Pulsar Astrophysics at Jodrell Bank: Rolling Grant 2009-2014

射电脉冲星是非常紧凑、旋转、质量大、密度大的恒星，主要由中子组成，沿着它们的磁轴发射射电束。当一颗比太阳重8倍的恒星耗尽燃料，经历灾难性的坍缩，形成超新星时，它们就会留下。坍缩过程中的巨大压力将比太阳稍重的物质挤压成半径只有10公里的球体，将物质转化为中子。当脉冲星旋转时，狭窄的射电波束扫过地球，我们探测到辐射脉冲，很像宇宙版的灯塔，因此得名脉冲星。极高的密度，相当于把整个地球塞进一个高尔夫球，这意味着脉冲星的旋转非常稳定。每个脉冲就像时钟的滴答声，其精度可以与地球上最好的原子钟相媲美。我们可以利用脉冲星像时钟一样的特性来进行令人难以置信的重力实验，这些实验只有在脉冲星上才能实现。例如，如果一颗脉冲星与另一颗恒星处于双星系统中，那么爱因斯坦的广义相对论预测，由于引力波的发射，两颗恒星相互绕轨道运行的时间将随着时间的推移而减少。赫尔斯和泰勒首先用脉冲星测量了轨道周期的减少，并因此获得了1993年的诺贝尔物理学奖。这有力地证明了爱因斯坦的理论是正确的，但这只是引力波存在的间接证据，直接探测还没有完成。脉冲星代表着可观测宇宙中物质密度最大的物体，它也有极大的磁场，大约是我们在地球上能产生的最强磁场的3000万倍。正是在这些磁场中，产生了被视为精确脉冲的无线电发射。这个过程的工作原理测试了我们对物理理解的极限，并允许我们在不可能在实验室中重现的环境中测量物质和等离子体的特性。为了能够进行所有这些令人兴奋的研究，我们首先必须找到足够数量和正确类型的脉冲星来进行这些实验。我们的小组是世界上发现射电脉冲星最成功的小组，但更极端的物体仍有待发现。为了找到这些丢失的珠宝，我们计划使用世界上最大的望远镜，以及我们开发的大型计算机和复杂的软件。我们预计会发现大约1000颗新的脉冲星！我们还继续使用目前最著名的物体来检验爱因斯坦的引力理论。这包括我们在2003年发现的独特的双脉冲星的计时。在这个天体中，两颗脉冲星以每小时100万公里的速度在不到2.5小时的时间内相互绕转。使用它，我们已经测试了爱因斯坦至少在99.95%的水平上是正确的。为了第一次直接探测到引力波，我们正在计划一个独特的实验，以最高的精度测量脉冲星脉冲的到达时间。这需要我们记录并整合欧洲5个最大望远镜的信号，形成一个相当于世界上最大望远镜的巨大天线。我们研究的一个重要因素是使用洛弗尔望远镜的可能性。作为世界上最大的脉冲星望远镜之一，我们跟踪了600多颗脉冲星的旋转，得到了许多意想不到的令人兴奋的结果，包括中子星的性质、致密物质，甚至是大质量恒星坍缩过程中的事件。我们提出的所有实验都需要尖端的定制技术，其中大部分是我们自己开发的。这项拨款中提出的最先进的软件和硬件使我们能够对这些极端物体进行令人难以置信的精确测量。

Radio pulsars are very compact, rotating, massive and dense stars, consisting mainly of neutrons, that emit a radio beam along their magnetic axis. They are what remains when a star about eight times heavier than our Sun runs out of fuel and undergoes a catastrophic collapse that results in a supernova. The immense pressure during that collapse squashes matter slightly heavier than our Sun into a sphere of just 10 kilometre in radius, converting the matter into neutrons. As the pulsar rotates, the narrow radio beam sweeps across the Earth and we detect a pulse of radiation, much like a cosmic version of a lighthouse, and hence the name pulsar. The extreme density, equivalent to fitting the entire Earth into a golf-ball, means that pulsars rotate very stably. Each pulse is like a tick from a clock which has an accuracy which rivals that of the best atomic clocks on Earth. We can use this clock-like nature of pulsars to perform incredible experiments on gravity which are only possible with pulsars. For example, if a pulsar is in a binary system with another star, then Einstein's theory of General Relativity predicts that the time that it will take the two stars to orbit each other will decrease with time due to the emission of gravitational waves. The predicted decrease in the orbital period was first measured by Hulse & Taylor using pulsars, for which they were awarded the Noble Prize in Physics in 1993. This was strong proof that Einstein's theory was correct but this was only indirect evidence for the existence of gravitational waves, a direct detection has not yet been made. Representing objects with the largest matter density in the observable Universe, pulsars also have extremely large magnetic fields, about 30 million times that of the strongest magnetic we can produce on Earth. It is in these magnetic fields that the radio emission is generated which is seen as the accurate pulses. How this process works tests the limits of our physical understanding and allows us to measure the properties of matter and plasma in environments which are impossible to recreate in a laboratory. In order to be able to perform all of these exciting studies, we first have to find sufficient numbers, and the right sort, of pulsars on which to do these experiments. Our group is the world's most successful one in finding radio pulsars but even more extreme objects are still to be found. In order to find these missing jewels, we are planning to use some of the largest telescopes in the world, together with a very large computer and sophisticated software that we have developed. We expect to find about 1000 new pulsars! We also continue to use the currently best known objects to test Einstein's theory of gravity. This includes timing the unique Double Pulsar, which we discovered in 2003. In this object, two pulsars orbit each other in less than 2.5 hours, moving with speeds of 1 million km/h. Using it, we have tested Einstein to be correct at least the 99.95% level. To directly detect gravitational waves for the first time, we are planning a unique experiment to measure the arrival time of pulses from pulsars with the highest precision yet achieved. This requires us to record and combine the signals of the 5 largest telescopes in Europe to form an enormous dish equivalent to the largest in the world. An important factor in our research is the possibility to use the Lovell telescope. As one of the greatest pulsar telescopes in the world, we track the rotation of more than 600 pulsars, leading to many unexpected and exciting results about neutron star properties, dense matter and even the events during the collapse of a massive star. All of our proposed experiments require cutting edge bespoke technology much of which we develop ourselves. This state-of-the art software and hardware proposed in this grant allows us to perform these incredibly precise measurements of these extreme objects.