18. ESLAB symposium on X-ray astronomy ccheveningen, W Ha*>e (Netherlands) 5-9 Nov 1984 CEEA-CONF—771A 4

X-RAY AND OPTICAL VARIABILITY AT THE HOUR TIMESCALE FOR IE 0630+178 (GEMINGA) AND ITS PROPOSED OPTICAL COUTERPART

L. vigroux1, J. A. Paul Service d'Astrophysique, Centre d'Etudes Nucléaires de Saclay 91191 Gif-sur-Yvette CEDEX, France

P. Delâche Observatoire de Nice, BP 139, 06003 Nice CEDEX, France

G. F. Bignami, P. A. Caraveo Istituto di Fisica Cosmica del CNR Via Bassini 15, 20133 Kilano, Italy

L. Salotti Dipartimento di Fisica, Université di Milano via Celoria 16, 20133 Milano, Italy

ABSTRACT. Einttem and EXOSAT data on the soft X-ray source IE 0630+173, the proposed counterpart of the -r-ray source GEMINGA, are analyzed for variability on the time scale of one to three hours. The EXOSAT September 1983 data, with an uninterrupted strech of over 10 he-.-s offer the most interesting case. In parallel, a similar analysis is presented for the first time, for the optical data of the m ~2i proposed counterpart. About 30 CCD exposures, of 15 min. each, taken over two consecutive nights at the 3.6 m CFH telescope, yield evidence of variability, when compared to the data of similar nearby objects in the field.

1. INTRODUCTION

The high-energy (£50 Mev) 7-ray source 2CG 195+04 (GEMINGA), discovered by the $AS-2 satellite and accurately measured by the COS-B mission, has recently been identified with an Eimtatn/EXOSAT soft X-ray source through its periodic emission at -59 s, Which had been seen in the SAS-2 data, subsequently confirmed and later retracted by the COS-B team as non significant (see Bignami, Caraveo, and Paul 1984, and references therein). The problem of its optical identification thus shifted down to the Einnam High Resolution Imager (HRI) -7x7 arc sec. error box (Bigna

visiting astronomer at the Canada-France-Hawaii Telescope, operated by the National Concil of Research, Canada, the Centre National de la Recherche Scientifique, France, and the University of Hawaii. interesting candidate has already been proposed with m -21 (Caraveo »t ti. 1984b, Caraveo «r •/. 1984c). Here, we report on repeated X-ray and CCD observations of this candidate GEMINCA counterpart, performed vjth the Einsftn and EXOSAT satellites, and at the Canada-France-Hawai (CFH) telescope.

2. THE X-RAY DATA

For a detailed history of the X-ray observations, tht reader is referred to Bignami, Caraveo, and Paul (1984). For a study of medium term variability.

looo uooo TIf€ (SEC.)

9e » b

80 • 7e ' (A 60 50 • 40 30 ' 20 10

1 1 1 ^ . s 10 1$ I 9 25 3 0 Tine (KUOSEC)

Figure l. Time profiles for IE 0630+178. a: Einn tin time profiles for IE 0630+178 (solid line) and background (broken line), b: EXOSAT time profiles for IE 0630+178 (solid line) and star SAO 095804 (broken line). only the Einttein Imaging Proportional Counter (IPC) March 1981, and the EXOSAT September 1983 observations , are long enough to be useful. The shorter IPC 1979 and HRI 1981 observations yield the same time-averaged flux of the other two, within the 30% or so accuracy with which the comparison is possible. Figure 1 shows the time profile of the Einstmn IPC March 1981 data and of EXOSAT CMA September 1983 data (for a description of the derivation of the curves in Figure lb, see Caraveo et ai. 1984). In both cases, the source IE 0630+178 is compared to a non-variable reference, in the IPC case, the counter background, and in the EXOSAT case, a field star, the apparent source variability on the hour timescale has been quantified with both x* -nd Kolmogovov-smiznof tests: for both data sets, the probability that the source is variable is -.94%.

Figure 2. CCD image of the GEMINGA field. This image has been taken at the primary focus of the CFH 3.6 m telescope with a R filter centered at 660 nm and 110 nm wide. The £imt»m HRI error box is indicated, with the m -20.3 object (labelled as G) in it as the only candidate. The star il (m -20.1) and the star 17

(mQ~l9.8) are also indicated. 3. THE OPTICAL DATA

3.1. Observations and method of analysis

The CCD observations were made on the nights of 7 and 8 January 1984, at the prime focus of the CFH 3.6 m telescope. The CCD used was a thin backside illuminated chip, with a corresponding pixel length of 0.412 arc sec. This resolution allows a good photometric accuracy, together with a short exposure time. In the adopted 15 min. exposures performed with a R filter (centered at 660 nm and 110 nm wide), we have recorded in each pixel -1.2x10* photoelectrons from the nigth sky alone, and -9x10* photoelectrons from the GEMtNGA counterpart candidate. A constant bias was substracted to each frame. Also, 10 exposures were obtained frorr. the morning twiligth sky at 1/2 to 2/3 of the CCD saturation level, in order to flat field the CCD. Moreover, since the used chip exhibits a quite high dêxk current, even in a -15 min. exposure, we have performed several long darn exposures for a total of ~4 hours, from which we have scaled the dark current that is suitable for a 15 min. exposure. Figure 2 shows a CCD image of the field with the HRI error box, centered at o(lt,0)=6h 30m 59.15s, 6,x ,§0,=17°48'33.0*, and with a size of 7x7 arc sec. The candidate reported by Caraveo et ai. (1984a) is seen at the HRI box boundary. Absolute positions for this object are given in Halpern et at. (1984) and in Sol et al. (1984). The variability in the F magnitude was investigated as follow: a set of about 20 stars (19 in the first night, 23 in the second night) was selected as an intensity reference all over the repeated exposures. These stars have been chosen as well isolated to eliminate crowding errors in the intensity determination. Their magnitudes ranges from m =16.5 to 18.5. Moreover, 10 stars with magnitude comparable to that of the GEMINGA counterpart (m »19 to 21) were also selected to make an estimate of the standard deviation of the magnitude measurement of such faint objects. The validity of the extrapolation from bright to faint stars has been checked on stars in the open cluster NGC 2682. No systematic deviation from linearity could be found on a dynamic range extending over ~6 mag. Since the maxim' - magnitude difference between the calibration stars and the GËMINGA counterpart does not exceed ~5 mag, the brightness reference can be determined with a good precision (SO.01 mag) over the successive images, without adding any appreciable systematic effect. Thirteen 15 min. exposures were obtained during the first night. The seeing was extremely good (<0.8 arc sec.) such that the standard deviation of the magnitude measurement of faint stars turns to be laio.03 mag. Eighteen 15 min. exposures were performed during the second night, but with less attractive seeing conditions (Si.5 arc sec). Moreover, 4 frames were affected by a low level fringe pattern induced by the atmospheric 630 nm CI line. The dispersion appears to be larger (laio.05 mag), it even reaches 0.09 mag in the two frames with the largest fringing. Due to the small number of fringed frames, it was not possible to determine an accurate fringe pattern, so it was not attempted to correct the disturbed frames. An other way to determine the measurement error is to make estimate of the noise sources which are the night sky photon shot noise, plus a pattern noise due to incorrect flat fielding. Analysis of black sky zones showed that the i .a* -.-14-1 H- !_.._. If!

.12

ItCO 171* !•£• çeminga time ( min. )

.12

.M

> • .»* • • 1 H • .** r • • ' r ! t I

tût* £0* l*'.0 IV.* tb^i» i^'.*J U-»» star 17 time t min. )

Figure 3. Time profile of tne observations of tne two labelled objects. For each observation, tne magnitude deviation is plotted with respect to tne set of - 20 reference stars. total noise can be estimated to «150 photoelectrons per pixel, tnat is 50% more than tne photon snot noise alone. A star with m ~20 extends over ~15 pixels in the first night exposures, and over «30 pixels tne night after. In these conditions, the total noise in tne case of a GEMINGA like star turns to be 0.024 and 0.046 mag in the first and second night respectively. These figures are almost identical to those derived from the observed dispersions.

3.2. Results and discussions

Figure 3 snows tne time profile of the magnitude dispersion of the GEMINGA counterpart with respect to the references stars, together with that of a : I ! ! M -F-1—

* 49 à» 12» It» 20» 24* folding period : 160 min. time ( m in. )

Figure 4. Time profile of the GEMINGA counterpart observations, folded modulo 160 min. For each observation, the magnitude deviation is plotted with respect to the set of - 20 reference stars. nearby star (star 17, m asi9.8). Clearly, the GEMINGA optical candidate is seen to vary more than the adjacent star. A xJ test was performed against the assumption of constancy: it yielded a chance probability of £3x10-' for the case of the GBMINGA counterpart, and of 0.5 for the case of star 17. Inspection of Figure 3 [GEMINGA case) shows that the distance between minima and maxima could be suggestive of a periodic behaviour. The data were then folded modulo 160 min., a period value considered in Bignami, Caraveo, and Paul (1984). Figure 4 shows the result of this folding, which covers an elapsed time of -.1800 min., i.e. more than eleven 160 min. periods. This folding modulo 160 min. exhibits a clear pattern characterized by a rather sudden drop and a subsequent slower recovery. Obviously, more copious data are needed to confirm the above findings. We feel however, that the optical candidate proposed by Caraveo et ai. (1984b), remains the most promising one: in particular, the association, now firm, of the 7-ray and X-ray source with a compact object (or system) periodically variable, makes for a strong case in favour of a variable star-lilce optical counterpart. It should be also possible to perform most accurate fast optical periodicity searches, even before the advent of the Space Telescope.

We thank the entire CFH staff at Hawaii for their help throughout the observations. We thank Sandro Mereghetti, Christiane Berthonnet and Irène Fondeur for their assistance in processing the data. REFERENCES

Bignami, G.F., Caraveo, P.A. and Lamû, R.C. 1983, Ap. j. (Letters), 272, L9. Bignami, G.F., Caraveo, P.A. and Paul, J.A. 1984, Nature, 310, 464. Caraveo, P.A., Bignami, G.F., Giommi, P., Mereghetti, s. and Paul, J.A. 1984a, Nature, 310, 481. Caraveo, P.A., Bignami, G.F., vigroux, L. and Paul, J.A. 198413, Ap j. (Letters), 276, L45. Caraveo, P.A., Bignami, G.F., Vigroux, L., Paul, J.A. and Lamb, R.C. 1984c, Adv Space Res , 3, 77. Halpern, J.P., Grindlay, J.A., Bignami, G.F., Caraveo, P.A. 1984, IAU Circ. n° 3907. Sol, K., Tarenghi, M., Van de Riest, C., Vigroux, L. and Lelièvre, G. 1984, Astron. Astrophys., in press .