= An additional positional improvement. in X-rays with 8 00 uncertainty came from the ROSAT High

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1 A&A manuscript no. (will be inserted by hand later) Your thesaurus codes are: 06( ; ; ; ) ASTRONOMY AND ASTROPHYSICS A candidate optical and infrared counterpart for GRS J. Mart 1;2, I.F. Mirabel 1,P.-A. Duc 3, and L.F. Rodrguez 4 1 CEA/DSM/DAPNIA/Service d'astrophysique, Centre d' Etudes de Saclay, Gif/Yvette, France 2 Departament d'astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, E Barcelona, Spain 3 European Southern Observatory, Karl-Schwarzschild-Strae 2, D Garching bei Munchen, Germany 4 Instituto de Astronoma, UNAM, Apdo. Postal , Mexico D.F., Mexico Received 1996 October 9; Accepted 1996 December 16 Abstract. The accurate radio position of the hard X-ray transient GRS has been observed with dierent ESO telescopes in a search for optical and near infrared counterpart. A candidate counterpart has been detected in all lters observed within less than 1 00 of the radio position. No significant optical nor near infrared photometric variability is observed within 0.2 mag in a time scale of several weeks. Our spectral type and reddening analysis appears to be consistent with either a normal massive X-ray binary or a low mass X-ray binary with a giant companion, both at roughly the galactic center distance. The a priori probability ofachance superposition in a eld as crowded as that of GRS is estimated to be 14%. However, this decreases to less than 1% if the source belongs to any of the spectral types and luminosity classes proposed. Key words: X-rays: stars, Infrared: stars, Radio continuum: stars, Individual: GRS Introduction The hard X-ray transient GRS (l =0. 66, b =1. 17) was rst reported during a galactic center observation by the coded mask telescope SIGMA on board of the satellite GRANAT starting on 1996 March 18(Paul et al. 1996). The original 3 0 SIGMA position was later improved in softer X- rays to 1 0 thanks to the TTM experiment on board of the Mir-Kvant Space Station (Borozdin et al. 1996), which also provided an estimate of the hydrogen absorption column density of4:10: cm 2.From the hardness of the spectrum in these early observations, it was soon realized that the source could be a black hole X-ray nova. Radio observations with the VLA in a Target of Opportunity (TOO) context provided the detection of the GRS radio counterpart as a variable radio source just inside the TTM error box (Hjellming et al. 1996). The time scale of the radio variability appeared consistent with the synchrotron radiation usually observed in X-ray transients and X-ray binaries. The position of the VLA radio source could be measured with subarcsec accuracy yielding Send oprint requests to: J. Mart the following J2000 coordinates: =17 h 42 m 40 ṣ 030 ṣ 02 and = An additional positional improvement in X-rays with 8 00 uncertainty came from the ROSAT High Resolution Imager (Dennerl & Greiner 1996), in full agreement with the detected radio counterpart. Finally, the RXTE satellite also succeeded in observing and detecting the source at 2-12 kev (Takeshima et al. 1996). In this paper, we present in detail our optical and near infrared identication and follow up observations of the GRS counterpart that were reported earlier as an IAU circular (Mirabel et al. 1996) Observations After a precise radio position for GRS became available (Hjellming et al. 1996), we started a search for optical and infrared counterpart in deep images obtained with several European Southern Observatory (ESO) telescopes at La Silla (Chile) in the context of a TOO program. The rst observations were on 1996 April 1 and they were continued for an interval of several weeks, until 1996 May 9. Useful data could be collected for a total of ten nights, although with a rather irregular time coverage. The observations consisted basically of optical and near infrared imaging in order to identify the source and to carry out dierential photometry by comparison with several nearby stars in the same eld. The main telescopes and instrumentation involved were the 2.2 m telescope with IRAC2b, the 1.5 m telescope with DFOSC and the NTT with EMMI. All frames were reduced using standard optical and infrared CCD techniques based on the IRAF image processing system. 3. Astrometric results and discussion Accurate astrometry was initially carried out on a deep R band image obtained with the 1.5 m telescope with DFOSC on 1996 April 16th. This image is shown in Fig. 1. A set of 10 unsaturated reference stars present in the eld were taken from the Guide Star Catalogue (GSC) for the Hubble Space Telescope 1 Based on observations collected at the European Southern Observatory, La Silla, Chile

2 2 R (0.70 mu) position is again within acceptable astrometric error. We can also consider what is the probability that the radio transient is falsely identied with a random star in such a crowded eld. From the number of stars as bright as or brighter than the candidate present in the eld (0.045 stars arcsec 2 ), the probability of nding one inside a circle of 1 00 radius around the radio position is about 14%. We will see later that the proposed candidate is likely to be a middle G/early K giant star or a early/middle B main sequence star. From the star numbers in Allen (1973), we can estimate upper limits to the surface density on the celestial sphere for each of these classes of stars. As a result, the probability of a chance superposition inside a circle of 1 00 radius drops to less than 1% and 0.4% for the giant and main sequence case, respectively. These are signicantly lower values and we conclude from our astrometric analysis that the detected optical and near infrared objects are very likely to be the same source as the radio counterpart of GRS Consequently, in the following we assume that this coincidence is real. 4. Photometric results and discussion Fig. 1. Optical counterpart candidate for GRS as seen in a R band image obtained with the La Silla 1.5 m telescope with DFOSC on 1996 April 16th. The radio position is indicated by the central cross and the proposed optical counterpart is the star closest to it. North is at the top and East to the left. The size of the eld is The central cross gap is larger than the radio position accuracy to better show the optical source. (Ta et al. 1990; Russel et al. 1990), which is based on the standard epoch J2000. The XY coordinates in pixels for these stars were measured through a Gaussian t. The average residuals obtained in the astrometric solution were in right ascension and in declination. An optical counterpart candidate was soon identied in the eld at J2000 =17 h 42 m 40 ṣ 06 and J2000 = The osets of these optical coordinates with respect to the radio position reported by Hjellming et al. (1996) are cos = and = The coincidence of both the optical and radio counterpart is therefore well within the astrometric error. In this same R band image, we also measured the position of up to 5 fainter anonymous stars to be used as secondary reference stars for the near infrared images obtained with the 2.2 m telescope. These secondary stars were carefully selected among the brightest and most isolated objects, identiable in both optical and infrared, and surrounding the immediate vicinity of our GRS counterpart candidate. Some of these images are presented in Fig. 2 where we show the appearance of the GRS eld in the J, H and K bands. The residuals of this secondary t are usually very small, less than A near infrared counterpart candidate was also detected in all lters observed and for all dates. The measured osets relative to the radio position for the near infrared images in Fig. 2 are very similar to those in the optical image. For example, the J osets found were cos = and = Taking into account the total resulting error in the secondary t, the coincidence of this near infrared source with the radio Dierential photometry has been carried out on several days for the near infrared counterpart of GRS The lters used were J, H, K 0 and K but not all of them could be observed every night. Several comparison and check stars were chosen among anonymous nearby and well isolated objects in the same eld of GRS The results given here correspond to best behaved comparison and check star pair, that showed a fairly good stability of their respective magnitude dierences during the whole observing run. For J, K 0 and K, the position of the comparison star is =17 h 42 m 39 ṣ 06 and = , as given by the same astrometric solutions of previous section. Based on observations of the standards HD (Carter & Meadows 1995) and FS35 (Casali & Hawarden 1992) on a few dierent nights, the adopted magnitudes of the comparison star are J =12:5, K 0 =10:5 and K =10:4. Similarly, the check star used is located at =17 h 42 m 39 ṣ 83 and = , with magnitudes J =12:3, K 0 =10:3 and K =10:3. Unfortunately, these comparison and check stars were not within the eld of view of H band images obtained during the rst night because of uncorrect telescope pointing. Therefore another set of comparison and check star have been used for H band photometry during the consecutive rst and second observing nights, the only ones where this lter is available. The coordinates of the selected H band comparison star are = 17 h 42 m 42 ṣ 62 and = , while the H band check star is located at =17 h 42 m 39 ṣ 69 and = Their corresponding magnitudes are H =13:8 and H =12:6, respectively. The errors for all absolute infrared magnitudes quoted here are about 0.1 mag, while the magnitude differences between comparison and check pairs are found to be stable usually within less than 0.1 mag. The optical V, R and I observations could be obtained on different single nights only and no direct variability information is available for these lters. The corresponding zero points for VRI photometry, with about 0.2 mag accuracy, were derived by using other dierent standard stars, namely, MARK A in V band, SA in R band (Landolt 1992) and HD in I band (Cousins 1980).

3 3 J (1.25 mu) H (1.65 mu) K (2.2 mu) Fig. 2. Images of the GRS near infrared counterpart candidate obtained with the La Silla 2.2 m telescope using IRAC2b. The proposed counterpart appears as the closest object to the radio position indicated by the central cross with a gap. From left to right, the lters used correspond to the J, H and K bands. The J and K images were taken both on 1996 May 1 while the H image shown here is from 1996 April 2. The eld size and orientation are the same as those of the previous R band gure. Fig. 3. Dierential J, H, K 0 and K photometry of GRS as a function of time. The squares at the bottom of each panel represent the magnitude dierences between GRS and the comparison star. The circles at the top correspond to the magnitude dierences between the comparison and check star, that remain acceptably constant within 0.1 mag. Error bars not shown are smaller than the symbol size.

4 Search for variability The results of dierential photometry are presented in Fig. 3. It is clear from this gure that the GRS counterpart did not experience signicant photometric variability, at least in the near infrared. Here, the observed magnitude dierences remain consistent with a constant brightness source within the 2-3 error level of photometric measurements. Optical observations are available only for single separate dates at each lter observed. Therefore, we cannot rule out variability in the optical. However, the fact that the source is well detected in R and I observations separated by twoweeks suggests that, at least, the source was not fading quickly in the optical during our observations. A rst explanation for this apparent absence of variability may be the fact that our observations were started nearly two weeks past the maximum of the X-ray outburst, so that the source could have faded signicantly before we rst observed it. On the other hand, for normal X-ray transients the bulk of optical luminosity usually comes from re-processing of the X-rays in an accretion disk. As time passes and the accretion disk gradually fades away, so does the optical output of the system. Only for systems where the optical luminosity is not fully dominated by the accretion disk, namely massive X-ray binaries and low mass X-ray binaries with a giant companion, one should expect no signicant photometric variability correlated with the X-ray outburst. From the reddening and spectral type analysis as follows in the next subsection, this second alternative interpretation seems to be quite likely the case of GRS Reddening and spectral type The average near infrared magnitudes completed with those observed occasionally in the optical are listed in Table 1. If a good estimate of the hydrogen absorption column density towards GRS were available, this information would be useful to constraint the spectral type and luminosity class of the system. This is, of course, provided that the bulk of optical luminosity does not originates in a temporary accretion disk but comes from a real stellar companion instead, as the long term lack of variability in GRS would suggest. Borozdin et al. (1996) have reported a column density of N(H)= 4:1 0: cm 2 from a power law t to the 2-27 kev spectrum obtained with the TTM X-ray experiment. This is a rather large value, corresponding to a visual extinction in the range A V = magnitudes (Predehl & Schmitt 1995), and it would not be surprising if GRS actually lies in the galactic bulge. However, by using a standard interstellar extinction law (Rieke & Lebofsky 1985), the resulting de-reddened colors from the magnitudes in Table 1 are then hardly consistent with any known normal stellar type (Johnson 1966). In order to avoid this inconsistency, a column density with smaller value is clearly needed. We note here that Borozdin et al. (1996) point out that a disk blackbody plus a power law model t, with smaller column density, gives better results for one of their observation dates. The column density that they derive in this case is 2:30: cm 2 (Borozdin 1996) and the equivalent visual extinction would be then something between A V = magnitudes. Although this second more complicated model t has more physical sense, the authors point out that the sensitivity of the TTM instrument does not allow to place strong restrictions on parameter values and additional evidence should be required to prefer it in comparison with a simpler power law t. Table 1. Magnitudes of the GRS counterpart Filter Observation Date Telescope Magnitude JD V NTT + EMMI R m+DFOSC I NTT + EMMI J Average all dates 2.2 m + IRAC2b H Average all dates 2.2 m + IRAC2b K' Average all dates 2.2 m + IRAC2b K Average all dates 2.2 m + IRAC2b More recently, Greiner et al. (1996) have reported improved column density estimates for GRS resulting from ROSAT observations. By using dierent modeling techniques, their full possible range of N(H) values covers nearly the entire interval cm 2, with their best estimate being 2:0 0: cm 2, i.e., consistent with the Borozdin (1996) smaller value. However, since there are still signicant uncertainties in the N(H) determination, we have explored the dierent plausible values in the ROSAT range in order to see which ones provide the de-reddened colors and absolute magnitudes more in agreement with real stars. We nd in particular that this happens only for N(H) cm 2, where the de-reddened colors of GRS are consistent with a wide rainbow of stellar types. At this point, we can still use an additional piece of information. Since GRS is located in the celestial sphere (l =0. 66, b=+1. 17) quite close to the position of the galactic center, it appears reasonable to expect that its distance should be close to the 8.5 kpc value if the source is actually there. Under this assumption, a supergiant star of any spectral type is completely ruled out since it should be well outside the Galaxy to appear with apparent visual magnitude of 23.2, even for the highest allowed TTM column density value. On the contrary, an acceptable consistency of the de-reddened colors and absolute magnitudes with a 8.5 kpc distance is only found for two dierent combinations: a) N(H)=1:3 0: cm 2 and a giant stellar companion. This would imply a visual extinction of A V = magnitudes, with de-reddened colors V R = 0:8, V I = 1:0, V J =1:5, V K =1:8, R I =0:2, R J =0:7, R K =1:0, J K = 0:3, etc. The plausible errors for these gures are typically 0.3 mag. Given such uncertainties, from Johnson (1966) we nd that a giant star with spectral type from middle G to early K could be consistent with this result. This late spectral classication would suggest GRS to be a low mass X-ray binary having a giant companion. Only a few of such systems are known in the Galaxy, with Cygnus X-2 being a good representative of this class. b) N(H)=1:8 0: cm 2 and a main sequence star. The extinction in this case would be A V = magnitudes and the de-reddened colors V R = 0:1, V I = 0:4, V J = 0:5, V K = 0:7, R I = 0:5, R J = 0:6, R K = 0:8, J K = 0:2, etc., with similar uncertainties as before. From Johnson (1966), we nd here that a main sequence star with early/middle B spectral type could be ac-

5 5 ceptably consistent with them. This alternative early spectral classication would place GRS as a normal B type massive X-ray binary. The distances derived from the corresponding extinction, visual apparent magnitude and plausible absolute magnitude ranges are 8-11 kpc for the case of a giant companion and 6-10 kpc for the case of a main sequence companion. Considering the roughness of these estimates, both distance ranges include the 8.5 kpc galactic center distance value. Although it is dicult to further discriminate among them, only for the main sequence star option the required hydrogen column density overlaps well with the ROSAT N(H) best estimate by Greiner et al. (1996). Therefore, a massive X-ray binary interpretation for GRS should be tentatively preferred until additional more accurate soft X-ray, optical photometric and spectroscopic observations become available. In any case, the existence of possibilities a) and b) shows that it is perfectly possible that GRS may actually belong to the swarm of microquasars and related X-ray binary systems that are known to cluster in the central region of our Galaxy. 5. Conclusions After observing the arcsecond accurate radio position of the galactic center hard X-ray transient GRS , during a TOO campaign with dierent ESO telescopes at La Silla, the following results have been obtained: 1) An optical and near infrared counterpart candidate for GRS has been discovered in coincidence with the radio position within astrometrical error. 2) In spite of the uncertainties in the determination of the hydrogen column density towards GRS , we nd that the de-reddened colors and absolute magnitudes are in agreement with those of a luminous early/middle B type main sequence star or, alternatively, a middle G/early K giant star in both cases at a distance close to that of the galactic center. 3) The estimated probability of a false identication with a background/foreground star is, a priori, of the order of ten percent given the crowdness of the eld and this reduces to less than one percent for an object of any of the spectral types and luminosity classes proposed above. 4) No signicant photometric variability, larger than 0.2 mag, has been observed in the near infrared bands during the whole duration of a month long observing campaign. Possibly, the lack ofvariability also occured in the optical. We cannot rule out that this could be due just to the time delay of our observations since the X-ray outburst took place. References Allen C.W., 1973, Astrophysical Quantities, 3rd. Edn., The Athlone Press, London Borozdin K., 1996 (private communication) Borozdin K., Alexandrovich N., Sunyaev R., 1996, IAU Circ Carter B.S., Meadows V.S., 1995, MNRAS, 276, 734 Casali M.M., Hawarden T.G., 1992, JCMT-UKIRT Newsletter, 3, 33 Cousins A.W.J., 1980, MNASSA, 39, 22 Dennerl K., Greiner J., 1996, IAU Circ Greiner J., Dennerl K., Predehl P., 1996, A&A (in press) Hjellming R.M., Rupen M.P., Mart J., Mirabel I.F., Rodrguez L.F., 1996, IAU Circ Johnson H.L., 1966, ARA&A, 4, 193 Landolt A.U., 1992, AJ, 104, 340 Mirabel I.F., Mart J., Duc P.-A., Rodrguez L.F., 1996, IAU Circ Paul J., Bouchet L., Churazov E., Sunyaev R., 1996, 1996, IAU Circ Predehl P., Schmitt J.H.M.M., 1995, A&A, 293, 889 Rieke G.H., Lebofsky M.J., 1988, ApJ, 288, 618 Russell J.L., Lasker B.M., McLean B.J., Sturch C.R., Jenkner H., 1990, AJ, 99, 2059 Ta L.G., Lattanzi M.C., Bucciarelli B., Gilmozzi R., McLean B.J., et al., 1990, ApJ, 353, L45 Takeshima T., Cannizzo J.K., Corbert R., Marshall F.E., 1996, IAU Circ Acknowledgements. JM acknowledges nancial support from a postdoctoral fellowship of the Spanish Ministerio de Educacion y Ciencia as well as partial support from DGICYT (PB ) at the early beginning of this work. JM also thanks F. Comeron (ESO) for both his hospitality and support during the data reduction process. LFR acknowledges support from DGAPA, UNAM and CONACyT, Mexico. K. Borozdin (Space Research Institute, Russia) is also acknowledged for providing information prior to publication and S. Chaty (SAp, CE- Saclay) for careful reading and discussion. This research has made use of the Simbad database, operated at CDS, Strasbourg, France. The ESO astronomers that obtained most of the TOO observations are also acknowledged. This article was processed by the author using Springer-Verlag LaT E X A&A style le 1990.