The low-frequency radio emission and spectrum of the extended SNR W44: new VLA observations at 74 and 324 MHz
The low-frequency radio emission and spectrum of the extended SNR W44: new VLA observations at 74 and 324 MHz
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DOI：
10.1051/0004-6361:20077062
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发表时间：
2007-02
影响因子：
6.5
通讯作者：
G. Castelletti;G. Dubner;C. L. Brogan;N. Kassim
中科院分区：
文献类型：
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作者：
G. Castelletti;G. Dubner;C. L. Brogan;N. Kassim
目标。我们提出了新的超大型阵列(VLA)无线电图像在74和324兆赫的信噪比W44。VLA图像对于这种低频(HPBW 37“在74 MHz，13”在324 MHz)具有前所未有的角分辨率和灵敏度，已与现有的1442 MHz无线电数据、Spitzer IR数据以及ROSAT和Chandra X射线数据结合使用，以研究这种SNR的形态和光谱连续特性。方法：研究方法。观测是用VLA同时进行的，A和B配置中的74和324 MHz，C和D配置中的324 MHz。通过直接比较74、324和1442 MHz的组合数据，得出了射电连续谱折射率分布。此外，为了分离和鉴定不同的光谱成分，进行了层析光谱分析。结果。我们分别在74 MHz和324 MHz测量了W44的634Jy和411Jy的总通量密度，并通过对22~10700 MHz之间已公布的值的仔细评估得出了全球积分连续谱指数$α=-0.37\pM0.02$。空间分辨光谱指数研究表明，无论是在信噪比附近还是在信噪比两端，亮丝的频谱都是在74-1442兆赫兹之间的直线频谱，α-sim为-0.5兆赫兹，但有两个明显的例外：信噪比边缘的一小部分在东南方，α在0到+0.4%之间变化，在西边有一条明亮的弧线，光谱在300兆赫兹附近断裂并凹陷。我们的结论是，在壳层和沿内丝，负责同步辐射的电子在激波时被加速，这是一个简单的扩散激波模型。正光谱对应于SN激波进入分子云的位置，视线与HII区域的光解离区相交，存在年轻的恒星物体。这种光谱反转是热吸收的典型特征，无论是来自震后区域的电离气体，还是来自HII区域本身，或者两者兼而有之。最西边明亮弧线上的弯曲光谱被解释为震后强密度和SN激波与重合分子云相互作用后磁场增强的结果。没有发现光谱指数痕迹表明相关的脉冲星PSRB1953+0.1与周围的壳层之间有任何联系，也没有发现信噪比与被认为与W44有关的3EG 1853+0114γ射线源之间的任何联系。324兆赫兹图像与斯皮策获得的4.5MHZ米红外图像的比较强调了W44北侧和西侧的辐射之间令人印象深刻的对应，而与ROSAT和钱德拉图像的比较证实，同步加速器射电辐射围绕着热X射线辐射。
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 $\alpha=-0.37 \pm 0.02$. 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 $\alpha\sim -0.5$, 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 μ m 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.