of statistical nature, this larger uncertainty in weak GRBs does not significantly affect the results. The very fact that there is a significant positional association between UHECRs and GRB s with angular separation < 5” puts a limit on the distance to these sources as < 1.5 kpc for an interstellar magnetic field of 2 PG. Further, the UHECR and GRB observations refer to two different times of observations. In particular, while the GRB observations are for the last five years or so, the UHECR catalogues refer to observations from 1960 onwards. Though the sample size of GRBs for the association is larger than the pre GRO study (V&a and Rao 1991), the significance level of association has not increased very much. Nevertheless, the observed association implies that both the GRB and the UHECR sources must be repeating to obtain the observed association. We estimate a repetition rate of about once in 10 years. A similar value for the repetition rate was derived for the GRBs using the GRB and FTX association (Vahia and Rao, 1990). The independent significant association between the faint and bright GRBs shows that GRBs as a class are heterogeneous and consists of several classes with widely separated intrinsic luminosities, According to our hypothesis, GRBs originate from widely different classes of objects with the same inherent production mechanism: magnetic reconnection on inter binary scale. Using diverse range in the intrinsic luminosity it is possible to explain the observed isotropy (Rao and Vahia, 1994a). There are, however, direct evidences to support this hypothesis. One of the highly localized GRB error box included an active star (Rao and Vahia 1994b). Another highly localized GRB was identified with a Galactic stellar object (Hurley et al, 1996) Further confirmation of our hypothesis would come from 1) improved statistics of brighter GRBs with CGRO/BATSE which should improve the significance of the association between GRBs and UHECRs and 3) the large duty cycle RXTE/ASM should reveal many more FTX like events coincident with GRBs.

104

A. R. Rao and M. N. Vahia

REFERENCES Atteia J.L. et al., A Second Catalogue of Gamma-Ray the Interplanetary Network, ApJS, 64, 305 (1987). Golenetskii,

S.V. et al., A.F. Ioffe Physical-Technical

Hurley, K. et al., Possible Association Ap. J., 464, 3423 (1996). Linsley, J, Catalogue

of a Quiescent

Bursts: Institute

1978 - 1980 Localization Preprint

from

Number 1026, (1987).

X-ray Source with a Gamma-ray

Burster,

of Highest Energy Cosmic Rays, ed. WDC-C2 for cosmic rays, p5 (1980).

Milgrom, M. and V. Usov, Possible Association of Ultra-High-Energy Strong Gamma-Ray Bursts, Ap. J., 449, L37 (1995).

Cosmic Ray Events with

Ottmann, R. and J.H.M.M. Schmitt, Rosat Observation of a Giant X-ray Flare on Algol: Evidence for Abundance Variations ?, A&on. Astrophys., 307, 813 (1996). Pagano, I., R. Ventura, M. Rodono, G. Peres, and G. Micela, A Major Optical Flare on the Recently Discovered X-ray Active dMe Star G102-21, Astron. Astrophys., submitted (1996). Pye, J.P. and I.M. McHardy,

Fast Transient

Rae, A.R. and M.N. Vahia, Fast Transient Astrophy., 188,109 (1987).

X-ray Sources, MNRAS 205, 875 (1983).

X-rays From Flare Stars and RS CVn Binaries Astron.

Rao, A.R. and M.N. Vahia, Observational Constraints on the Inter-Binary for the Gamma-Ray Bursts A&on. Astrophy., 281,L21 (1994a).

Stellar Flare Hypothesis

Rae, A.R. and M.N. Vahia, On the Possible Source of GRB 930131, Astron. Astrophy., 287,L34 (1994b). Reid, R.J.O., A.A. Watson, Catalogue rays, p71 (1980).

of Highest Energy Cosmic Rays, ed. WDC-C2 for cosmic

Stanev, T., R. Schaefer, and A. Watson, Search for Correlation Between the Highest Cosmic Ray Showers and Gamma Ray Bursts, Astroparticle Physics, 5,75 (1996). V&a,

Energy

M.N. and A.R. Rao, Origin of the Gamma Ray Bursts, Astron. Astrophy., 207,55 (1988).

Vahia,

M.N. and A.R. Rae, Cosmic Rays from Magnetically Astrophy., 250,424 (1991).

Active Stellar

Systems

A&on.

of Highest

Energy

Vietri, M., Ap.J., 453, 883 (1995). Waxman,

E., Phys. Rev. Lett., 75, 386 (1995).

Winn, M.M. J. Ulrichs, L.S. Peak, C.B. McCusker, L. Horton, Cosmic Rays, ed. WDC-C2 for cosmic rays (1986).

Catalogue