Visualizing RFI with AARTFAAC

© Bharat Kumar Gehlot

Identification and excision of Radio Frequency Interference (RFI) is a critical hurdle in experiments that seek to measure the 21-cm signature of neutral hydrogen from the Cosmic Dawn and the Epoch of Reionization. The AARTFAAC Cosmic Explorer (ACE) project utilises the AARTFAAC-12 imager of the LOFAR telescope to search for the 21-cm brightness temperature fluctuations (at z~18) caused by the first generation of stars that appeared during the Cosmic Dawn.

While performing data quality and ionospheric activity checks on a small ACE dataset (15mins, 2.7MHz) from November 7, 2018, we found RFI activity from various sources. The GIF presented here loops over 15 snapshots produced using the A12-dataset, and each snapshot (1-minute duration) shows maximum amplitude along the frequency direction of the residual emission after subtraction of frequency smooth galactic emission and compact sources.

We see significant RFI from airplanes flying over or close to the LOFAR core. The observed RFI is narrowband and is possibly due to the reflection of radio signals from the airplanes or communication channels between airplanes. There is more frequent activity near the western horizon, which is the direction of the Schiphol airport relative to the LOFAR-core.

The RFI activity is mostly benign towards the northeast, east, and south-east of LOFAR-core, where most windmill farms and solar parks are located. The two bright blobs persistent throughout all snapshots are residuals from the bright A-team sources, Cas A and Cyg A.