T. Daishido

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.

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.

Quick-Look Data Analysis in the Nasu Radio Transient Search Project

We have developed a new technique, called quick-look data analysis, for radio transient searches at the Nasu Interferometer. Quick-look data, a color map according to the radio source signal-to-noise ratio (S/N), were generated spatially and temporally from drift-scanning data. A steady source appears at the same right ascension with the same color every day. In contrast, transient objects such as radio bursts appear irregularly in the sky, and the resulting map will have sporadic patches. Radio transient WJN J1443+3439 has already been detected using this technique. The quick-look data were created using processes that we call the fringe-detecting method and color image creation, which we describe here, and we show actual color images of Cyg A, 3C 48, Mrk 501, and the transient radio source WJN J1443+3439 found in 2005.

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.

Two 1.4 GHz Sky Atlases at Declination 32° to 42° in Nasu Radio-Transient Search Project

We present results of two 1.4 GHz radio surveys of a +32° < δ < +42° area with four interferometers in the Nasu Radio-Transient Search Project. The observable area was 7.0% of the whole sky. The surveys were carried out between 2004 and 2005. All observed interferometric data were transformed to color images to construct color atlases of the 1.4 GHz sky. From these atlases, 144 radio sources were detected by comparison with the 1.4 GHz NRAO VLA Sky Survey Catalog, excluding the Cygnus region. The observed data will be used in a transient search alert system.

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.