MAMBO-2 1.2 mm (250 GHz) observation of GRB031026

Frank Bertoldi  2.11.2003

Analysis of the prompt X-ray and gamma-ray emission of GRB 031026 (Butler et al; GCN 2429) has shown that this burst has a very hard spectrum. This is unusual for relatively faint, long GRBs like GRB 030126. Based on the spectral properties of its prompt high energy emission, Atteia et al. (GCN 2432) computed a high "pseudo-redshift" of 14 for GRB 031026 using the prescription of Atteia (2003) (see AA, 407, L1, for the definition of pseudo-redshift, or pseudo-z). This value is the highest one amongst the 40 HETE GRBs for which pseudo-z's have been calculated; only 2 others (of the 40 now) have pseudo-z's greater than 4: GRB 020305 with 5.9, and GRB 010612 at 9.5.

The afterglow of GRB 031026 was not located yet, neither in the optical nor radio. If it truly lies at a very high redshift, its optical afterglow could be undetectable as a result of absorption by the Lyman alpha forest, yet X-ray and/or IR observations might reveal a counterpart.

The unusual nature of GRB 031026 motivated us to try locate it with a MAMBO map, within our ToO program (Frail et al.) at the IRAM 30m. Butler et al. give a WXM localization expressed as a 90% confidence circle with a radius of 15 arcminutes, centered at:

WXM-Ground: R.A. = +03h 18m 42s, Dec. = +28d 21' 58" (J2000).

Centered at this position, we have mapped this region with MAMBO-2 on 2003 Oct 30 and Nov 1-2 taking three maps with mapping parameter (times are beginning of map):

set wobbler 42
map 1200 8 1200 31 /time 150 

             Date     Julian Date   UT    LST    Elev  tau_z Scan          min
1 GRB031026  30/10/03 2452943.4432  22:38 0.9981 59.57 0.32  20031030_5050 47
2 GRB031026  01/11/03 2452945.4165  21:59 0.4884 53.50 0.14  20031101_5112 105
3 GRB031026  02/11/03 2452945.6592  03:59 6.3283 51.28 0.19  20031102_5139 105

The first map was canceled after 47 minutes (19 subscans) due to strong wind. The first 2 maps are likely to have a pointing error of 3 arcsec (beam is 10.7") due to the strange structure of the pointing source we had taken, J0329+313, which appears to have extended structure leading to wrong pointings. The coadded data of all three maps (scale arcsec and mJy) is shown in the images below, where the upper images shows the data at 10.7 and 12 arcsec resolution, and the lower images show signal/noise maps smoothed to 12 and 14 arcsec. The rms noise level is 5 mJy per 10.7 arcsec beam (4.0 mJy per 12" beam in the smoothed map) in the center of the field, and it is nearly constant within a 10 arcmin radius and beyond that increases outward.

No source is detected at a 4 sigma level >16 mJy in the field. There are a number of ~4 sigma peaks in the map. None of them has an NVSS counterpart, and conversly, no NVSS source in the field shows significant mm emission. Given the size of the map, the appearance of a number of 3-4 sigma sources is consistent with noise.

Based on the MAMBO deep field surveys, we would expect to detect ca. one 1.2 mm background source with flux density >16 mJy in a field with radius 12 arcmin (number density ca. 0.002 arcmin^-2).

No further observations are planned.

MAMBO 1.2mm map at original resolution of 10.7 arcsec. Scale is arcsec and mJy. RMS noise level in central 10 arcmin is 5 mJy.

Same as above, smoothed to 12 arcsec. RMS noise level in the central 10 arcmin is 4 mJy.

Signal/Noise map at 12 arcsec resolution.

Signal/Noise map at 14 arcsec resolution.