HST Data Suggest Proper Motion for the Optical Counterpart of GRB

arXiv:astro-ph/9707163v1 15 Jul 1997 hstigrdanme fotclsace nteerror the in an searches particular, optical In of number box. a a triggered from 1997a,b,c). al, This afterglow et (Costa the X-ray BeppoSAX to satellite thanks an Italian-Dutch accurately positioned time, and detected first was GRB the For (GRBs). ta,19;Ganeie l 97 ol o edetected be not could 1997) al, et Guarnieri 1997; al, et box error the ena h rnin oaincagdb 0 by changed location transient the at seen edopitrqet to requests offprint Send of history the in out stands 970228 GRB Introduction. 1. words: Key Optical the Caraveo A. for P. 970228. Motion GRB of Proper Counterpart Suggest Data HST utwudsrnl upr h aatcntr fGRB of nature galactic re- the this support 970228. confirmed, strongly absolute If would average pixels. sult 0.09 and than zero less to with displacements the displacement, close significant in residuals any show sources astrometric not adjacent do of field four motion same comparison, proper a By to corresponding mas/year. days, 12 in els Abstract. Telescope, Space .... Accepted ....; Received 4 3 2 1 httepsto ftefitpitlk bet( object point-like faint the find of We 970228. position GRB the of that nature the understanding of pose R/pia rnin einwr ae nMrh26 March on 7 taken April the and were of region images (HST) transient Telescope GRB/optical transient. Space optical Hubble an public of Two pos- detection made successful This the accurately. sible it positioned and 970228 GRB nFb28 Feb BeppoSAX satellite on Italian-Dutch the astronomy, (GRB) 13.07.1 are: codes thesaurus later) Your hand by inserted be (will no. manuscript A&A gni pzaeIain,Rm,Italy Roma, Italiana, Spaziale Agenzia USA York, New University, Columbia Italy Milano, Gar CNR, Physik, für del Extraterrestrische Max-Plack-Institute Cosmica Fisica di Istituto ♠ ae nosrain ihteNS/S Hubble NASA/ESA the with observations on Based th th fe ure facnuyof century a of quarter a After 97dtce otXryatrlwfrom afterglow X-ray soft a detected 1997 , 97 hyaeaaye ee ihtepur- the with here, analyzed are They 1997. , am a us,otcl GRB970228 optical, Burst, Ray Gamma ∼ 1 .P Mignani P. R. , 0husatrteeet(a Paradijs (van event the after hours 20 .Crvo [email protected] Caraveo, P. : m V 21 = . 2 betfudinside found object 3 .Tavani M. , . 40 γ γ ± a bursts ray ryburst -ray m 0 1 . v , 0pix- 10 3 ∼ ∼ hn,Germany ching, n .F Bignami F. G. and 550 26) th ♠ utfidsne ssonb aae ta 19)i the neu- in Geminga (1996) the of al (d star displacement et tron parallactic Caraveo the by then of is shown case search as at motion since, stellar proper moving a A justified star, or nature. different 1994), neutron of Lorimer, local & Itobject (Lyne nearby. a speeds be pulsar be typical also example, would for optical event. source could, burst extended point-like the the of WFPC2 remnant all. and The the at tracing galactic, be cosmological might be emission be would extended not the it of then Rather, variability would time which of might emission, evidence This for 1997b). al, taken et be (Metzger away to faded 2 has according April ulosity that, on appear taken measurements now ground-based would it signal- lower Indeed, an significantly im- to-noise. to I-band best, owing the also while At inconclusive, image, is V-band galaxy. age the point-like in a visible off-centered is of an source existence surrounding emission the extended show clearly to oe.Hwvr epHTiae ftefil Sh et (Sahu 26 field March the on of taken ”cos- images 1997a,c), fa- the HST al, strongly 1997), be deep al, would inter- However, et GRBs is vored. for Paradijs this scale” (van If distance 1997a). galaxy mological al, a et as Metzger preted 1997; al, et (Groot eodsto S mgswstkno pi 7 April on taken was images HST of set second A reduction. Data 2. objects. faint for even pixel a of than tenth better to one displacements relative measure can Camera faint a of showed ( counterpart images object the deeper extended be Subsequent to 970228. transient GRB optical the proposed ( fteotcltaset h bevn taeywsiden- was strategy coordinates observing the The transient. on 35”, optical centered x the was 35” res- of arcsec) View angular 0.0455 of highest size (Field pixel the Camera hours achieve Planetary few To the after 1997b). olution, public al, made et and (Sahu UT, 3:42-7:45 between m V ≥ 23 . 6) ∼ ∼ 4 6 cand pc 160 , 1 asltr a aaise l (1997) al. et Paradijs van later. days 8 m R ∼ m 4 ttetasetslocation transient’s the at 24) V 25 = th . ihteWP2 fail WFPC2, the with ) h S Planetary HST the 5), ASTROPHYSICS ASTRONOMY 21.10.2018 AND nd h neb- the , th 1997 , 2 P.A. Caraveo et al.: HST Observations Suggest Proper Motion for GRB 970228

used in the computation of the centroid of the presumed counterpart of GRB970228. Although it is not the pur- pose of this paper to study in detail the diffuse emission discussed by Sahu et al.(1997c), close attention has been given to its possible influence on the GRB counterpart centering. A blow-up of the region in the March frame, where such a nebulosity is more apparent, is shown in figure 2a where green identifies backgroung pixels, averag- ing 8 counts, while yellow covers the range 8.1-8.4 counts (read noise ∼ 0.75 and gain ∼ 7.12). For comparison the peak source intensity is 14 counts/pixel. The contribution of the diffuse emission to the source counts is indeed neg- ligible. The faintness of the diffuse emission stems clearly from the two orthogonal tracings of Figures 2b(row) and 2c (column). The shape of the point-source is clearly un- affected by the very faint nebulosity. Therefore, we are confident that the GRB centering is not polluted by it. Table 1 lists magnitudes and Gaussian widths measured in the March and April observations for all our stars. While objects 2,3,4 and the GRB 970228 counterpart are point- Fig. 1. HST/WFPC2 4700 sec exposure of the field of GRB like with very similar width of the Gaussian profiles, ob- 970228 using filter F606W taken on March 26th, 1997. Numbers ject 1 is slightly extended. However, we are confident that 1 to 4 identify the field stars used for relative astrometry. The its centering can be accurately performed. We note that faint point source at the center of the field, labelled as GRB, the width values, measured for the point-like sources, in is the proposed counterpart of GRB 970228. April are larger than the March ones. This must be due to some systematic effect. However, since all the sources are affected in the same way, such a broadening does not tical to that used for the March observation, i.e. four hamper our astrometric comparison. In the presence of F606W (an extended V filter centered at λ = 5843A˚ with such a systematic difference between the star profiles in ∆λ = 1578A˚) images for a total exposure time of ∼ 4700 the two observations, the use of a variable Gaussian pro- s, and two F814W (roughly equivalent to I, centered at file for accurate star positioning is certainly superior to the λ = 8269A˚ with ∆λ = 1758A˚) images for a total time of standand PSF fitting. The centering errors of the reference ∼ 2400 s. First, the March and April data sets have been stars are between 0.02 and 0.07 pixels, while for the candi- cleaned for cosmic ray hits, using the IRAF/STSDAS task date counterpart of GRB 970228 the uncertainties are ∼ combine. Next, to overcome the limitation on star cen- 0.07 pixels. The values of all the centroids were corrected tering accuracy induced by the PC undersampling of the for the instrument geometrical distorsion, following the Point Spread Funtion, a 3x3 box smoothing is applied. detailed procedure outlined by the WFPC2 team (Holtz- Although based on a simple algorithm, this procedure is mann et al, 1995). very effective in restoring the astrometric potential of the PC data (see Caraveo et al, 1996). Figure 1 shows the Obj. mag FWHMx FWHMy March 26 V-band field image. The point-like source seen 1 24.3 ± 0.1 6.6 [6.2] 5.0 [5.2] on March 26th is well detected on April 7th. Our measured 2 22.9 ± 0.05 3.2 [4.7] 3.2 [4.1] ± ± V-band magnitude is 26.1 0.2 (26.0 0.3, according to 3 25.8 ± 0.2 3.0 [4.4] 2.8 [4.6] Sahu et al, 1997b), as opposed to the value of 25.7 ± 0.3 . ± . th 4 23 8 0 1 3.1 [4.3] 3.1 [4.3] measured on March 26 (Sahu et al, 1997a). The conser- GRB 25.7 − 26.1 ± 0.2 3.5 [4.6] 3.1 [4.2] vative errors leave room for significant fading. However, more data are required to define a clear trend. The shape Table 1. The table lists the magnitudes of the stars labelled of the diffuse emission is difficult to assess, due to higher in Fig.1. For the GRB counterpart the two values correspond background in the new observations as compared to the to the March and April observations, respectively. In columns previous ones. In the following, we concentrate on the V- 3 and 4 the FWHM components of the gaussian functions best band images (which show the best signal to noise) to ob- fitting the stars’ profiles are given (in pixels, 1pix=0.0455 arc- tain a very accurate relative astrometry of all point-like sec). Numbers in brackets refer to the April observation. objects present in both observations. The centroid of each star was computed in the MIDAS environment. A Gaus- sian function was used to fit the source data, yielding the The two observations cannot be immediately superim- best positions and associated errors. Particular care was posed because they have been obtained with slightly differ- P.A. Caraveo et al.: HST Observations Suggest Proper Motion for GRB 970228 3

was used in establishing the translation factors. Since nei- ther the net shifts nor their errors change when object #1 is excluded, all reference objects were used. To eval- uate globally the errors introduced by the procedure de- scribed above, we compare the coordinates of our stars in the two images. In the absence of signiﬁcant proper mo- tions, they should be the same, within errors, i.e., their residuals should cluster around zero.

3. Results. The results of the centroid cross-comparisons are shown in Figure 3 where, for each star, we plotted the April position relative to the March one, taken as zero point. While the positioning errors originate from the original star centering uncertainties (plus translation accuracy), the actual values of the displacements provide a measure of the pre- cision achieved in the image superposition. (a) Pixel displacements of stars 1,2,3,4 are: ∆α = −0.08 ± .09, 0.03 ± .08, 0.07 ± .10, −0.02 ± .04, ∆δ = −0.05 ± .09, 0.02 ± .08, 0.05 ± .10, −0.01 ± .04, yielding total April vs. March residuals < 0.1 pixels. This gives confidence in the overall correctness and accuracy of our procedure and provides a global estimate of the final uncertainty. The behaviour of the candidate counterpart of GRB 970228 is different: it shows an angular displacement of ∆ α = 0.30 ± 0.1 pixels and ∆ δ = −0.26 ± 0.1 pixels, for a total displacement of 0.40 ± 0.10 pixels equiv- (b) alent to 18±4.5 mas to the South-East. We note that star #3, of m606 = 25.8, does not show any displacement to within a global error budget of 0.1 pixel.

4. Discussion. In summary, although in need of conﬁrmation, our comparison of the two available HST images shows evidence that the point-like counterpart of GRB 970228 is moving. (c) Taking the result of our method at face value, the angular

th displacement found for the optical counterpart of GRB Fig. 2. (a) Blow-up of the F606W March 26 image centered 970228 is ∼ 550 (± 140) mas/yr for a constant speed, on the GRB 970228 counterpart (pixel size is 0.0455 arcsecs). and implies a transverse velocity-distance relation of the The square shape of the point source is an artifact due to the × color scale used to display the very faint nebulosity (b) Tracing type : v(km/sec)=2.7 d(pc). For v = c, a value of d of the above image along the row passing through the GRB ∼ 100 kpc is obtained. In this case, the optical emission maximum. X-axis: each tic corresponds to 5 PC pixels, Y-axis: seen by Hubble 36 days after the burst would be due to count/pixel (c) idem for column of maximum a relativistic plasmoid ejected from a compact source in a jet-like geometry. A transverse velocity of few hundreds km/sec, typical of radio pulsars (Lyne & Lorimer, 1994), ent roll angles and pointing directions. Thus, precise align- would correspond to a distance of ∼ 100 pc. ment of the two images is needed. This has been performed The WFPC2 has already measured comparable V-band following the procedure used by Caraveo et al (1996) to ﬂuxes from nearby neutron stars such as Geminga (Big- measure the parallactic displacements of Geminga. To ac- nami et al, 1996) and PSR0656+14 (Mignani et al, 1997), count for image rotation, we have rotated the source co- two middle-aged isolated neutron stars (INSs) with faint ordinates according to the known telescope roll angles, optical emission. However, we note that the colors of the ending up with the X-axis aligned with increasing right GRB candidate may not follow a simple Rayleigh-Jeans ascension and the Y -axis aligned with increasing declina- extrapolation of the spectrum, also owing to possible cir- tion. Thus, the statistical weight of the reference sources cumstellar nebulosity. Moreover, diﬀerent emission mech- 4 P.A. Caraveo et al.: HST Observations Suggest Proper Motion for GRB 970228

an isotropic and homogeneous spatial distribution. The relatively long duration of GRB 970228 (∼ 80 s) and its likely low value of the average spectral hardness (Costa et al, 1997) are consistent with being a burst belonging to the homogeneous population detected by BATSE (e.g. Kouveliotou et al, 1996). Alternatively, a relativistic plasmoid at d ∼ 100 kpc may be consistent with observations. Relatively rapid fading within a time scale of 1-2 months after the burst is expected for a relativistic plasmoid. A slower decay of emission from the point-like object may result from residual surface or disc emission of a compact object. More interpretative work is needed to clarify these issues. In any case, any proper motion of the GRB 970228 counterpart would prove its galactic nature. The reported fading of the surrounding optical nebulosity seems to support a galactic origin of the source. However, this fading has now been put in question (Fox et al, 1997). GRB 970228 may be representative of a large fraction, if not all, of GRB sources. In view of the importance of the topic and of the preliminary nature of our result, which stretches HST ca- pabilities to their limit, further observations are needed. They will both extend the time span for the proper motion measurement and gauge the source luminosity evolution. More X-ray afterglows from other GRBs may be detected in the near future by BeppoSAX, and rapid optical follow- up observations will further constrain the nature of GRB sources.

Fig. 3. April 7th vs March 26th, 1997 displacements of reference star centroids and of the GRB 970228 proposed counter- References part. The March 26th positions (identified with filled circles) have been used as zero point. Open circles identify the posi- Bignami G.F. et al, 1996, Ap.J. 456, L111 tions measured on April 7th. While the reference stars show no Briggs M.S. et al, 1996, in Proc. of the 3rd BATSE Symposium, angular displacements, the proposed counterpart of GRB does. AIP 384, 335, eds. C. Kouveliotou, et al. North to the top, East to the left. Axis units are in arcsec. Caraveo P.A. et al, 1996, Ap.J. 461, L91 Costa E. et al, 1997a, IAU Circ. 6572 Costa E. et al, 1997b, IAU Circ. 6576 anisms (see Tavani, 1997) should also be considered, pos- Costa E. et al, 1997c, Nature 387,783 sibly in the context of older and cooler isolated neutron Fishman G.J. & Meegan C.A., 1995, Ann. Rev. A&A. 33, 415 Fox D.W. et al., 1997 IAU Circ. 6643 stars, emitting optical radiation because of a burst-driven Groot P.J. et al, 1997, IAU Circ 6588 heat release or residual accretion. Guarnieri A. et al, 1997, submitted to A&A Constraints on a galactic population of compact objects Holtzman J. et al, 1995, P.A.S.P. 107, 156 producing GRBs can be deduced from the isotropic and Kouveliotou C. et al, 1996, in Proceedings of the 3rd BATSE apparently non-homogeneous distribution of GRBs ob- Symposium AIP 384, 42, eds. C. Kouveliotou et al. tained by BATSE (e.g. Fishman & Meegan, 1991). Argu- Lyne A.G. & Lorimer D.R., 1994, Nature 369, 127 ments against a local disk population of GRBs (Paczynski, Mao S. & Paczynski B., 1992, Ap.J. 389, L13 1991; Mao & Paczynski, 1992) are mostly based on the dif- Meegan C.A. et al, 1992, Nature, 355, 143 ficulty of reconciling the observed GRB isotropic and non- Metzger M.R. et al, 1997a, IAU Circ 6588 homogeneous distribution (Meegan et al, 1992; Briggs et Metzger M.R. et al, 1997b, IAU Circ 6631 al, 1996) with compact objects distributed as Population Mignani R.P., Caraveo P.A. & Bignami G.F. ,1997, The Mes- senger 87, 43 I stars. However, a recent analysis of BATSE data indi- Paczynski B., 1991, Acta Astron., 41, 157 cates that non-homogeneity may not apply to the whole Sahu K. et al, 1997a, IAU Circ 6606 sample of GRBs (e.g. Kouveliotou et al, 1996). Several Sahu K. et al, 1997b, IAU Circ 6619 classes of GRB sources with different spatial distributions Sahu K. et al, 1997c, Nature 387, 476 may exist. A fast moving counterpart of GRB 970228 can Tavani M., 1997, Ap.J. Lett. - in press (astro-ph/9703150) be ascribed to a local population of compact objects with van Paradijs J. et al, 1997, Nature 386, 686 P.A. Caraveo et al.: HST Observations Suggest Proper Motion for GRB 970228 5

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