Her X-1 (Hercules X-1)

In January, 1972, Her X-1 was the 2nd binary pulsar discovered. It has important similarities to Cen X-3:

  • Close binary system that has about 15% of orbit in eclipse (1.7 d orbit)
  • Slowly rotating pulsar (1.23 sec)
  • Accelerating rotation: pulsar spinning faster with time

I was responsible for the analysis of the data from Uhuru, that produced these results (binary system, eclipsing, accelerating spin rate), along with the data and results from Cen X-3, and as a result was extremely familiar with all the significant characteristics of the data.

The most significant result of the analysis (besides demonstrating that the systems were binaries) is that the pulsar period is getting shorter, which means the neutron star’s spin rate is accelerating. This result was completely different from any of the other pulsars known in early 1972. Most of this site is focused on understanding the implications of binary pulsar spin-up and how it intrinsically contradicts the commonly assumed theory that neutron stars are produced by supernova explosions. The spin-up property conclusively proves that neutron stars cause supernova explosions and are present long before the explosion actually occurs.

However, Her X-1 demonstrated some additional properties, that I, at least, did not understand until recently. Specifically:

  • In addition to its 1.7 day orbital period, Her X-1 has a 35-day period covering roughly 21 orbital periods. Her X-1 “turns on at time zero, is visible (in X-rays) for about five 1.7-day periods, then disappears for the remaining sixteen 1.7-day orbital periods. It then repeats this cycle every 35 days.
  • There is a “dip” in intensity of the X-rays, a few hours before it goes into eclipse, and it then re-appears again just before entering the eclipse. There are still x-ray emissions during this dip, but they are significantly lower in intensity.

These properties have always been a mystery to me, however, with my recent increase in understanding of the NS-Capture theory, I have been able to come up with plausible explanations for both.

The 35-day period

On the 35-day period, what I have realized (esp based on Bhattacharya & van den Heuvel) is that not enough attention has been paid to the effect of the neutron star pulsar on its companion.

The main thing to consider is that by generating 10**37 -> 10**39 ergs/sec while immersed in the atmosphere of the companion, the companion is absorbing all this energy as heat, and, naturally the companion gets hotter and balloons out to larger radius.

In the case of Her X-1, what we are observing is that the companion is expanding and contracting with a 35-day period. i.e. the 75% of the time that Her X-1 is not visible, it is actually enveloped by the atmosphere of the companion. When this occurs the companion absorbs all the energy that can possibly be emitted by the accretion process, which approaches 10**39 ergs/sec.

So, basically, one can think of Her X-1 as huffing and puffing its companion which will eventually cause its atmosphere to gravitationally destabilize and then literally be blown away. Undoubtedly, not all at once, but inexorably over and over again, ultimately until there is nothing left of the companion.

Unlike Cen X-3, in the case of Her X-1, the companion may not explode as a supernova, but more likely first have the atmosphere blown away, then later the remaining core will ultimately be evaporated by the pulsar.

The “dips”

As far as the dips are concerned, just yesterday, I was flipping through my copy of “Pulsar Astronomy (Fourth Edition)”, when I came across some old data analysis I had produced on p174 fig 12.3 for Her X-1.

I said to myself: “Oh yeah, the ‘dips'”. Just before the eclipse (which can be seen in extreme detail in Giacconi, Gursky, Kellogg, Levinson, Schreier, and Tananbaum: Ap J. 184, 227-236, 1973, August 15).

Unfortunately, at the time, I had no idea what was causing them. and in the above paper the speculation was that it was absorption by gas, but, as stated in the conclusion of that paper:

“It is clear that the behavior of the dips bears some relation both to the 35-day cycle and to the 1.7-day orbital phase, but we have not yet been able to establish any simple relationship between these phenomena.”

Well, right here and now, possibly for the first time, I will state the simple relationship of the dips to the 1.7-day and 35-day cycles.

What is happening is that the Her X-1 pulsar is dragging a big cloud of gas behind it.
Therefore, when the pulsar approaches 270 degrees in its orbit, when it is heading directly away from us, it is obscured by the cloud of gas behind it.
Amazingly, just before the eclipse, the pulsar is just far enough ahead of the trailing cloud that it gets to peek through between the cloud and the companion just before the eclipse.
The reason for the shape of the intensity curve: rapid rise, with long slow decline is that the rapid rise occurs when the contraction of the companion, suddenly exposes the pulsar. When the billowing expansion begins, the pulsar slowly gets enveloped by more and more of the atmosphere until it finally disappears until the next contraction.