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HST Data Suggest Proper Motion for the Optical Counterpart of GRB

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HST Data Suggest Proper Motion for the Optical Counterpart of GRB A&A manuscript no. (will be inserted by hand later) ASTRONOMY AND Your thesaurus codes are: ASTROPHYSICS 13.07.1 21.10.2018 HST Data Suggest Proper Motion for the Optical Counterpart of GRB 970228. ♠ P. A. Caraveo1, R. P. Mignani2, M. Tavani1,3 and G. F. Bignami4,1 1 Istituto di Fisica Cosmica del CNR, Milano, Italy 2 Max-Plack-Institute f¨ur Extraterrestrische Physik, Garching, Germany 3 Columbia University, New York, USA 4 Agenzia Spaziale Italiana, Roma, Italy Received ....; Accepted .... ♠ Based on observations with the NASA/ESA Hubble (mV ≥ 23.6) ∼ 8 days later. van Paradijs et al. (1997) Space Telescope, proposed the optical transient to be the counterpart of GRB 970228. Subsequent deeper images showed a faint Abstract. After a quarter of a century of γ-ray burst extended object (mR ∼ 24) at the transient’s location (GRB) astronomy, the Italian-Dutch satellite BeppoSAX (Groot et al, 1997; Metzger et al, 1997a). If this is inter- on Feb 28th, 1997 detected a soft X-ray afterglow from preted as a galaxy (van Paradijs et al, 1997), the ”cos- GRB 970228 and positioned it accurately. This made pos- mological distance scale” for GRBs would be strongly fa- sible the successful detection of an optical transient. vored. However, deep HST images of the field (Sahu et Two public Hubble Space Telescope (HST) images of the al, 1997a,c), taken on March 26th with the WFPC2, fail GRB/optical transient region were taken on March 26th to clearly show the existence of a galaxy. At best, an and April 7th, 1997. They are analyzed here, with the pur- extended emission surrounding an off-centered point-like pose of understanding the nature of GRB 970228. We find source is visible in the V-band image, while the I-band im- that the position of the faint point-like object (mv ∼ 26) age is inconclusive, also owing to significantly lower signal- seen at the transient location changed by 0.40 ± 0.10 pix- to-noise. Indeed, it would now appear that, according to els in 12 days, corresponding to a proper motion of ∼ 550 ground-based measurements taken on April 2nd, the neb- mas/year. By comparison, four adjacent sources in the ulosity has faded away (Metzger et al, 1997b). This might same field do not show any significant displacement, with be taken for evidence of time variability of the extended astrometric residuals close to zero and average absolute emission, which would then not be cosmological at all. displacements less than 0.09 pixels. If confirmed, this re- Rather, it would be galactic, and the extended optical sult would strongly support the galactic nature of GRB emission might be tracing the remnant of the burst event. 970228. The WFPC2 point-like source would also be nearby. It could, for example, be a local neutron star, moving at arXiv:astro-ph/9707163v1 15 Jul 1997 typical pulsar speeds (Lyne & Lorimer, 1994), or a stellar object of different nature. A proper motion search is then justified since, as shown by Caraveo et al (1996) in the Key words: Gamma Ray Burst, optical, GRB970228 case of the parallactic displacement of the Geminga neu- tron star (d ∼ 160 pc and mV = 25.5), the HST Planetary 1. Introduction. Camera can measure relative displacements to better than GRB 970228 stands out in the history of γ ray bursts one tenth of a pixel even for faint objects. (GRBs). For the first time, an X-ray afterglow from a GRB was detected and positioned accurately thanks to the 2. Data reduction. Italian-Dutch satellite BeppoSAX (Costa et al, 1997a,b,c). th This triggered a number of optical searches in the error A second set of HST images was taken on April 7 , 1997 between 3:42-7:45 UT, and made public after few hours box. In particular, an mV = 21.3 object found inside the error box ∼ 20 hours after the event (van Paradijs (Sahu et al, 1997b). To achieve the highest angular res- et al, 1997; Guarnieri et al, 1997) could not be detected olution, the Planetary Camera (Field of View 35” x 35”, pixel size 0.0455 arcsec) was centered on the coordinates Send offprint requests to: P. Caraveo, [email protected] of the optical transient. The observing strategy was iden- 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 fig- ure 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.
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