A pulsar in eclipse: the key to understanding the Shapiro delay effect from general relativity

NASA's RXTE satellite has discovered the first X-ray pulsar eclipsed by a companion star. This special star system helps in understanding the states of very high density matter, very compressed matter in the universe. In this way, it is an important astrophysical laboratory for understanding Einstein's theory of general relativity

For a long time, Einstein's theory of general relativity was not in the scientific news. And here she is again making headlines with the NASA device called the Rossi X-ray Timing Explorer (RXTE) or in Hebrew, Rossi X-ray Timing Explorer. The RXTE was launched at the end of 1995 and is the second largest after the Hubble telescope. It is used for astrophysical missions. RXTE discovered the absorbing millisecond pulsar SAX J1808.4-3658 in 1998 and continues to provide a unique window into the environment of neutron stars and black holes while continually focusing on new discoveries. In 2006 the RXTE together with the Swift Burst Alert Telescope (BAT) discovered a new binary system called Swift J1749.4-2807. This system has recently provided new findings.

Neutron star - heavy average neighborhood
An article published a month ago said that the RXTE had discovered the first x-ray pulsar eclipsed by a companion star. This special star system helps in understanding the states of very high density matter, very compressed matter in the universe. In this way, it is an important astrophysical laboratory for understanding Einstein's theory of general relativity.

A pulsar is a neutron star that rotates at an extremely fast rate - the core of a massive star that has collapsed. The star itself exploded a long time ago as a supernova. Neutron stars are very massive but unlike our sun they are compact stars, packed into a ball that is about 60,000 times smaller. Their size ranges from 10 to 15 kilometers, around a residential neighborhood in northern Tel Aviv.

Unlike the contents of an average north Tel Aviv neighborhood, it is difficult to determine the mass of pulsars or neutron stars, even when they are found in a binary system. It is difficult to know with the desired level of accuracy what the internal composition and sizes of neutron stars are.
The pulsar and the star - the devourer and the concealer
And here the system of the pulsar and the star that covers the pulsar and causes it to be eclipsed appeared. It seems that this system could have provided astrophysicists with a great opportunity to measure the mass of the pulsar and from it could also learn about the companion star.
The system is called the AMPX pulsar - that is, Accretion-Powered Millisecond X-ray pulsar. But this is the first AMPX system to show x-ray defects. The astrophysicists have named this system: Swift J1749.4-2807 because it was discovered in June 2006 by Shady and his team with the Bate telescope on the Swift satellite. The system was given a short name: J1749. The system was discovered when NASA's Swift satellite noticed a small eruption. The observations made by Swift as well as by RXTE and other sources led to the conclusion that it is a source that is part of a binary system located 22,000 light years away in the constellation Sagittarius (1).
The J1749 system is a binary system of a pulsar and a star, where the pulsar is actively gobbling up material from its stellar partner and at the same time the star causes its pulsar partner to be eclipsed. The adsorption coalesces into a disk of matter around the neutron star. Due to the adsorption, the binary system undergoes eruptions due to instability in the adsorption disc, when part of the gas collides with the neutron star. The magnetic field of the pulsar directs the gas falling towards it towards the magnetic poles. It means that the released energy appears in hot spots and they rotate together with the neutron star and produce fast x-ray pulses.

The system rotates 518 times per second (that is, the pulse signal was detected at a frequency of 518 Hz and it comes and goes, comes and goes...). The system can be imagined as an entire city that rotates as if it were a mixer or blender on the kitchen counter. In addition, this rotational motion imparts regular changes in the frequency of the x-ray pulses. These changes indicate that the two stars in the binary system orbit each other every 8.8 hours.
Einstein as usual enters every system
Why are neutron stars interesting astrophysical laboratories for physics? They have an enormous mass compressed to a radius of about 10 kilometers, their density is enormous and they present extreme conditions that require physics that cannot be tested in national laboratories. The astrophysicists expect that "gravitational wave astronomy" will finally be discovered. Observations of gravitational waves have implications for neutron star physics. Whereas the information gathered will help in understanding the state of matter in extreme densities. NASA designed the LIGO detector and the GEO600 detector searched and searched - all of them were designed to hunt for one and only Einstein's prediction, gravitational waves. Meanwhile, clear gravitational waves have not been detected.

But neutron stars are important for other reasons, which of course are also related to Einstein. Craig Marquardt and Todd Stromeyer of NASA's Goddard Space and Aeronautics Center reported last June the discovery of defects in the x-ray range in the J1749 system. This is the first discovery of x-ray defects in a system of this type. The team summarized its findings in the July 10 issue of The Astrophysical Journal Letters.

The RXTE instrument observed J1749 in an outburst that occurred between April 14, 2010, and April 20, 2010. The instrument observed three eclipses (each lasting 36 minutes) that occurred when the neutron star passed behind the normal star in the system. The instrument also detected three bursts or pulses in the x-ray range this week, and these identified the neutron star as a pulsar. He even recorded the changes in the pulses and thus the rotational movement of the neutron star could be noted.

The discovery of the defects should allow the exact mass measurement of the neutron star when studying its companion star in the binary system. The mass of the neutron star was estimated to be between 1.4 and 2.2 solar masses. The mass (radius) of the neutron star is important for understanding the equation of state of highly compressed matter. With the help of the information about the defects, the team was able to obtain information about the size and mass of the companion star to the neutron star with a very high level of accuracy. The team compared the observations made with the RXTE instrument to the theoretical mass range for neutron stars. From this comparison they determined the mass of the normal star in the binary system J1749: the star was found to be 70 percent of the mass of the Sun. Or the result was about 0.7 solar masses (between 0.6 and 0.8 solar masses).

The eclipses indicated that the normal star should be 20 percent larger than its apparent mass and apparent size. The team interprets this as the star's surface being inflated by x-rays from the pulsar, which is only a few million kilometers from the star. This additional heating is probably interfering with the surface of the star. One data in any case is missing to determine the mass of the pulsar accurately: it is necessary to observe the normal star with telescopes in the optical or infrared range to measure its movement and obtain the information about the pulsar from the data of the normal star to the same extent that the data of the normal star was obtained from the pulsar data.
Shapiro's delay and general relativity
One of the results of general relativity is that a signal - such as a radio wave or an x-ray pulse - lags slightly in time as it passes by a massive body. Irwin Shapiro of the Massachusetts Institute of Technology MIT suggested that this result from Einstein's general relativity would be a new test of Einstein's theory. The delay in question called "Shapiro's delay" (gravitational time delay) has been repeatedly demonstrated with the help of radio signals returned from Mercury and Monos and in experiments related to communication between space vehicles (2).

In the calculations, Marquardt and Stromeyer introduced the Shapiro delay effect for the J1749 system: the photons arriving from the pulsar would undergo a delay due to the gravitational potential from the companion star equal to 21 microseconds (or equal to 10,000 times the blink of the human eye). Such a delay is within the range of uncertainty of the RXTE device and can observe it.
What to do? The authors tried to keep the disturbing noise away by making adjustments on the 15th, 16th, 17th, 18th and 20th of April. The researchers adjusted the parameters so that the expected Shapiro effect would be minimal. Then the researchers added the Shapiro element and saw that it was not significant and did not change their calculations much. However, when they assumed the existence of the Shapiro effect, they placed an upper limit on the mass of the neutron star and found it to be: 2.2 solar masses.
The team believes this is the first time a realistic limit has been set for the Shapiro retardation effect at x-ray wavelengths for a system outside our solar system. They provided a technique to measure relativistic effects on a mission like NASA's RXTE. If the binary system J1749 erupts again - and if it erupts for a longer time, this will allow a better study of the defects and the delay of Shapiro. This will make it possible to better measure the mass of the neutron star, the two authors conclude.

For information on the NASA website
for research in ARCIVX

Another explanation for the phenomenon
to the Swift satellite website

Schady P., Beardmore AP, et al., 2006, GRB Coordinates Network, 5200, 1.

(2)

Shapiro, II, Ash, ME, Ingalls, RP, Smith, WB, Campbell, DB, Dyce, RB, Jurgens, RF, & Pettengill, GH 1971, Physical Review Letters, 26, 1132.