Possible science cases of transient source searches in mm/submm:

gamma-ray burst orphan afterglows, and fast radio bursts

戸谷 友則 (TOTANI, Tomonori)

Department of Astronomy, Univ. of Tokyo

The 3rd workshop onLarge Aperture mm/submm Telescopes in the ALMA era

NAOJ, Mitaka, Tokyo 2015/Mar/10

Contents✦ orphan afterglows of gamma-ray bursts (GRBs)

✦ as a possible science case for a wide-field, large submm telescope

✦ some comments on fast radio bursts (FRBs)✦ related on the (still largely unexplored) science chance by wide field monitoring of radio sky

the Picture of (long) GRBs

GRB Jets and Orphan GRB Afterglows✦ Suggestions of jet/collimation of GRBs

✦ Too large isotropic energy > 3×1054 erg✦ Breaks in light curves of afterglows

✦ Prediction of the jet hypothesis: orphan afterglows without prompt GRBs✦ A test of GRB jet/collimation✦ no convincing detection so far

GRB Afterglows and theoretical modeling

✦ Panaitescu & Kumar (2001)“ultra-relativistic supernova remnant”electron synchrotron emission

parameters: Ejet, θjet, next, p, εe, εB

Break in light-curve: evidence for jet?✦ Achromatic break in optical afterglow light curve consistent with jet-like explosion

✦ Light-curve fitting suggests (4π/Ω)>103 for half of GBRs

(GRB 990510, Harrison et al. 1999)

Dynamics of Jets and afterglow emission (1)

✦ γ: bulk Lorentz factor of afterglow jet (decreasing with time)✦ typical initial value: 100-1000

✦ Stage 1: γ> 1/θjet

✦ only a small fraction of jet opening angle is visible for an observer

1/γ

θjet radiation

Dynamics of Jets and afterglow emission (1)✦ Stage 2: γ< 1/θjet

✦ gamma-rays can reach outside the jet opening angle✦ sideway expansion of jet material becomes significant

θjet

1/γ

θobs

Off-Axis Light Curves of GRB Afterglows: optical

✦ θobs = 1, 3, 5, 10, 20, 30°✦ scaled to z = 1 for all bursts

Totani & Panaitescu 2002

Off-Axis Light Curves of GRB Afterglows: X and radio

Detection Rate Prediction in optical & radio

on-axis

orphan

optical radio

Summary of the Predictions in 2002

Totani & Panaitescu 2002

Orphan afterglow search by a Large Aperture mm/submm Telescope?

✦ Possible merits:✦ no contamination of supernovae compared with a search in optical

✦ less contamination of AGNs compared with GHz-radio✦ GRB afterglows have harder spectrum than AGNs in radio

✦ GRB: α = 1/3 to 2 （Ｆν∝να）✦ AGN: α= -1 to 0

✦ effective sensitivity gain for GRB afterglows in submm than GHz

✦ not annoyed by interstellar scintillation ✦ A GHz-radio search for variable objects generally suffers from this

GRB Afterglow SEDs✦ expected break around 10 GHz

✦ upper ν: α=1/3 (synchrotron low frequency limit)

✦ lower ν: α=2 (synchrotron self absorption)✦ Fν∝να

Sari+’98

afterglow vs AGN radio spectra

GRB 970508, Galama+’97

Taylor+’98

Trippe+’10, AGNs

-α

typical GRB radio index:α~1/3 - 2 (fν∝να)

Past Radio Transients Survey at ~GHz✦ FIRST survey transient search (Thyagarajan+’11)

✦ 8444 deg2, down to 1 mJy @ 1.4 GHz✦ 1627 variable sources detected, more than half unidentified!

✦ MOST 22-yr transient object search (Bannister+’11)✦ 2776 deg2, down to 14 mJy @ 843 MHz✦ 57 highly variable and 15 transient sources✦ most of them suspected to be scintillating AGNs

Expected Event Rate in submm Wide Survey✦ ~10 deg2, 0.1 mJy (5σ) @ 2 mm by ~ month scale survey✦ LST info from Kohno-san

✦ Fν(150 GHz) / Fν(5 GHz) ~ 10✦ Fν(5 GHz) ~ 0.01 mJy

✦ Nexp ~ 30000 in all sky✦ Nexp ~ 7

✦ afterglow variability time scale: 100-1000 days

✦ strong orphan effect at Fν(150GHz) <~ 1 mJy ✦ Nexp/Non-axis ~ 50 at 0.1 mJy

✦ A deep and large survey by LST should be able to detect orphan afterglows!

LST FIRST

FRBs

Fast Radio Bursts: A New Transient Population at Cosmological Distances

✦ the first one event reported by Lorimer et al. (2007), confirmed by Thornton et al. (2013)✦ ~Jy level flux at GHz frequency ✦ intrinsic pulse width <~ 1 msec (observed width broadened by scattering)✦ dispersion measure indicates z ~ 0.5-1✦ ~ 104 /sky /day ~ 4×104 yr-1 Gpc-3 (z < 1, 〈z〉=0.75)✦ many proposed scenarios:

✦ SGR giant flares, delayed collapse of hyper massive NS, NS-NS mergers, WD-WD mergers, ...✦ a unique probe for ionized IGM (=missing baryon!)

Thoronton+’13

FRB search in submm/mm?✦ similar FoV of LST to Parkes

✦ a similar FRB detection rate if the sensitivity to msec burst is same?

✦ nothing known for SED of FRBs ✦ coherent emission✦ perhaps similar to pulsars?✦ still variety in radio spectrum

✦ short time delay by dispersion✦ difficult to measure DM?✦ difficult to discriminate from noise events?

✦ better angular resolution✦ ~15 min at Parkes, ~10 arcsec by LST✦ good to identify counterparts/host galaxies

some general thoughts... ✦ FRBs as a good lesson:

✦ discovery of z=0.5-1 sources first by a big radio telescope with a FoV of ~1 deg2

✦ in optical, this is similar to a search for z~1 supernovae by Subaru!✦ the bright, transient radio sky seems to be largely unexplored.

✦ Bohdan Paczynski’s words on optical astronomy ✦ "We do not know what is happening in the sky in real time, even at the bright end. This is a huge gap in astronomical research, which can be filled only with small, wide-angle instruments."

✦ "It's sort of embarrassing to the astronomical community that the brightest sky is so poorly mapped"✦ from "The Future of Small Telescopes in the New Millennium", ed. T.D. Oswalt

✦ Same can be said for radio, or even worse✦ discussion of large mm/submm telescopes after ALMA started to work!

✦ Many exciting new phenomena yet to be discovered by wide field radio surveys?