Spectra of the HB 21 supernova remnant: Evidence of sprectrum flattening at the low frequencies

 

D. Borka,¹ V. Borka Jovanović,¹ and D. Urošević²

 

¹ Atomic Physics Laboratory (040), Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia. (dusborka@vinca.rs, vborka@vinca.rs).

² Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia. (dejanu@math.rs).

 

Received 2011 June 9.
Accepted 2011 October 19.

 

RESUMEN

Utilizamos observaciones de la emisión en radiocontinuo a 1420, 820, 408, 34.5 y 22 Mhz con objeto de estimar las temperaturas de brillo medias en cinco frecuencias para el remanente de supernova HB 21. También presentamos el índice espectral medio de Hb 21. Estimamos los espectros de HB 21 mediante diagramas de temperatura media versus frecuencia, para 1420, 820, 408, 34.5 y 22 Mhz. Presentamos asimismo diagramas T — T para tres pares de frecuencias, entre 142034.5, 1420–22, 34.5–22 MHz. Notamos que los índices espectrales son menores para frecuencias por debajo de 408 Mhz. Probablemente esto se debe a la absorción por plasma térmico a bajas frecuencias.

 

ABSTRACT

We use observations of the continuum radio emission at 1420, 820, 408, 34.5 and 22 MHz to estimate the mean brightness temperatures of the HB 21 supernova remnant (SNR) at five frequencies. We also present mean spectral index of HB 21. The spectra of HB 21 are estimated from mean temperatures versus frequency plots for 1420, 820, 408, 34.5 and 22 MHz. We also present T — T plots of three frequency pairs: between 1420–34.5, 1420–22, 34.5–22 MHz. We notice flatter spectral indices at frequencies below 408 MHz. Probably this is due to the absorption by thermal plasma at low frequencies.

Key Words: ISM: individual objects (HB 21) — radiation mechanisms: non–thermal — radiation mechanisms: thermal — radio continuum: general — surveys.

 

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