LOFAR Science (Single Station)

Single Station Science Projects of the Low Frequency Array

While the LOFAR array routinely operates as a single telescope (the ILT) with all stations connected to the central processing facility, there are times when the long baselines of the international stations are not needed for ILT observations. At these times the international stations can be used as single stations, detached from ILT control. In Germany this is operated as the GLOW mode, which allows observations with any German station from anywhere within the German LOFAR network.

In principle the GLOW mode allows to use the German stations as a small long-baseline array. Using this is planned for some future projects, but current projects use the German stations as single stations.

Single stations have a very poor spatial resolution, so science is constrained to fields where spatial resolution is not critical. Current research projects pursued with German stations are all-sky imaging (MPIfR Bonn) and pulsar monitoring (Bielefeld University).

 

All-Sky Imaging

Using the Effelsberg LOFAR station, wide-field observations of the full accessible sky were made. From the 330 hours of observations performed over a 9-month period in 2013 maps of the northern sky at several frequencies were created. Two examples of the resulting brightness temperature maps are shown below. The map created from observations with the Low-Band antennae is centered on the frequency 45 MHz. The corresponding map based on observations with the High-Band antennae is centered on 130 MHz. The bandwidth used for the 45 MHz map is 1 MHz and for the 130 MHz map is 10 MHz. Both maps are similar in appearance with the brightest radio regions along the equator at b=0o latitude. This is  the plane of the Milky Way. The center of the Milky Way is at longitude 360o and latitude b=0o. The two bright sources left from the radio bright Galactic Center Region are the supernova remnant Cas A (l=111o), and the radio galaxy Cyg A (l=76o).  On the 130 MHz map the supernova remnant Tau A and the radio galaxies Her A and Vir A stand out as faint point sources. The plume extending from the galactic plane at l=360o northward is called North Polar Spur, and is also a supernova remnant. 

 

45GHz all-sky map130 GHz allsky
 
Maps of the northern sky observed with the LOFAR station in Effelsberg at 45 MHz and 130 MHz. The maps are shown in in Galactic coordinates with the plane of the Milky Way along the major axis. The black area is the part of the sky not accessible from Effelsberg. The intensity of the radio radiation is shown in temperature units, with high temperatures meaning high intensities (Köhler, 2016, PhD thesis).
 

 

Pulsar monitoring

Pulsars are highly magnetized, rapidly rotating and extremely dense stars that emit a beam of electromagnetic radiation. This radiation is seen only when the beam sweeps over the Earth, hence the pulsed appearance of the emission. The pulses last from milli-seconds up to several seconds. The pulses are affected by the electrons and magnetic fields of the interstellar medium (ISM), while they are traveling from their origin to Earth. Single stations can observe pulsars at different frequencies measuring the arrival time of the pulses. The arrival times at different frequencies vary due to the influence of the ISM. The differences in arrival time ("dispersion measure (DM)") yield information about the electron density and the magnetic field along the line of sight. Small-scale variations of the electron density generate variations of the dispersion measure. Hence monitoring of pulsars in many directions of the sky allow to study the small-scale structure of the electron density in the ISM. Studies of the interplanetary medium in the solar system and of the modes of pulsar rotation are further topics followed with data taken with LOFAR in single-station or GLOW mode. Read more...

 

 
Source: Lorimer and Kramer 2005 (Handbook of pulsar astronomy)