Kuniyuki Asuma

THE CLOSELY POSITIONED THREE RADIO TRANSIENTS IN THE NASU 1.4 GHz WIDE-FIELD SURVEY

In 2004, we started the Nasu 1.4 GHz wide-field survey to study the dynamic universe by simultaneously performing the surveillance of the wide-field sky. We have detected six radio transients with flux densities larger than 1 Jy. In this paper, we report the detection of three new radio transients in the drift-scanning observations at δ ~ +417. Based on their positions in the sky and flux densities, we considered the candidate types of astronomical objects for transients. As a result, it is suggested that the three radio transients could be massive radio flares associated with known extragalactic sources, such as active galactic nuclei, which are usually very faint in X-ray and radio wavelengths. The three transients were found in the sky of 2° × 1°, while nine transients, including the three transients reported in this paper, are distributed at both high and low Galactic latitude. According to the observational results, we suggest that the wide-field survey simultaneously performed would be useful in searching for radio transients to study the dynamic radio sky.

Pulsar huge array with Nyquist-rate digital lens and prism

An 8 X 8 equals 64 element digital interferometer was developed at Waseda University (1989 - 1995). It was an equally spaced two dimensional array of 2.4 m dishes at 10.6 GHz, and a 2D FFT processor was used for Nyquist rate imaging. Recently a spatial-temporal (2 + 1)D FFT processor has been developed for pulsar survey (1996 - 2000), in which a first 2D spatial FFT transforms the electric fields of coordinate represented to those of momentum represented. We obtain the electric fields in each 64 direction at Nyquist rate by the 2D spatial Fourier transform. The following temporal 1D FFT gives the spectrum of 64ch or 256ch to 64 directions respectively also at Nyquist rate. The total bandwidths of each direction are 20 MHz and the resulting frequency resolutions are 20/64 MHz or 20/256 MHz. For sensitive pulsar survey, we also have developed an interferometric array of 20 m spherical dishes in Nasu flat, 160 km north of Tokyo. Five dishes are completed at present. Normal temperature receivers of high electron mobility transistors (HEMT) are used at 1.4 GHz observation and the receiver noise temperatures are below 40 K. Expected rms detection limit (Delta) T is about 10-2K under the condition of 20 MHz band widths and 1s integration time. Preliminary results of zenith survey at (delta) equals 40 deg using a single dish show (Delta) T equals 0.03 K without switching. Sub-reflectors and feed horns were designed so as to observe 5 deg off directions from zenith and they rotate around Az axis. It enable us to survey the declination zone of 32

The Automatic Radio Burst Search System at Nasu Observatory

In order to search for radio bursts in radio transient objects, we have developed an automatic radio burst search system. The system is being used at Nasu Pulsar Observatory of Waseda University in Jiyu‐Gakuen Nasu Farm, 160 km north of Tokyo. The drift‐scan fringe data from four different declinations are simultaneously obtained with an eight‐element interferometer. Using the automatic radio burst search system, some radio transients were detected. Here we discuss its fringe‐finding algorithms and the application in burst search observations. The application is similar to that of the blink comparator in optical telescopes to search for novae.

High and Low Galactic Latitude Radio Transients in the Nasu 1.4 GHz Wide-Field Survey

We report the detection of two radio transients in the Nasu 1.4 GHz wide-field survey. In the survey, we use four pairs of the two-element interferometer aligned east-west to monitor the wide-field sky and simultaneously survey the region at +32° < δ < +42° in drift scanning. In 27 days of continuous observation at a declination between +41° and +42°, we have detected two radio transients of 1 Jy intensity. Since they appeared on only a single day during the 27 days, we consider these detections to be 1 Jy class bursts that brightened and faded within 2 days and have constant emission smaller than 200 mJy, the detection limit of the 27 days of integrated data. While one transient was in low Galactic latitudes, the other transient was detected in high Galactic latitudes and has counterparts only in γ-ray databases. Therefore, the high Galactic latitude transient might be one of the active galactic nuclei that are normally very faint in X-ray and quiet in radio wavelengths.

Method for Finding Variable Radio Sources in Drift-Scan Interferometric Data from the Nasu Observatory

We have developed an analysis method for finding flux variabilities of the fringes in drift-scan interferometric data at 1.4 GHz obtained from the Nasu observatory. This method uses the expectation data of the Nasu observation generated from a radio sky survey catalog at 1.4 GHz, such as the NRAO VLA Sky Survey (NVSS) catalog. The variabilities are detected by comparing the power scales of the fringes in the observation data with those in the expectation data. A difference in both data fringe scales indicates the possible presence of variabilities. By using the technique described above, we analyzed the Nasu drift-scan survey results at δ = 32.5° ± 0.3°. Thus, we succeeded in finding the fringe variabilities in the data. This result indicates that our method is effective in detecting fringe variabilities in the Nasu interferometric data.

Calculation of False-Detection Rate for Nasu Interferometric Sky Survey

This article proposes a method for calculating the false-detection/alarm rate for the Nasu interferometric sky survey. This method considers both spectral intensity and time width of a fringe in the time-frequency domain. It detects periodic waves with a given time width in a noise band and calculates the distribution of the spectral intensity of the waves. Using this distribution, we can calculate the false-positive rate of fringe detection and determine the reliability of the detection results as an objective probability. The proposed method can be applied to interferometric observations such as the Nasu sky survey whose observation results depend entirely on the existence of fringes.

A design for digital signal processing in a large field radio patrol camera

An FFT processor is being developed for the present pilot system of the large field radio patrol camera. A design of the processor is discussed.

Rapid fringe detection technique for discovering radio transients in nasu drift-scan data

This article describes a rapid fringe detection technique for detecting fringes that appear transiently in Nasu observational data. It can quickly and accurately detect fringe waveforms that are not derived from steady sources in the drift-scan data according to constant numerical criteria. A fringe is considered to be a radio transient fringe if the signal-to-noise ratio of each data point is above a threshold for a specific period of time. Such fringes are searched for in the region unaffected by fringes due to steady sources in the observational data, which is determined by using the expectation data generated from an earlier survey catalog. Using the proposed technique, a computer can detect radio transient fringe using numerical criteria, thereby shortening the time required to discover transient signals. Therefore, by reducing the analysis time, we will be able to identify temporal flux variations in radio transients by follow-up observation immediately after detection.

Nasu 1.4 GHz Interferometer Transient Radio Source Survey and Improvement in Detection of Radio Sources

We have surveyed 1.4GHz transient radio sources in Nasu Pulsar Observatory. To investigate such sources, both immediacy and accuracy are severely maintained. We have developed Data Transfer System and improved antenna control system. Now we have received the fringe data from transient radio source candidates. To get reliable information, we carefully analyze with Fringe Band Pass Filter software and Fringe Fitting method.

EGRET Unidentified Source Radio Observations and Performance of Receiver Gain Calibration

Last year, we have developed the receiver gain calibration system by using Johnson-Nyquist noise, for accuracy flux measurement, because we have been starting radio identification program of transient radio sources, blazars and radio counterpart of The Energetic Gamma Ray Experiment Telescope (EGRET) unidentified γ-ray sources in Waseda Nasu Pulsar Observatory. It is shown that there are a few low correlation data between receiver gain and ambient temperature around receiver for anything troubles of receiver, because we can detect gain and ambient temperature through a day by developed system. Estimated fluctuations of daily data of steady sources decrease by removing low correlation data before analysing. As the result of our analysis by using above system, radio counterpart of EGRET identified source showed fading light-curve for a week.