Study of temporal evolution of emission spectrum in a steeply rising submillimeter burst

Abstract

In the paper the spectral temporal evolution of a steeply rising submillimeter (THz) burst occurred on 2003 November 2 was investigated in detail for the first time. Observations show that the flux density of the THz spectrum increased steeply with frequency above 200GHz. Their average rising rates reached a value of 235 sfu/GHz (corresponding spectral index α of 4.8) during the burst. The flux densities reached about 4,000 and 70,000 sfu at 212 and 405 GHz at maximum phase, respectively. The emissions at 405 GHz maintained continuous high level that they exceed largely the peak values of the microwave (MW) spectra during the main phase. Our studies suggest that only energetic electrons with a low-energy cutoff of ∼1 MeV and number density of ∼10 6 -10 8 cm −3 can produce such strong and steeply rising THz component via gyrosynchrotron (GS) radiation based on numerical simulations of burst spectra in the nonuniform magnetic field case. The electron number density N, derived from our numerical fits to the THz temporal evolution spectra, increased substantially from 8 × 10 6 to 4 × 10 8 cm −3 , i.e., N value increased 50 times during the rise phase. During the decay phase it decreased to 7 × 10 7 cm −3 , i.e., decreased about five times from the maximum phase. The total electron number decreased an order of magnitude from the maximum phase to the decay phase. Nevertheless the variation amplitude of N is only about one time in the MW emission source during this burst, and the total electron number did not decrease but increased by about 20% during the decay phase. Interestingly, we find that the THz source radius decreased by about 24% while the MW source one, on the contrary, increased by 28% during the decay phase.

Diagnostics from three rising submillimeter bursts

“…The emissions at 405 GHz maintained a continuous high level so that they largely exceeded the peak values of the MW spectra during the main phase. Our studies suggest that such a strong and steeply rising THz component can be produced by energetic electrons with a low-energy cutoff of 1 MeV via GS radiation in the case of a magnetic dipole field (Li et al 2016). The electron number density N , derived from our numerical fits, increased substantially from 8×10 6 to 4×10 8 cm −3 at the rise phase, i.e., the N value increased 50 times at the rise phase (see Table 3).…”

“…Observations show the flux density of the THz spectrum was steeply rising and the rising rate of the flux density of the THz spectrum attained a value as high as 342 sfu GHz −1 in the maximum phase. Its mean rising rate also reached a value of 235 sfu GHz −1 (corresponding to a spectral index α of 4.8) during the burst (Li et al 2016). The flux densities reached about 4000 and 70 000 sfu at 212 and 405 GHz at the maximum phase respectively.…”

“…It is found from the numerical fit that the two slowly rising and one steeply rising submillimeter spectral components can be produced, respectively, by energetic electrons with energy ranges of 30 keV-10 MeV and 1-10 MeV in a compact source (having a radius of about 0.5 ′′ ) with strong local magnetic fields varying from 4590 to 780 G via the GS emission. The photospheric magnetic field of 5000 G would be possible in an observation of a compact source (Li et al 2016). The associated microwave spectral components can be produced by energetic electrons with 10 keV-5 MeV and with a mean local magnetic field strength in an extended source with a radius of 25 ′′ -40 ′′ .…”

“…Their average rising rate only reaches 20 sfu GHz −1 (corresponding to a spectral index α of 1.6) for the two events. The rising rates of the steeply rising THz burst from the 2003 November 2 event were estimated (Li et al 2016). Its average rise rate could attain a value of 235 sfu GHz −1 (α=4.8) for the 2003 November 2 burst, which is about one order of magnitude higher than that for the two slowly rising THz bursts.…”

“…In this case we have to take a smaller source size of 0.38 ′′ , 0.35 ′′ , or even 0.25 ′′ to fit these spectra in the decay phase for the three THz bursts, i.e., the THz source radius decreased by 20%-50% in the decay phase. The effect of the emission source size on the GS emission spectrum in the THz range is given (Li et al 2016). On the contrary, we also found that the MW source size obtained from the spectral fit increased from 25 ′′ to 32 ′′ at the decay phase of the November 2 burst, i.e., the MW source radius increased by 28% .…”