Brief History of the BDA Project:

Summary:

A team of Brazilian scientists are co-coordinating efforts to develop the Brazilian Decimetric Array (BDA) which is a 38-element radio telescope employing modern radio interferometric techniques and working in the frequency range of 1.2-6.0 GHz. The final baseline of the interferometer will be 2.27 km in the East-West and 1.17 km in the South directions, respectively. This instrument will obtain radio images from the sun with a spatial resolution ~4x6 arc seconds. A prototype of the BDA interferometer (PBDA), consisting of five antennas having base lines up to 220 meters in the East-West direction, operating from 1.2- 1.7 GHz has been successfully put into operation for solar and non-solar observations at Cachoeira Paulista, CP- INPE (Latitude 45o 00' 20" West and Longitude 22o 41' 19" South) in the months of November - December 2004. The 6th antenna is currently being installed. Both, hardware and software were successfully tested for almost one year using observations of strong southern declination radio sources like Cygnus-A and the Crab Nebula. One dimensional brightness temperature maps of the sun at 1.6 GHz have also been obtained. In this report we give details of the procedures for amplitude and phase calibrations, estimated minimum flux detectable/beam, dependence of spatial resolutions on length of base lines, and a brief description of the hardware and its specifications. The estimated sensitivity of the PBDA consisting of 5 antennas of 4 m diameter each, is of about 3.5 Jy/beam for 1 minute of integration time for galactic and extragalactic observations at 1.4 GHz. In case of the Sun, the estimated sensitivities/beam for time resolution of 100 ms is around 140 SFU/beam (1 SFU = 10000 Jy). The BDA, when completed, will be open to the entire scientific community for use for both solar and non-solar observations and studies of space weather phenomena.

Scientists and engineers of INPE's Interplanetary Physics Group have, in the past, developed radio telescopes of international standards using available Brazilian technology. These telescopes are the Millimeter spectroscopes known as the Variable Frequency Millimeter Wave Radiometer, in operation since 1988 and the Digital Decimetric Brazilian Solar Spectroscope with a 9-meter polar mount antenna, in regular operation since 1990, at the INPE campus, The Brazilian Solar Spectroscope - BSS. The group has also initiated a Space Weather Prediction program using spectral tomography techniques in 1997. Realizing the importance of imaging spectroscopy to solar terrestrial relationships and forecasting of space weather to the Brazilian space science program, a team of scientists/engineers initiated the design and planning of the BDA project in 1996.

The BDA is the result of efforts of Brazilian scientists in collaboration with famous international astronomers, namely:

* Prof. Govind Swarup, National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research, India,
* Prof. Kiyoto Shibasaki, Nobeyama Radio Heliograph, Japan,
* Dr. K. R. Subramanian, Indian Institute of Astrophysics, India,
* Prof. W. J. Welch, University of Berkeley, USA, and
* Prof. D. E. Gary, from the New Jersey Institute of Technology, USA.

These efforts aim at constructing a bi-dimensional interferometer with the shape of a "T", dedicated for investigations on solar, galactic and extra-galactic astrophysics research. It will be a unique instrument in the Southern Hemisphere (Sawant et al., 2003). The collaboration also contributed to dramatically reduce the budget for development of the project.

The BDA, which is under development, will have high spatial and time resolutions of ~5 sec of arc (at 5.6 GHz) and 100 ms respectively. It is planned to employ modern technology at low cost to build the BDA. The BDA will have the capability of observing both solar and non-solar phenomena in the following protected radio bands viz. 1.2 - 1.7, 2.8 and 5.6 GHz. The sensitivity estimates show that the BDA will have an rms of 3 mJy at 21 cm for a system temperature of 50 K. The final version of the BDA will be an interferometric array consisting of 38 parabolic antennas of 4 meters diameter each with a compact "T" shaped array at the centre having 32 antennas. The BDA will provide solar radio images to be used in a spectral tomography technique being developed for application to space weather forecasting. Using extensive Interplanetary scintillation (IPS) observations from the 4 station solar wind observatory at Toyokawa, Japan coupled with a tomographic mapping technique, some work has already been carried out in studying long lasting (> 24 hours) low density anomalies observed at 1 AU. The analysis of such data will lead to a better understanding of the fundamental problems in solar physics and space weather. The BDA will also be very useful for galactic and extra-galactic investigations of the Southern hemisphere sky which is not accessible to VLA.

BDA - Phase-I:


The development of the phase I of the Brazilian Decimetric Array (Prototype - PBDA) started on December, 2001 (Sawant et al., 2000a, b). The main purpose of the development of this 5 element prototype was to optimize all the extensive engineering tests to be done on each sub-system (Sawant et al., 2002). This instrument was in operation at the campus of INPE in São José dos Campos (Sawant et al., 2005a; Cecatto et al., 2004). In 2004, the whole infra-structure for the site of the BDA at the campus of INPE in Cachoeira Paulista was set up and the array was transferred according to the original work plan. The 5 elements were installed along a maximum baseline of 216 m in the East-West direction. The PBDA was then put in operation, with the implementation of the digital correlator system and the software for calibration and production of one-dimensional maps. The first solar map was obtained by the instrument in December, 2004 (Sawant et al., 2005b). The phase I (PBDA) was concluded in 2006 (Sawant et al., 2006) and the instrument has capability for scientific production at present. As highlights, it is also worth mentioning the transfer of technology of various sub-systems to local industries, such as mechanical structure, tracking system, and PLO type receiver, the applications for receiving satellite signals and the formation of human resources in radio interferometry technique, specially the formation of 2 Ph. D. students, 1 M. Sc. student and undergraduate scholars.

There is a lack of interferometers in the Southern Hemisphere which are dedicated to solar observations in the decimetric wavelength range, and intended for investigations of the fundamental problems of Solar Physics, such as the release of energy in solar flares, the coronal heating, the triggering of CMEs, and effects on the space weather. Along with other radio heliographs, the development of the BDA will allow the realization of solar observations, on a continuous basis for 24 hours per day, as shown in Figure 1a.

Figure 1a: Locations of Radio Heliographs around the globe.

The high angular resolution of the BDA will contribute towards improving galactic and extra-galactic observations made with the Parkes radio telescope, thereby contributing towards improving the observations of galactic and extra-galactic radio sources in the Southern Hemisphere. Within a one decade time frame, interferometers like SKA, ALMA and LOFAR will start operating, and so, with the BDA, the Brazilian researchers will be trained to use these telescopes that will be the largest in the world. Dominating the interferometry technology is very important for the Brazilian space program.

BDA Phase - II

The roadmap for the development of the BDA is shown in Figure 1b.

.. Figure 1b: Participating institutes and funding agencies in the development of the BDA.

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Thirty Month Timetable:

Table-I: Thirty month timetable for the implimentation of Phsae-II of the BDA >>Click here<<

Science with the BDA:

Solar Physics:

It is well known that the emission at decimeter wavelengths takes place in the solar corona at the heights were soft X-rays and energetic particles are accelerated or heated during solar flares (Moore et al., 1980; Barta and Karlicky, 2005). The comparison between emission at the decimetric radio range and other wavelength ranges are topics of interest at present in Solar Physics research (Bastian et al., 1998a). The possible investigations based on solar observations in radio and X-rays are presented in detail in recently published reviews (Bastian et al., 1998b and Ramaty and Mandzhavidze, 2000).

There are few investigations about the regions where the decimetric radio bursts take place, such as some observations done with the Very Large Array (VLA) (Gopalswamy et al., 1995). To fulfill this gap, efforts are being made at INPE to build the Brazilian Decimetric Array (BDA) (Sawant et al., 2000a,b, 2002), a radio heliograph that operates in the frequency range 1-6 GHz, and will supply the lack of proper instrumentation for more detailed solar studies. At present, there is no any radio interferometer dedicated for solar observations operating in this frequency range. Shown in Figure 2 is a comparison of the angular resolutions of the radio heliographs presently operating, showing that the angular resolution of the BDA is comparable to that of the VLA-C configuration.

Figure 2: A comparison of the angular resolutions of the radio heliographs presently operating.

The BDA is planned to complement the observations made at the Northern Hemisphere by the radio heliographs at Nobeyama, operating at 17 and 35 GHz (Nishio et al., 1995), Nancay operating at 160, 327 and 408 MHz, and Gauribidanur, operating at the frequency range 40-150 MHz (Subramanian et al., 1994). The combined investigations will allow the study of solar phenomena at different heights at the solar atmosphere. The BDA will be part of a large world network to continuously monitor the solar emission at radio frequencies, allowing studies of the evolution of active regions. These studies will lead to the forecast of eruptive phenomena such as solar flares and Coronal Mass Ejections (CME) (Robbrecht and Berghmans, 2005) that are the main causes of geomagnetic perturbations on Earth. At the present space era, it is important to know the release of energetic particles location in order to prevent satellites, spacecrafts, airplanes, etc, from the damage caused by radiation, in addition to predicting the effects at the near-Earth environment such as geomagnetic storms, and blackouts at radio communications and electricity networks. Efforts are being made at INPE, in collaboration with UFSCar to develop a parallel architecture machine for the BDA data acquisition. Software for applying spectral tomography techniques to model the forecast of eruptive phenomena (solar flares and CMEs) is under development at the LAC/INPE, aiming at the prediction of space weather (Rosa et al., 1999; Mucheroni et al., 2000; Moron et al., 2000). The technique of pattern recognition is based on the use of simultaneous images at several wavelengths, obtained with high time- and angular- resolutions (Rosa et al., 1997). It is intended to apply spectral tomography to study active regions using high angular resolution X-ray images from the RHESSI and Solar-B (Kosugi, 2006) satellites, and UV images from the SoHO satellite (Gabriel, 1997). We plan to use the radio images, obtained in real time at various frequencies with the BDA operating in the frequency range 1-6 GHz, to perform spectral tomography of solar active regions and for real time space weather forecast. There is particular interest in the detailed investigations of the dynamics of Coronal Holes observed in X-rays and related to the dynamics of active regions observed in radio. Such a study will help understanding the complex events known as CHARCS (Coronal Holes, Active Regions, Current Sheets) (Gonzalez et al., 1996; Srivastava et al., 1998; Balasubramanian, et al., (2003); Janardhan et al., (2005), associating Coronal Hole and Active Region activity with the occurrence of geomagnetic storms.

The following fundamental questions of Solar Physics will be investigated with the BDA:

1. Transient Energetic Phenomena:

* energy release;
* electron acceleration and plasma heating;
* particle transport; and,
* creation and instability of large-scale structures.

2. Nature of Coronal Magnetic Fields:

* estimate of coronal magnetic fields;
* time-evolution of coronal magnetic fields.

3. Solar Atmosphere:

* coronal heating;
* structure of the quiet solar atmosphere.
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Galactic and extra-galactic investigations:

The operation of the BDA will allow surveys to be conducted at the Southern Hemisphere at 1.4, 2.8 and 5.6 GHz, complementing other surveys made at the Northern and Southern Hemispheres. The telescope can also be used to study time-variability of quasars, radio galaxies and to study the morphological structure of extended radio sources. There are few research centers in the world that carry out research in quasar variability, active galactic nuclei and BL Lacertae objects in the radio range of the electromagnetic spectrum. The Brazilian group dedicated to study variability since 1979 uses instrumentation operating at the frequencies 22 and 43 GHz at the Itapetinga radio telescope, in Atibaia. There are also few researchers who have been studying the variability at high frequencies (22-230 GHz) at the Southern Hemisphere. It is remarked that using the BDA it is possible to detect low flux density radio sources (<60 mJy), better than the sensitivity of other studies being conducted at present. It is possible to use the BDA to study the variability of quasars, active galaxies and BL Lacertae objects at 1.4, 2.8 and 5.6 GHz to test the existing models by comparing the data obtained with those at higher frequencies. The selected radio sources will be extracted from the SEST survey (Tornikoski et al.,1993; Tornikoski et al., 1996) and from the catalog by Kuhr et al. (1981).

Molecular Lines:

Using the BDA with a spectral resolution of 8 KHz and angular resolution better or equal to 3 arc minutes, it is intended to observe the neutral Hydrogen emission (=21 cm) from various dense molecular clouds at the Southern Hemisphere as well as high speed clouds. Given the high angular resolution of the BDA, observations of HI and OH in the direction of Corona Australis, Chamaeleon and of the cometary globules at the Vela region might provide important information about those objects. The emission peaks of the transition J=1-0 of the C16O observed in the direction of these clouds (Vilas-Boas et al. 1994; 2000) are proper positions to be observed through the transition 1(1,0)-1(1,1) of H2CO, that demands high excitation densities. In the same sense as in the dark clouds, it will also be possible to study the dense condensations of gigantic molecular clouds through these transitions. The positions in the direction of these regions may be selected from the H2CO surveys conducted at Parkes (Whiteoak and Gardner, 1974) with angular resolution of the order of 15 arc minutes. The high speed clouds will be selected from the HI surveys at the Southern Hemisphere (Bajaja et al., 1985; Putman and Gibson, 1999). These clouds will be mapped up to the limit of 60 mJy. Only those presenting very intense HI emission peaks will be mapped with longer integration times. Surveys of the H2CO line at 4830 MHz will be conducted in the direction of these peaks, in order to check if these Hydrogen clouds also have associated condensations with densities higher than 104 cm-3. Using this instrument it is also intended to search for CH lines (Rydbeck et al, 1976) between 3.2 and 3.4 GHz, in the direction of many of these clouds. A series of other surveys may be done through the OH lines, specially in the direction of stars and proto-stellar objects. The BDA will also be used to search for neutral hydrogen emission in the direction of extra-galactic sources such as star-burst type galaxies, quasars and to map the sources of HI observed at Parkes with a flux density higher than 60 mJy.

Cosmic Background Radiation:

The experimental cosmology group at INPE has already performed observations of the galactic synchrotron emission with the GEM (Galaxy Emission Mapping) telescope at 5 frequencies (408 MHz, 1.465, 2.3, 5 and 10 GHz) with low angular resolution (of the order of a few degrees) and covering a wide region in the sky. Our proposal is to use the BDA to study in detail some specific regions which were observed by GEM. The BDA observatory is an optimal complement to the studies carried out by GEM, since both are located at the same site and observe at very near frequencies and the BDA may be used for interferometric observations of target sources detected by GEM. For example, Watson et al. (2005) present evidence for anomalous emission in the direction of the Perseus molecular cloud, showing a crescent spectrum peaking at 17 GHz (flux density ~42 + 4 Jy at 22 GHz). Such a flux density at this frequency is roughly an order of magnitude higher than what could be explained in terms of the normal emission processes. Considering the already published spectral indexes for this anomalous emission, studies about this component may be easily carried out with the BDA at 1.4 GHz. As most of the results are obtained from observations performed at the Northern Hemisphere, the combination of GEM and BDA measurements may be an interesting complementary research field to study the anomalous galaxy emission using Southern Hemisphere targets.

Selected Publications:

Some selected publications are listed below. A more exhaustive list of publications can be accessed here or via the link in the left hand column at the top of this page.

1. Brazilian Decimetric Array.
Sawant, H. S., Subramanian, K. R., Ladke, E., Sobral, J. H. A., Swarup, G., Fernandes, F. C. R., Rosa, R. R., Cecatto, J. R. Advances in Space Research, 25, No.9, 1809-1812, 2000c.

2. A low cost steerable radio-telescope.
Sawant, H. S., Neri, J. A. C. F., Fernandes, F. C. R., Cecatto, J. R., Sankararaman, M. R., Faria, C., Stephany, S., Rosa, R. R., Andrade, M. C., Alonso, E. M. B., Ladke, E., Subramanian, K. R., Ramesh, R., Sundararajan, M. S., Ananthakrishnan, S., Swarup, G., Boas, J. W. V., Botti, L. C. L., Moron, C. E., Saito, J. H. Advances in Space Research, 32, No. 12, 2715-2720, 2003.

3. Brazilian Decimetric Array (Phase-1): Initial Solar Observations.
Ramesh, R., Sawant, H. S., Cecatto, J. R., Faria, C., Fernandes, F. C. R., Kathitavan, C., and Suryanarayana, G.S. Advances in Space Research, 39, 1453-1455, 2007.

4. Prototype of the Brazilian Decimetric Array.
Sawant, H.S. R. Ramesh, J.R. Cecatto, C. Faria, F.C.R. Fernandes, R.R. Rosa, M.C. Andrade, F.R.H. Madsen, S. Stephany, L.B.T. Cividanes, C.A.I. Miranda, L.C.L. Botti, J. W.S.V. Boas, J.H. Saito, C.E. Moron, N.D. Mascarenhas, K.R. Subramanian, M.S. Sundararajan, E. Ebenezer, M.R. Sankararaman. Solar Physics, 242, 213-220, 2007.