NGRG Science
- Bruce Grossan of the University of California Space Sciences
Lab
Science Requiring Rapid-Response
- Emission Mechanisms
Synchrotron emission in the internal shock scenario is expected to have a spectrum Fν ∝ ν+1/3 (when electrons are in a slow cooling regime) or Fν ∝ ν–1/2 (for fast-cooling; Ghisellini, Celotti, & Lazzati, 2000; Piran, 1999; Mészáros, 2002) below the peak frequency. This mechanism would produce X/γ and OIR emission correlated in time, the one extrapolated from the other with a single spectral index. However, if unabsorbed prompt OIR emission were much brighter than the extrapolated low-energy γ-ray spectrum, then this would provide evidence that the γ-ray emission was produced by synchrotron-self Compton (SSC;Piran, Sari, & Zou, 2009). Combining OIR data with X/γ data during the burst, one can determine electron thermal Lorentz factor and magnetic field strength. This, in turn, would allow determination of the jet as magnetic- or baryon-dominated. If unextinguished emission in the OIR were fainter than ~ 2% of the 15 keV flux, this would indicate that the synchrotron self-absorption frequency is at least a few eV. This knowledge of absorption frequency has implications for the distance from the central engine where γ-rays are produced, and also on electron thermal Lorentz factor and magnetic field strength in the jet (Shen & Zhang, 2009).
The photospheric emission mechanism (Mészáros & Rees, 2000; Pe'er, Mészáros, & Rees, 2006), whereby γ-ray photons come from multiple inverse Compton scatterings within the Thompson photosphere of the jet, is of great interest because of relatively recent Fermi data and fitting (Ryde, 2004; Pe'er, Ryde, Wijers, Meszaros, & Rees, 2007; Ryde, et al., 2011; Veres, Zhang, & Mészáros, 2013). This mechanism would produce very faint OIR emission, as this would be the Rayleigh-Jeans tail of this thermal emission. Given faint OIR emission with a Rayleigh-Jeans spectrum, if dust extinction can be shown to be small, and if the emission is fainter than that expected for SSC, and the OIR emission were correlated with γ-band emission, this would argue for photospheric production of both spectral components.
If OIR emission is not correlated in time with X/γ emission, then separate mechanisms or locations must be invoked for the two spectral components. X/γ emission shows high variability at all time scales, down to ~ ms, and therefore is often ascribed to internal shocks where fast moving material from the central engine collides with slower material ejected at an earlier time (Rees & Meszaros, 1994). Bright, beamed OIR emission with uncorrelated variability could then come from a reverse shock synchrotron emission, and a t-1.5 decay would be predicted (Meszaros & Rees, 1993; Sari & Piran, 1997; Sari & Piran, 1999). Identification of such a mechanism would also require a baryon-dominated jet (a reverse shock traveling into a magnetic jet produces weak emission undetectable in OIR; Zhang & Kobayashi, 2005; Narayan, Kumar, & Tchekhovskoy, 416; Giannios, Mimica, & Aloy, 2008). Alternatively, OIR emission could come from interaction with the ISM, in which case a t–1 decay slope would be observed.
Finally, if OIR and X/γ emission are similarly variable, but uncorrelated, this would suggest either two separate jets with similar mechanisms, or an as yet unknown mechanism.
- An Independent Measure of Bulk Lorentz Factor
Measurement of the bulk Lorentz factor (BLF) in the GRB jet is an important diagnostic of jet conditions. The interaction of the jet and the ISM often produces an optical and X-ray afterglow peak; a simple, nearly model-independent argument applied early by Molinari et al. (2007) shows that the BLF can be measured from the time of this peak (but see also Nava, Sironi, Ghisellini, Celotti, & Ghirlanda, 2012). As pointed out above, a large fraction of optical light curves record only the afterglow decay phase, i.e. the optical response was too slow to catch the peak. Therefore, the available optical BLF distribution is incomplete, i.e. biased toward low BLFs. Measurement of a correlation (or not) with γ-ray measured BLFs (e.g. Ackermann, et al., 2013, Abdo, et al., 2009), would support (or not) a scenario with emission in the two different bands produced in the same jet. A separate optical BLF measurement would allow comparison of the optical and γ-ray BLF for the same GRB, a test rarely, if ever, made.
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Dynamic Dust Measurements In Individual, High-z Progenitor Systems
Measurement of the rapid evolution of the optical-IR slope provides rich information on processes and environment local to the GRB. GRBs are associated with massive stars, with high dust and gas columns. X-ray afterglow observations often show significant gas absorption columns (equivalent NH ~ 1022 cm-2; e.g. Galama and Wijers 2001; Stratta et al. 2004; Schady et al. 2007; Perley, et al., 2009), yet typical IR-UV observations show less extinction than predicted from these columns using typical Local Group dust-to-gas ratios (e.g. Prochaska et al. 2009). This discrepancy may result from the very-rapid destruction of the circumburst dust by a prompt optical-UV flash (e.g., Waxman and Draine 2000; Perna et al. 2003). If this process occurs, rapid early-time color and brightness evolution would be observed as the radiation “burns” its way through the dust, changing from extremely red to blue with the brightening of the optical emission. Direct detection of this process would open new avenues for studying the GRB environments and progenitors, separately from any host galaxy dust. This process gives perhaps the only tool to study dust in individual star systems independent of host dust, and it can be used to extraordinary red shifts. Most current observations respond too slowly to fully measure this phenomenon, as dust destruction should happen almost completely 60 s after the burst, again requiring rapid-response. Evidence for photodestruction of a modest amount of dust (AV~ 1 mag) has been presented (Morgan, et al., 2013), providing evidence that this process does occur, and can be observed. The modest amount of dust may be due to responding too late, and catching only the very end of the process.
Dr. Bruce Grossan
Mailing Address:
Lawrence Berkeley Lab 50R-5005
1 Cyclotron Road
Berkeley, CA 94720
Office Location for visits: 50-5005D
bruce [at] singu[dot]lbl[dot]gov [anti-spam format address]
![[PHONE]](../phone.xbm) 510.486.5489
Fax 510.486.7149
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Last Update: 2014 Feb 21 |