IR and Optical Observations of GRB from Campo Imperatore

1. IR and Optical Observations of GRB from Campo Imperatore R. Speziali, F. D'Alessio, L.A. Antonelli, A. Di Paola, L. Burderi, F. Fiore, G.L. Israel, D. Lorenzetti, F. Pedichini, L. Stella & F.Vitali Osservatorio Astronomico di Roma

2. Abstract In this poster we present the Infrared and Optical observations of GRB afterglows from the Campo Imperatore Station where two complementary instruments are available: the AZT-24 (1.1m Ritchey Cretien) equipped with the NIR camera SWIRCAM that is one of the few IR telescope working in the northern emisphere, and the Schmidt telescope (60/90/180) now renewed and equipped with a 2Kx2K back illuminated CCD. A description of the Station facilities, suitable for very fast reaction to GRB triggers, and the IR detection of the GRB 000926 are reported.

3. Introduction Comprehending the nature of the Gamma-Ray Bursts (GRBs) has been a long-standing problem of modern astrophysics since their discovery in the late of sixties (Klebesadel et al., 1973, ApJL, 182, L85). The BeppoSAX satellite made important steps forward thanks to its capability to locate the bursts with an unprecedented accuracy of 1-3 arcmin within few hours from the event. This led to the discovery of the X-rays, optical, IR and radio afterglows of GRBs. Optical spectroscopy yielded the first redshift measurements therefore proving the cosmological nature of the GRB phenomenon (Costa et al., 1997,Nat., 387, 783; Van Paradijs et al., 1997, Nat., 386, 686). Much progress in the study of GRBs has been achieved over the last three years from detailed multi-wavelength observations of the afterglows. Yet the origin and the physics of GRB phenomenon is still actively debated. The most widely discussed theoretical models for GRBs consider vastly different scenarios both in terms of the progenitors and environment. For example, if the GRBs originate from neutron star-neutron star or neutron star-black hole mergers (Meszaros & Rees, 1997, ApJ, 482, L29) then the explosion should be located very far from the place where the progenitor binary system was formed and most probably in a low-density interstellar medium. On the contrary if GRBs originate in hypernova (Paczynski 1998, ApJ, 494, L95) or supranova (Vietri & Stella, 1998, ApJ, 507, L45) events, being the immediate progenitor a very high mass rotating star, the explosion should take place in a high density medium, probably a star-forming region. So a positive detection of an optical and IR afterglow and its study can help us to constrain and better understand the physics underlaying these phenomena through their spectral energy distribution endevolution (Galama et al., 1998, ApJ, 500, L97).

4. Introduction In particular near-infrared observations are very important to ascertain whether the burst goes off in a dense medium of a star-forming region or even in the ejecta of the pre-supernova star. An important contribution in the infrared band might also be due to the deceleration of the expanding blastwave in a high density absorbing medium. The presence of an infrared afterglow and the lack of an optical one, as in the case of GRB990705 (Masetti et al., 2000, A&A,354,473), may suggest the existence of a high density circumburst medium. Another indication of the presence of a dense sorrounding medium can be seen in the earlyer steepening of the decay law as envisaged by Dai & Lu (1999) and as observed in the case of GRB990705 (Masetti et al., 2000), in the case of GRB990123 (Castro-Tirado et al.,1999, Sci.,283,2069), GRB990510 (Stanek et al.,1999, ApJL, 522, 39) and, GRB000315c (Jensen et al., 2000, astro-ph/0005609). The NIR band vs the optical one are fundamental importance in the study the GRB phenomenon to address the physics of GRBs and the environment in which they go off. The prompt triggers from BeppoSAX (1-3 hours) and the Wide Field Cameras error-circles (~ 3') provide at the moment the best opportunity for successful follow-up observations. Usually the positions are refined within few hours at 1-1.5 arcmin level by the BeppoSAX Narrow Field Instruments follow-up observations. In the near future (October 2000) the quality of the trigger will be significantly improved with the launch of HETE2. This satellite will detect to GRBs performing fast detection (few seconds) to an accuracy of 3 arcmin. More refined GRB positions (10''-2'' error-circle radius depending on the Burst intensity) will be obtained shortly after few minutes. HETE2 will provide the astronomical community with the GRBs coordinates through an automatic distribution system, so faster and more accurate follow-up observations will be possible with ground-based telescopes allowing to investigate the GRB afterglows.

5. The Schmidt Telescope + ROSI The Schimdt telescope, installed in 1958, is now placed under the second renewing phase. The old mechanics was completely overhauled and the same control system of the AZT-24 allow remote operations now (Di Paola et al, 2000, SPIE 4009, 317-326). The very fast optics (F# = 3) and the large FOV make this telescope a unique instrument among the 1m class telescopes, both for fast photometry and the search of optical GRB counterparts with large error boxes. ROSI (Speziali et al. 2000, SPIE, 4008, 389-395) is the new camera of the Schmidt telescope. Based on the 2K  2K thinned EEV chip cooled down to 180K, has a FOV of 55x55 arcmin. The high Q.E. of the array allow to reach mv>22 in a few minutes.

6. The AZT-24 telescope + SWIRCAM The 1.1 m AZT-24 (V. Abalakin et al., 1998, SAIT) is the new telescope of the station. Placed in the East dome was opened in the end of 1997 and has been fully operative for two years. Highly automatic it’s the only telescope in Italy dedicated to the near infrared observations. SWIRCAM (F. Vitali et al., 1997, OAR/IR5 - F. D’Alessio et al. 2000, SPIE 4008, 748-758) is based on a the 256x256 PICNIC array. Equipped with a set of standard NIR filters (J,H,K,K’) has a FOV of 4.5’x4.5’. The camera is equipped with two grism (R. Speziali, F. Vitali, 1997, OAR/IR6) that will also allow to work with a low resolution (R=300) spectroscopic mode.

7. GRB with the Schmidt The first detection of a GRB from a ground based telescope is represented by the observation of the famous optical afterglow of GRB 970228. The field was imaged 16 hours after the burst with the old camera (Pedichini et al., 1997, A&A 327, L36-38) and five hours before the well-known observation of this GRB (Van Paradijs et al., 1997). The OT in the first image has magnitude mV=17.0.

8. GRB with the AZT-24 This is the first detection of a GRB with the AZT-24. In the image, taken on September 28th, is clear the IR counterpart of the GRB 000926 (Gorosabel et al., 2000 , GCN 803). This image was obtained in the J band with a 30 min. exposure (A. Di Paola et al., 2000, GCN 816) measuring mJ=18.6 ± 0.3 with a S/N=5. The object is not present in the K band were it was only possible to determine an upper limit of mK17.0 with a 30 min. exposure.

9. The Alert machine GCN WEB page opt. F.C. USNO cat. S/W developed by GROAR Team. Some scripts are from DAMA project (http://argos.mporzio.astro.it) RA, DEC Error circle trig. time Observing strategy Alert CI mail,sms,fax Observable Yes

10. Future perspectives • HETE2 will observe about 35 bursts per year improving the possibility to observe both optical and NIR afterglow of GRBs with the telescopes of Campo Imperatore. • SWIFT (2003) will cover a wide band ranging from optical up to 300 keV. It will observe about 300 bursts per year so NIR observations with AZT-24 will be very important to have a wider band coverage.