Gamma-ray burst afterglows with VLBI: a sensitivity quest Ylva Pihlström University of New Mexico

Outline Science with Gamma-Ray Burst (GRB) afterglows Current capabilities and results (HSA) The future: –Continuum –Spectral line What we want more of

Impact of discovery of afterglows Afterglows have provided crucial info to test GRB models Measurement of redshift –Confirmation of cosmological distances –Distance allowed accurate calculations of energy (causality arguments indicate sizes <100 km, time scales ~ seconds) –Fireball shock model suggested (relativistic expanding debris shells) Identification of candidate host galaxies Fireball shock model illustration

Afterglows at radio wavelengths Flux density fluctuations (t ~ hours) attributed to interstellar scintillations => indirect size estimates –Rare (in the diffractive regime only for a few days) Direct measurements of size (as a function of time) can be provided by VLBI in nearby GRBs –Measure expansion and proper motion Size as a function of time allow specific tests of GRB models (jet-break, cannon-ball, external medium) VLBI can pinpoint locations within galaxy, and provide parallax measurements

Status of VLBI observations Afterglows have been studied by VLBI in a few sources –GRB & GRB (det.), GRB (no det., variability), GRB (amb, southern dec) Taylor et al. (1998) GRB a unique observing opportunity –2nd nearest burst in the northern sky at z = (only GRB at z = nearer) –Brightest afterglow detected to date (10mJy at 5GHz 30 days after burst) Greiner et al. (2003)

GRB flux density Flux density history (VLA and WSRT data) Day well described by power law: F  t -1.23± VLBI epochs, up to 806 days after the burst Berger et al. (2003), Frail et al. (2005), van der Horst et al. (2005), Pihlström et al. (2007)

GRB expansion The HSA (GBT, Ef, Ar, WSRT, 25m MkII Jb) 1Gb/s recording, 15  Jy rms noise ±0.09 mas (twice as large as day 217) Continued growth, and gradual decrease in apparent expansion speed

GRB evolution fit Data constrains external density profile (R>10 18 cm)  =Ar -k Favor a uniform density medium (k=0) over a stellar wind (k=2) Pihlström et al. (2007)

Proper motion limits Determines accurate position Implied limit on proper motion in the plane of the sky <0.38 mas (<1.08pc) Consistent with narrow, double-sided jet Rules out e.g. cannon- ball models Taylor et al. (2004, 2005), Pihlström et al. (2007)

In the future: the external medium Fireball models imply presence of external medium - unknown properties Density profile: current models suggest density governed by mass-loss of progenitor star (  r -2 ) A few observations seem to point to uniform medium (e.g. Berger et al 2003) to explain light curve Ionization likely to vary with time, causing time variable EW (optical) and relative fraction of atomic and molecular gas (radio)

Arecibo absorption experiments On-source t int 30/50 min N H <9x10 20 cm -2 N OH <1.4x10 15 cm -2 (I cont ~3.5mJy) Not particularly constraining! Completely ionized? –Might be more beneficial looking for excited transitions of OH. Taylor et al. (2004) HI OH

Future absorption experiments A good molecular tracer of GRB phenomenon could be the NH 3 molecule: –f OH may be high both in a shocked region as well as in a PDR, but f NH3 small in PDRs. –A shock can increase f NH3 to A detection would be a clear sign of gas shocked by GRB (and rule out foreground host galaxy gas) Simple estimates indicates possible detection for a few mJy bright continuum …bright radio continuum GRBs rare, unfortunately

Arecibo In our HSA observations, inclusion of Arecibo more or less doubled our sensitivity For spectral line experiments, the inclusion of a large collecting area might be crucial With a continuous frequency coverage, Arecibo can be used to trace redshifted molecular lines in absorption against afterglows (excited-states of OH)

Concluding remarks VLBI observations are needed for resolution Currently hampered by sensitivity –VLBA recording rate 0.5=>4Gb/s by end of 2009, for a similar experiment with VLBA ~3 times better sensitivity (2  Jy) –At that time, GRB will be ~70  Jy (gives ~5yr) Need improved sensitivities to: –Map appearance of counter-jet that (predicted by fireball models) –Constrain density and jet parameters –To be able to map more than one GRB afterglow! Want higher recording rate (at all VLBI antennas)

Low frequencies Cosmology: lower freq will be delayed wrt higher by an interval proportional to integrated column density of free electrons. This dispersion delay dominated by IGM at z~0.2. => information to discriminate between reionization histories? Solid lines: dispersion delay for the IGM, dotted curves local molecular clouds. Inoue (2004)