An international team of astrophysicists including Russian scientists from the Space Research Institute of the Russian Academy of Sciences (RAS), MIPT, and Pulkovo Observatory of RAS has detected an abrupt decrease of pulsar luminosity following giant outbursts. The phenomenon is associated with the so-called “propeller effect,” which was predicted more than 40 years ago. However, this is the first study to reliably observe the transition of the two X-ray pulsars 4U 0115+63 and V 0332+53 to the “propeller regime.” The results of the observations, the conclusions reached by the researchers, and the relevant calculations were published in Astronomy & Astrophysics.
The two sources studied, 4U 0115+63 and V 0332+53, belong to a rather special class of transient X-ray pulsars. These stars alternately act as weak X-ray sources, undergo giant outbursts, and disappear from sight completely. The transitions of pulsars between different states provide valuable information about their magnetic field and the temperature of the surrounding matter. Such information is indispensable, as the immensely strong magnetic fields and extremely high temperatures make direct measurements impossible in a laboratory on Earth.
The name of a pulsar is preceded by a letter designating the first observatory to discover it, which is followed by a numerical code containing the coordinates of the pulsar. The “V” refers to Vela 5B, a US military satellite that was launched to spy on the Soviets. As for the “4U” in the other name, it stands for the fourth Uhuru catalog, compiled by the first observatory in orbit dedicated specifically to X-ray astronomy. Following the discovery of the first pulsar, it was originally known as “LGM-1” (for “little green men”), because it was a source of regular radio pulses, leading scientists to believe that they might have received a signal from intelligent extraterrestrials.
An X-ray pulsar is a rapidly spinning neutron star with a strong magnetic field. A neutron star can be part of a binary system. In a process that astrophysicists call accretion, the neutron star can channel gas from its normal star companion. The attracted gas spirals toward the neutron star, forming an accretion disk, which is disrupted at the magnetosphere radius. During accretion, the matter penetrates to a certain extent into the magnetosphere, “freezes into it,” and flows along the lines of the magnetic field toward the magnetic poles of the neutron star. Falling toward the poles, the gas is heated to several hundred million degrees, which causes the emission of X-rays. If the magnetic axis of a neutron star is skewed relative to its rotational axis, the X-ray beams it emits rotate in a manner that resembles the way beacons work. For an “onshore” observer, the source appears to be sending signals at regular intervals ranging from fractions of a second to several minutes.
A neutron star is one of the possible remnants left behind by a supernova. It can be formed at the end of stellar evolution, if the original star was massive enough to allow gravitation to compress the stellar matter enough to make electrons combine with protons yielding neutrons. The magnetic field of a neutron star can be more than 10 orders of magnitude stronger that any magnetic field that could be achieved on Earth.
In a binary system, an X-ray pulsar is observed when the neutron star is accreting matter from its normal star companion—often a giant or a supergiant characterized by a strong stellar wind (ejection of matter into space). Alternatively, it can be a smaller star like our own sun that has filled its Roche lobe—the region beyond which it is unable to hold on to the matter attracted by the gravity of the neutron star companion.
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