A pulsar with double kick per orbit

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A pulsar with double kick per orbit
A pulsar with double kick per orbit

One pulsar with double kick per orbit

Pulsars are exotic creatures in the starry sky that are always good for a surprise. The latest mystery comes from a pulsar emitting X-rays twice each orbit as it orbits its companion star, instead of the expected once. Colleen Wilson of NASA's Marshall Space Flight Center, in an article accepted for publication in the Astrophysical Journal, describes the discovery of an accreting X-ray pulsar that is not visible in the visible spectral range.

When GRO J2058+42 is active, it appears to emit x-rays twice each orbit of its primary star. "This is unusual behavior," said the scientist."It differs from that of other accreting pulsars". These are pulsars that "suck in" matter from a larger parent star, emitting X-rays and gamma rays in the process. Twelve accreting X-ray pulsars without an optically visible companion have been discovered so far, but they all otherwise "transmit" once per orbit.

Wilson made their discovery using the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory and made further observations using the Rossi X-ray Timing Explorer.

Pulsars – rotating neutron stars – are among the most fascinating celestial bodies. They were discovered in 1965 when radio astronomers observed various objects emitting radio waves at precisely timed intervals. The sources were soon identified as rapidly rotating neutron stars with intense magnetic fields. Radio pulsars are characterized by a regularity comparable to that of a Swiss watch; accreting pulsars, on the other hand, tend to be more like cheap alarm clocks, which can quickly fly in and out - and fail when you least expect it.

Since the launch of BATSE, astrophysicists have discovered 20 of the 45 known pulsars and found another five new X-ray pulsars. These accreting pulsars are not to be confused with the mysterious gamma-ray bursts that appear to have originated at the edge of the universe. While they have enough energy for BATSE to register, they are not always immediately noticed.

In September 1995, when Wilson reviewed BATSE data, she encountered an explosion that was 140 milli-Crab bright (that's 140 thousandths the brightness of the Crab Nebula, which astrophysicists use as a benchmark). After processing the data with the computer, she found that the source flashed every 198 seconds. This indicated a massive, compact object that is spinning very quickly.

GRO J2058+42 (numbers represent approximate location: 20 hours and 58 minutes along the celestial equator, 42 degrees north) had signaled the beginning of a gigantic eruption that lasted 46 days. He peaked at 140 millicrab. BATSE also recorded minor eruptions for over 500 days.

All-sky observations by BATSE and Rossi showed outbursts every 54 days. With BATSE it could be proven that their brightness fluctuated in a 110-day cycle. It became clear that Wilson was on to something new.

For the other accretion-powered pulsars, emissions occur only once per orbit (if they occur at all). Sometimes they stay away for several weeks or months. J2058+42's twice-per-orbit outbursts, on the other hand, are new.

According to Wilson, one possible explanation is that J2058+42 is a binary star system composed of a Be-type star (a B star with emission lines) with a mass approximately eight to 15 times that the mass of the sun and a neutron star in an oblique orbit. The Be star ejects gas with an unusual solar wind that collects in a disk shape in the equatorial plane. Because the neutron star's orbit is tilted toward the equator, it travels through the disk of matter twice in 110 days. As a result, J2058+42 is quiet most of the time, emitting X-rays twice per orbit as the dense, magnetically intense neutron star weaves its way through the gas cloud. When the pulsar is near the periastron (closest to the star), the flares are more intense, reaching about 1.4 percent the brightness of the Crab Nebula. On the other hand, when the pulsar is close to the aastron (most distant from the star), they are fainter, less than one percent, which is close to the sensitivity limit of BATSE.

A more likely explanation, according to Wilson, is that the pulsar is winding up the periastron accretion disk and accelerating its ingestion of matter. In the apastron he only takes in some of the solar wind.

Assignment to the Be star-pulsar binary group is purely based on the behavior of J2058+42, which is consistent with other binaries. "In about half of the known high-mass binary systems containing X-ray pulsars, the primary star is likely to be a Be star," the scientist said. The high mass here refers to the companion star, not the pulsar. Unfortunately, nobody was able to look at the new object with other methods and thus visually confirm the considerations. According to Wilson, this is the sixth time BATSE has found an object for which no visible component was found.

"Part of the problem is that the margins of error are so large," she said. BATSE is designed to observe the entire sky, so it cannot locate a source with the same accuracy as a telescope pointed at a star. Pre-1995 data from BATSE and ESA's X-ray satellite also showed nothing to be a possible source. A study of optical images also failed to provide an explanation, possibly because interstellar dust simply swallows starlight before it gets here.

Although the Rossi observations have narrowed the estimated position to 4 arc minutes (about one-eighth the apparent diameter of the Moon), the margin of error is still quite large for optical telescope searches. Wilson estimates the distance to J2058+42 to be between 23,000 and almost 50,000 light-years (almost a quarter of the way across the galaxy).

Meanwhile, J2058+42 has become too quiet for BATSE to detect him. Wilson will continue to review the data and eagerly awaits the Rossi Explorer observations that should be made shortly.

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