Do X-Ray Binaries Emit Gamma Rays? P. M. Chadwick, M. R. Dickinson, N. A. Dipper, J. Holder, T. R. Kendall, T. J. L. McComb,

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1 Do X-Ray Binaries Emit Gamma Rays? P. M. Chadwick, M. R. Dickinson, N. A. Dipper, J. Holder, T. R. Kendall, T. J. L. McComb, K. J. Orford, J. L. Osborne, S. M. Rayner, I. D. Roberts, S. E. Shaw and K. E. Turver. Department of Physics, Rochester Building, Science Laboratories, University of Durham, Durham, DH1 3LE, UK Abstract Recent observations of Cen X-3 at GeV energies using EGRET on board CGRO and at TeV energies using the Mark 6 atmospheric Cerenkov telescope at Narrabri suggest that X-ray binaries are sources of high energy -rays. 1. Introduction In a series of measurements the Durham and Potchefstroom groups obtained evidence that X-ray binary systems produced TeV -rays. In 1986 the Potchesfstoom group presented data showing periodic emission from Vela X-1 in the form of count rate increases occuring in bursts every 283 secs the spin period of the pulsar for about 5 rotations of the pulsar (North et al. 1987). Between 1987 and 1993 the Durham group found evidence for a weak but apparently persistent periodic signal from Vela X-1 (Carrami~nana et al 1987, Bowden et al. 1992); in observations in 1992 evidence for the strong periodic outbursts lasting for 5 or 6 periods of the pulsar, similar to the original Potchefstroom result, was noted on two occasions separated by exactly 1 orbital period (Bowden et al. 1992). Observations of Cen X-3 by the Durham group lasting more than 300 hrs spread over 5 years suggested a periodic emission at the contemporary X-ray period, signicant at>3level, for the total dataset. Analysis of the data in intervals of binary orbital phase suggested that most of the emission originated in observations made at orbital phase around 0.75 and maybe 0.25 (Carrami~nana et al. 1989, Brazier et al 1989). The periodicity noted was that of the spin of the X-ray pulsar but the site of the emission did not coincide with the compact object; rather it was at a range of positions in the accretion disc close to the compact object. This analysis of the periodicity, allowing for an origin close to, but not coincident with the compact object, has been developed (Bowden et al. 1993, Raubenheimer and Smit 1997). The technique of analysing the timing information allowing for this variation in the production site is shown in Figure 1, where the Rayleigh probability of periodicity at the X-ray period is plotted for a range of positions in the accretion disc specied by values of a sin i and orbital phase (Bowden et al. 1993). Note that the band of possible emission sites depends upon the value of the pulsar spin period. 2. Observations of GeV Gamma Rays with EGRET In a recent paper Vestrand et al (1997) have reported the detection of an outburst of GeV emission from Cen X-3 using the CGRO=EGRET experiment. The outburst occurred in October 1994 during an interval of rapid spindown by the pulsar. The signicance of the detection

2 Rayleigh probability for periodicity at the X-ray pulse period of Cen X-3 plotted for a range of positions in the accretion disc specied by values of a sini and orbital phase. Fig. 1: of was > 4. A phase analysis of the EGRET data using the contemporary X-ray data from BATSE indicated modulation of the -ray emission at the pulsar spin period. The signicance was at a level > 99:5%. The phase averaged integral ux at E > 100 MeV during the outburst was (9:2 2:3) 10,7 cm,2 s,1, corresponding to a luminosity of erg s,1. This is the rst evidence for GeV -ray emission from an X-ray binary system. 3. TeV Measurements with the Mark 6 Telescope at Narrabri Observations were made with the Mk 6 telescope at Narrabri in March and June These are the rst observations of XRBs in the Southern hemisphere with an imaging telescope. The data were collected in the ON/OFF chopping mode with a segment duration of 15 min. The ON and OFF observation times were equal (37 ON/OFF scan pairs). No normalization of ON/OFF data was performed. Observations were at a range of phases in the 2.1 day orbit. The date were processed using the procedures adopted for the initial analysis of Mk 6 telescope data. Events were selected which had :- 1. a total Cerenkov light sample of > 200 dc ( 125 GeV of -ray energy) 2. the centroid of the image between 0:45 and 0:85 from the source position 3. values of D dist (the separation of centroids in the LEFT and RIGHT collectors of the Mark 6 telescope - see Chadwick et al (1997)) less than the median value. 4. ECCENTRICITY between 0.35 and WIDTH less than a sample dependent value in the range 0:12, 0:28 Finally, the ALP HA selection was made. On this basis, an excess in the number of events ON source over those OFF source was noted. The sample sizes at various stages in the analysis are given in Table 1. The signicances have been calculated following the method desribed in Gibson et al. (1982)

3 Table 1: The results of cutting using the selections described in the text. ON OFF Dierence Signicance No. of events (1) and (2) ,0:4 No. of events (1) - (5) :1 No. of events (1) - (5) plus ALPHA :4 The distribution in the pointing angle ALP HA of the events (ON-OFF) so selected is shown in Figure Number of events ALPHA (degrees) Fig. 2: The dierence in the ALP HA distributions for ON and OFF source events recorded during observations of Cen X-3. The position of the source of the excess counts is shown in results of the false source analysis in Figure 3; the signicance of the excess is shown for the those events with ALP HA < 22:5 when the source position is allowed to vary over region of 2 2 in RA dec around the position of Cen X-3. The peak signicance is Periodicity A search was made for periodicity in the ALP HA selected dataset, after correcting the event times to the barycentre of the solar system and assuming that the emission originated from the position of the pulsar. No indication at the contemporary BATSE spin period was found. An analysis allowing for an origin for the emission elsewhere in the accretion disc at the precisely known BATSE spin period has also been made. The -ray emission was assumed to originate anywhere in the range of orbital phase and at a distance out to 60 ls from the massive companion. No evidence was found for strong periodic emission of the type seen occasionally in previous experiments. 4. Comparison of TeV Results with RXTE Data The data from the All Sky Monitor on the RXT E satellite may be an indication of the activity of the X-ray binary. The daily average count rate of the ASM is shown in Figure 4 for the days

4 Right Ascension Declination Fig. 3: The result of a false source analysis for Cen X-3. The centre of the plot is the position of Cen X-3. when TeV measurements were made, indicating that the object was in an active state during the March observations, but less active in June Date (MJD) Fig. 4: The daily average count rate of the All-Sky Monitor on board RXT E. The VHE -ray observations considered here were taken during the intervals denoted by the grey shading. 5. Conclusions Recent observations of Cen X-3 at GeV and TeV energies have provided results which reinforce the earlier claims that X-ray binaries emit very high energy -rays. The ASM data from RXT E indicated a moderate level of X-ray activity during the TeV observations. The March 1997 data coincided with a strong burst of X-ray activity.

5 6. Acknowledgements We are grateful to the UK Particle Physics and Astronomy Research Council for support of the project and the University of Sydney for the lease of the Narrabri site. The Mark 6 telescope was constructed with the assistance of the sta of the Physics Department, University of Durham, and the eorts of Mr. P. Cottle, Mrs. E.S. Hilton and Mr. K. Tindale are acknowledged with gratitude. This paper uses quick look results provided by the ASM/RXT E team. References Bowden C. C. G., Bradbury, S. M., Brazier, K. T. S. et al. 1992, J. Phys. G., 18, L127 Bowden C. C. G., Bradbury, S. M., Chadwick, P. M. et al. 1993, in Proc 23rd ICRC, 1, 298 Brazier K. T. S., Carrami~nana A., Chadwick, P. M. et al. 1989, in Proc 23rd ESLAB Symposium, 1, 311 Carrami~nana A., Chadwick, P., Dipper, N. et al. 1989, in Timing Neutron Stars, ed. E.P.J. van der Heuvel and H. Ogelman, p. 369, Kluwer Academic Publishers, Dordrecht Carrami~nana, A., Chadwick, P., Dipper, N. et al. 1989, ApJ, 346, 967 Gibson A. I., Harrison, A. B., Kirkman, I. W. et al. 1982, Nature, 296, 833 North, A. R., Raubenheimer, B. C., De Jager, O. C. et al. 1987, Nature, 326, 567 Raubenheimer B.C. and Smit H.J. 1997, Astropart. Phys., 7, 63 Vestrand W.T., Sreekumar P. and Mori M., ApJ, 483, L49.