Possibility of Determining Intergalactic Gas Density by Radio Observations of Flares of Remote Sources
Possibility of Determining Intergalactic Gas Density by Radio Observations of Flares of Remote Sources
复制标题
通过远程源耀斑射电观测确定星系间气体密度的可能性
作者：
V. Ginzburg
星系际空间气体平均浓度（N mg）的测定是一个重要而又远未解决的问题。人们最终可能希望用不同的间接方法来确定N mg的值（例如，见参考文献1）。同时，通过测量频率ν的无线电信号延迟时间τ（ν）= t（ν）-t（∞），有可能直接找到N mg，其中t（∞）是相当高频率的信号到达的时间。在完全电离的气体中，折射率n = 1−（e 2 N/2πmν2），群速度u = cn，其中L是到信号源的距离，假设（1 − n）<$1;所用表达式的准确性很清楚（例如，参考文献2）。对于我们的银河系，在不太小的银河系纬度上，<$N gdl <$N g L g <$（1020 to 1021）cm−2。因此，在N mg <$10 −5 cm−3处，如果L>1025至1026 cm，则乘积N mg L>N g L g。因此，对于大于3 ~ 30 Mpc的河外源，在星系际空间中N <$10 −5处的信号延迟大于银河系中的信号延迟，并且可以被测量（我注意到，作为一个例子，在N <$10 −5，ν <$3 × 107 Hz和L <$1025处，时间τ <$100 s）。然而，为了应用这种在脉冲星3中广泛使用的方法，必须存在快速变化和强大的河外射电发射源;这些尚未被发现。
DETERMINATION of the mean gas concentration in intergalactic space (N mg) is an important problem which is still far from being solved. One may hope finally to determine the value of N mg in different indirect ways (see, for example, ref. 1). At the same time there is a possibility of finding N mg directly by measuring the radio signal retardation time τ(ν) = t (ν) − t (∞) for a frequency ν, where t (∞) is the time of the arrival of a signal of a rather high frequency. In a completely ionized gas the refractive index n = 1−(e 2 N/2πmν2), the group velocity u = cn and where L is the distance to the signal source assuming (1 − n)≪1; the accuracy of the expressions used is clear (for example, from ref. 2). For our Galaxy, at not very small galactic latitudes, ∫N gdl≡N g L g∼(1020 to 1021) cm−2. Therefore at N mg∼10−5 cm−3 the product N mg L>N g L g if L>1025 to 1026 cm. Thus, for extragalactic sources at more than 3 to 30 Mpc signal retardation in intergalactic space at N≳10−5 is greater than in the Galaxy and could be measured (I note, as an example, that at N∼10−5, ν∼3 × 107 Hz and L∼1025, the time τ∼100 s). To apply this method, which is widely used in the case of pulsars3, there must, however, exist rapidly varying and powerful extragalactic radio emission sources; these have not yet been found.