Fitting Formula for Flux Scintillation of Compact Radio Sources

We present detailed multifrequency, multiepoch radio observations of GRB140304A at z=5.283 from 1 to 86 GHz and from 0.45 to 89 days. The radio and millimeter data exhibit unusual multiple spectral components, which cannot be simply explained by standard forward and reverse shock scenarios. Through detailed multiwavelength analysis spanning radio to X-rays, we constrain the forward shock parameters to E k,iso ≈4., p≈2.6, and q jet ≈1°.1, yielding a beaming-corrected γ-ray and kinetic energy, g E ≈2.3×10 49 erg and E K ≈9.5×10 50 erg, respectively. We model the excess radio emission as due to a combination of a late-time reverse shock (RS) launched by a shell collision, which also produces a rebrightening in the X-rays at ≈0.26 days, and either a standard RS or diffractive interstellar scintillation (ISS). Under the standard RS interpretation, we invoke consistency arguments between the forward and reverse shocks to derive a deceleration time, t dec ≈100 s, the ejecta Lorentz factor, Γ(t dec )≈300, and a low RS magnetization, R B ≈0.6. Our observations highlight both the power of radio observations in capturing RS emission and thus constraining the properties of GRB ejecta and central engines and the challenge presented by ISS in conclusively identifying RS emission in GRB radio afterglows.

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“…toward this line of sight (Goodman & Narayan 2006). This is of the same order as the observing bandwidth, ≈1 GHz.…”

Section: Interstellar Scintillationmentioning

confidence: 55%

A VLA Study of High-redshift GRBs. II. The Complex Radio Afterglow of GRB 140304A: Shell Collisions and Two Reverse Shocks

Laskar

Berger

Margutti

et al. 2018

ApJ

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“…The shaded bands in Figure 6 show the expected strength of ISS in each of our radio epochs based on this simple picture, following Goodman & Narayan (2006) and including both diffractive and refractive contributions. Clearly, the standard approach cannot explain the full amplitude of the lowfrequency peak at 12 and 22 days, although some of the deviations from the RS models explored in Section 5.1 are likely explained by ISS.…”

Section: Interstellar Scintillationmentioning

confidence: 99%

A Reverse Shock and Unusual Radio Properties in GRB 160625B

Alexander

Laskar

Berger

et al. 2017

ApJ

133

164

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We present multi-wavelength observations and modeling of the exceptionally bright long γ-ray burst GRB 160625B. The optical and X-ray data are well fit by synchrotron emission from a collimated blastwave with an opening angle of 3 . 6 j q »  and kinetic energy of E 2 10»´erg, propagating into a low-density (n 5 10 5 »´-cm −3 ) medium with a uniform profile. The forward shock is sub-dominant in the radio band; instead, the radio emission is dominated by two additional components. The first component is consistent with emission from a reverse shock, indicating an initial Lorentz factor of 100 0  G and an ejecta magnetization of R 1 100 B » -. The second component exhibits peculiar spectral and temporal evolution and is most likely the result of scattering of the radio emission by the turbulent Milky Way interstellar medium (ISM). Such scattering is expected in any sufficiently compact extragalactic source and has been seen in GRBs before, but the large amplitude and long duration of the variability seen here are qualitatively more similar to extreme scattering events previously observed in quasars, rather than normal interstellar scintillation effects. High-cadence, broadband radio observations of future GRBs are needed to fully characterize such effects, which can sensitively probe the properties of the ISM and must be taken into account before variability intrinsic to the GRB can be interpreted correctly.

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“…At 5 GHz, the angular size of a source must be 135 µas for it to scintillate at amplitudes of m5 ≥ 0.02 on a timescale of 2 days. We derived this estimate based on the Goodman & Narayan (2006) fitting formula for ISS, assuming fiducial values of the scattering screen distance (500 pc), scattering screen velocity (30 km s −1 ), and transition frequency between weak and strong scattering (5 GHz). We also assume that the power spectrum of the electron density fluctuations in the ISM follow a power law consistent with Kolmogorov turbulence; this is observationally motivated (Armstrong et al 1995).…”

Section: Interpretation As Interdependence Between Iss and Intrinsic mentioning

confidence: 99%

“…We show curves for both cases in which m15 is derived from D(τ = 2d) and D(τ = 100d), the latter being more representative of the longer term 15 GHz OVRO measurements where D(τ ) has saturated. We calculated the structure functions using the Goodman & Narayan (2006) fitting function for ISS, which has the advantage of being also applicable to cases where the observing frequencies are comparable to ν0. In the left panel of Figure B1, m15 is larger than m5 in the lower left region of the model curves (corresponding to larger source sizes).…”

Section: Appendix B: Ruling Out Interpretation As Correlated 5 Ghz Anmentioning

confidence: 99%

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

Koay

Macquart

Jauncey

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We investigate the relationship between 5 GHz interstellar scintillation (ISS) and 15 GHz intrinsic variability of compact, radio-selected AGNs drawn from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory (OVRO) blazar monitoring program. We discover that the strongest scintillators at 5 GHz (modulation index, m 5 ≥ 0.02) all exhibit strong 15 GHz intrinsic variability (m 15 ≥ 0.1). This relationship can be attributed mainly to the mutual dependence of intrinsic variability and ISS amplitudes on radio core compactness at ∼ 100 µas scales, and to a lesser extent, on their mutual dependences on source flux density, arcsec-scale core dominance and redshift. However, not all sources displaying strong intrinsic variations show high amplitude scintillation, since ISS is also strongly dependent on Galactic line-of-sight scattering properties. This observed relationship between intrinsic variability and ISS highlights the importance of optimizing the observing frequency, cadence, timespan and sky coverage of future radio variability surveys, such that these two effects can be better distinguished to study the underlying physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud sources exhibit significantly higher 5 GHz ISS amplitudes than gamma-ray quiet sources. This relationship is weaker than the known correlation between gamma-ray loudness and the 15 GHz variability amplitudes, most likely due to jet opacity effects.

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Fitting Formula for Flux Scintillation of Compact Radio Sources

Cited by 32 publications

References 26 publications

A VLA Study of High-redshift GRBs. II. The Complex Radio Afterglow of GRB 140304A: Shell Collisions and Two Reverse Shocks

A VLA Study of High-redshift GRBs. II. The Complex Radio Afterglow of GRB 140304A: Shell Collisions and Two Reverse Shocks

A Reverse Shock and Unusual Radio Properties in GRB 160625B

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

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