Gamma Ray Bursts IV : Observational Techniques and some results Andrea Melandri INAF – Astronomical Observatory of Brera

Detect intense pulses of γ-rays Duration can be short or long Unpredictable (anytime and anywhere) Synchrotron emission Power-laws decays Observable at all wavelengths From early up to late times Many components (prompt + shocks + jet breaks + SNe) What we want to observe? Pulses GRB-SN Power-lawsLocationShort - Long

4 8 hour data gap 4 orders of magnitude Beppo-SAX needed at least 6-8 hours to perform an afterglow follow-up observation with its narrow field instruments. During this time, afterglow fades orders of magnitude. GRB-prehistory : the data gap

5 Burst Alert Telescope (BAT) – keV –FOV: 2 steradiants –Centroid accuracy: 1’ - 4’ X-Ray Telescope (XRT) – keV –FOV: 23.6’ x 23.6’ –Centroid accuracy: 5” UV/Optical Telescope (UVOT) –30 cm telescope –6 filters (170 nm – 600 nm) –FOV: 17’ x 17’ –24 th mag sensitivity (1000 sec) –Centroid accuracy: 0.5” BAT XRT Spacecraft UVOT BAT UVOT XRT Spacecraft Swift Mission Swift was designed to fill in the gap making very early observations of the afterglows, beginning approximately 1 minute after the burst.

6 BAT Burst Image T<10 sec  < 4’’ 1.Burst Alert Telescope triggers on GRB, calculates position on sky to < 4 arcmin 2.Spacecraft autonomously slews to GRB position in s 3.X-ray Telescope determines position to < 5 arcseconds 4.UV/Optical Telescope images field, transmits finding chart to ground BAT Error Circle XRT Image T<100 sec  < 5’’ T<300 sec  < 0.5’’ UVOT Image Observing Scenario

Now we need a plan …

GRB follow-up LT Over-ride & slew on GCN alert Optical imaging within 1-2 mins Identify OT automatically Subsequent observing strategy: – Optical imaging – IR imaging – Spectroscopy – Polarimetry Dependent on error box; LT or Swift XRT position used. Gomboc et al. 2003

LT-GRB Pipeline (Basics) Goal : to automatically & reliably detect GRB afterglow candidates in real time. Method 1 : New source detection in comparison with existing catalogues o Advantage: can identify possible OT from a single image, but it can not be used for identification of faint afterglows (R<20) due to catalogue in- completeness. Method 2 : Variable-source detection by image subtraction o Suitable for faint afterglows or rapid fades but sensitive to variable image properties. Method 3 : Comparison of source lists from several images o more sensitive than (1) & more robust than (2).

RAW image Automatic Reduction Automatic Astrometry Main Catalog: USNO B1.0 Images from GRB LT data set Automatic SEXtraction Conservative parameters Catalog vs. Extracted lists matching (RA, DEC, mag) Afterglow.....and other candidates Easy ? …

LT-GRB Pipeline (Problems) Trade-off between science needs (e.g. start with reddest filter) and software needs (e.g. avoid I filter) for first short exposures. Extraction parameters optimization and auto-adjustment in case of non-ideal observing conditions (e.g. seeing). CCD defects and fixed patterns. Extended objects and bright objects. Saturation, ghosts.. Catalog completeness and reliability Astrometric fit robustness and alternative solutions in case of failure Proper motion correction Single image, un-associated objects selection. Search radius, magnitude match, PSF check, distance to unmatched catalog objects... Ranked list for each image and search for false detections Image-to-image variability check and relation with expected afterglow behavior Automated intelligence non-trivial!

LT-TRAP Liverpool Telescope Transient Rapid Analysis Pipeline RINGO2 observations Decisions always comes down to some computational floating chart

… but when you do things properly …

GRB : the more distant T90 = 10.3 s f = 5.9 x erg cm -2 E iso ~ 9 x ergs z ~ 8.2 (record so far!)

GRB : the more distant TNG spectrum, Salvaterra et al. ‘09, Nature, 461, 1258

GRB : the more distant GROND, Tanvir et al. ‘09, Nature, 461, 1254

GRB : the more distant

GRB B : the brightest PI of the Sky PARITEL T90 ~ 57 s f ~ 6 x erg cm -2 E iso ~ 1.3 x ergs V mag ~ 5.3 (record!!) z = 0.937

GRB B : the brightest Racusin et al. 2008, Nature, 455, 183

GRB B : the brightest Racusin et al. 2008, Nature, 455, 183

GRB B : the brightest    5.3 = visual peak of GRB B  36.0 = faintest object observable with E-ELT (42m) = Sun 12.9 = 3C 273, brightest and first quasar 24.8 = faintest amateur pics, quasar CFHQS J = limit of stars observed by mean naked eye 7.8 = maximum brightness of Pluto 3.4 = M31 (Andromeda) Galaxy 3.0 = SN 1987A in LMC 0.0 = Vega, by definition the magnitudes zero point -7.5 = SN 1006, the brightest stellar event recorded

Do we really understand the nature of all the objects that we can observe?

……and what are those? GRB A ?? Swift Light Curves Rep., Evans et al. ‘07 T90 (main ~ peak at ~1870) > 1 hour !! X-rays bright but suddenly ended > 1000s Optical + UV very weird no certain host galaxy detection ! Optical Some objects collision? Minor object into a compact object? Something new?

……and what are those? GRB A ?? XRT + UVOT imageHSTChandra Only seen in the X-rays, very bright, very weird, still emitting in the X-rays, coincident with a faint galaxy (z=0.35), maybe visible in radio and slightly rising in the optical : tidal disruption of a star falling into a BH? Are we looking right inside the beamed jet? Credits clockwise: NASA/Swift/S.Immler; NASA/ESA/A.Fruchter; NASA/CXC/Warwick/A.Levan; NASA/Swift/Penn State/J.Kennea

Swift Mission achievements Filled the gap allowing fast and deep multi-wavelengths observations Helped to improve robotic telescopes technique (to follow up the triggers!) Behaviour of Prompt and Afterglow emission High accuracy for X-ray/Optical/UV observations (not only GRBs) Challenged the Fireball model (do we need a more complex model or a complete new simply model?) Full understanding of the Prompt and Afterglow emission? Open questions on : Dark GRBs, Optical flashes, Central engine, Short GRBs, polarization and jets, high energy behaviour, etc… Some very odd cases: maybe new class of objects?

"If you can't explain it simply, you don't understand it well enough”