IAFE   05512
INSTITUTO DE ASTRONOMIA Y FISICA DEL ESPACIO
Unidad Ejecutora - UE
artículos
Título:
The low-frequency radio emission and spectrum of the extended SNR W44: new VLA observations at 74 and 324 MHz
Autor/es:
CASTELLETTI G., DUBNER G., BROGAN G., KASSIM N.E.
Revista:
ASTRONOMY AND ASTROPHYSICS
Editorial:
Springer
Referencias:
Lugar: Paris; Año: 2007 vol. 471 p. 537 - 549
ISSN:
0004-6361
Resumen:
Aims.We present new Very Large Array (VLA) radio images at 74 and 324
MHz of the SNR W44. The VLA images, obtained with
unprecedented angular resolution and sensitivity for such low
frequencies (HPBW 37'' at 74 MHz, and 13'' at 324 MHz), have been used
in combination with existing 1442 MHz radio data, Spitzer IR data, and
ROSAT and Chandra X-ray data to investigate morphological and spectral
continuum properties of this SNR. Methods: The observations were
carried out with the VLA simultaneously at 74 and 324 MHz in the A and B
configurations and at 324 MHz in the C and D configurations. The radio
continuum spectral index distribution was derived through direct
comparison of the combined data at 74, 324, and 1442 MHz. In addition,
to isolate and identify different spectral components, tomographic
spectral analysis was performed. Results: We measured total flux
densities of 634 Jy and 411 Jy at 74 and 324 MHz, respectively, for
W44, and from a careful assessment of published
values between 22 and 10 700 MHz derived a global integrated continuum
spectral index α=-0.37 ± 0.02. The spatially resolved
spectral index study revealed that the bright filaments, both around and
across the SNR, have a straight spectrum between 74 and 1442 MHz, with
α -0.5, with two clear exceptions: a short portion of the
SNR limb to the southeast, with α varying between 0 and +0.4 and a
bright arc to the west where the spectrum breaks around 300 MHz and
becomes concave down. We conclude that at the shell and along the
internal filaments, the electrons responsible for the synchrotron
emission were accelerated at the shock according to a simple diffusive
shock model. The positive spectrum corresponds to a location where the
SN shock is running into a molecular cloud and the line of sight
intersects the photo dissociation region of an HII region and a young
stellar object is present. Such spectral inversion is a classic
signature of thermal absorption, either from ionized gas in the
postshock region, from the HII region itself, or both. The curved
spectrum on the westernmost bright arc is explained as the consequence
of strong post-shock densities and enhanced magnetic fields after the
interaction of the SN shock with a coincident molecular cloud. No
spectral index trace was found indicating any connection between the
associated pulsar PSR B1953+0.1 and the surrounding
shell, nor between the SNR and the 3EG 1853+0114
γ-ray source proposed to be associated with
W44. The comparison of the 324 MHz image with a 4.5
μm IR image obtained with Spitzer underscored an impressive
correspondence between emission both to the north and west sides of
W44, while the comparison with ROSAT and Chandra
images confirm that the synchrotron radio emission surrounds the thermal
X-ray radiation.