Gamma-ray burst afterglows with VLBI: a sensitivity quest

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

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

3. 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

4. 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

5. Status of VLBI observations • Afterglows have been studied by VLBI in a few sources • GRB970508 & GRB980703 (det.), GRB980329 (no det., variability), GRB980425 (amb, southern dec) Taylor et al. (1998) • GRB030329 - a unique observing opportunity • 2nd nearest burst in the northern sky at z = 0.1685 (only GRB060218 at z = 0.0331 nearer) • Brightest afterglow detected to date (10mJy at 5GHz 30 days after burst) Greiner et al. (2003)

6. GRB030329 flux density • Flux density history (VLA and WSRT data) • Day 59-806 well described by power law: F t-1.23±0.03 • 7 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)

7. GRB030329 expansion • The HSA (GBT, Ef, Ar, WSRT, 25m MkII Jb) • 1Gb/s recording, 15Jy rms noise • Size@day 806 0.347±0.09 mas (twice as large as day 217) • Continued growth, and gradual decrease in apparent expansion speed

8. GRB030329 evolution fit • Data constrains external density profile (R>1018 cm) =Ar-k • Favor a uniform density medium (k=0) over a stellar wind (k=2) Pihlström et al. (2007)

9. 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)

10. 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)

11. Arecibo absorption experiments • On-source tint 30/50 min • NH<9x1020 cm-2 • NOH<1.4x1015 cm-2 (Icont~3.5mJy) Not particularly constraining! • Completely ionized? • Might be more beneficial looking for excited transitions of OH. HI OH Taylor et al. (2004)

12. Future absorption experiments • A good molecular tracer of GRB phenomenon could be the NH3 molecule: • fOH may be high both in a shocked region as well as in a PDR, but fNH3 small in PDRs. • A shock can increase fNH3 to 10-6-10-8 • 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

13. 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)

14. 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, GRB030329 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)

15. 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)