Temporal Evolution Of Spectra Research Articles

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

The temporal evolution of a spectrum during a steeply rising submillimeter (THz) burst that 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 200 GHz. Their average rising rates reached a value of 235 sfu GHz−1 (corresponding to 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 the maximum phase, respectively. The emissions at 405 GHz maintained such a continuous high level that they largely exceeded 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 ∼106–108 cm−3 can produce such a strong and steeply rising THz component via gyrosynchrotron radiation based on numerical simulations of burst spectra in the case of a nonuniform magnetic field. The electron number density N, derived from our numerical fits to the THz temporal evolution spectra, increased substantially from 8 × 106 to 4 × 108 cm−3, i.e., the N value increased 50 times during the rise phase. During the decay phase it decreased to 7 × 107 cm−3, i.e., it decreased by 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 in 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 radius, on the contrary, increased by 28% during the decay phase.

Spectroscopic Monitoring Observations of Nova V1724 Aql in 2012

Spectroscopic and photometric monitoring observations of nova Apl 2012 (V1724 Apl) were conducted at Koyama Astronomical Observatory, Fujii-Kurosaki Observatory and Bisei Astronomical Observatory. The nova was initially considered as an outbursting pre-main-sequence young stellar object. Our monitoring observations have revealed the nova to be a Fe II type classical nova. The temporal evolution of spectra and light curves of the nova were similar to those of a slow nova (e.g., V1280 Sco and V5558 Sgr). We observed no evidence of molecule formation in V1724 Aql in contrast with V2676 Oph in which dust formation occurred after the molecular formation in the nova outflow.

Variability Pattern and the Spectral Evolution of the BL Lacertae Object PKS 2155−304

The TeV blazar PKS 2155-304 was monitored with the X-ray satellite ASCA in 1994 May as part of a multiwavelength campaign from the radio to X-ray bands. At the beginning of the two-day continuous observation, we detected a large flare, in which the 2-10 keV flux changed by a factor of 2 on a timescale of 3 × 104 s. During the flare, the increase in the hard X-ray flux clearly preceded that observed in the soft X-rays, with the spectral evolution tracking a "clockwise loop" in the flux versus photon index plane. Ascribing the energy-dependent variability to differential synchrotron cooling of relativistic electrons, we estimate the magnetic field B in the emission region. We tested two different methods of comparing the time series in various X-ray bands: (1) fitting the light curves to a Gaussian function and searching for the time shift of the peak of the flare, and (2) calculating the discrete correlation function. Both methods yielded a consistent solution of B ~ 0.1 G. We also found that the flare amplitude becomes larger as the photon energy increases, while the duration of the flare stays roughly constant throughout the ASCA energy band (0.7-7.5 keV). In the framework of the time-dependent synchrotron self-Compton model in a homogeneous region, we consider a flare where the maximum Lorentz factor (γmax) of the injected electrons increases uniformly throughout the emission volume. The temporal evolution of spectra as well as the light curves were reproduced with the physical parameters self-consistently determined from seven observables. We obtained B ~ 0.1-0.2 G and a region size R ~ 10-2 pc for relativistic beaming with a Doppler factor of δ ~ 20-30. We discuss the significance of light-travel time effects.