RCW 103: Young Magnetar Likely the Slowest Pulsar Ever Detected

RCW 103

Credit  X-ray: NASA/CXC/University of Amsterdam/N.Rea et al; Optical: DSS

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Using NASA's Chandra X-ray Observatory and other X-ray observatories, astronomers have found evidence for what is likely one of the most extreme pulsars, or rotating neutron stars, ever detected. The source exhibits properties of a highly magnetized neutron star, or magnetar, yet its deduced spin period is thousands of times longer than any pulsar ever observed.

For decades, astronomers have known there is a dense, compact source at the center of RCW 103, the remains of a supernova explosion located about 9,000 light years from Earth. This composite image shows RCW 103 and its central source, known officially as 1E 161348-5055 (1E 1613, for short), in three bands of X-ray light detected by Chandra. In this image, the lowest energy X-rays from Chandra are red, the medium band is green, and the highest energy X-rays are blue. The bright blue X-ray source in the middle of RCW 103 is 1E 1613. The X-ray data have been combined with an optical image from the Digitized Sky Survey.

Observers had previously agreed that 1E 1613 is a neutron star, an extremely dense star created by the supernova that produced RCW 103. However, the regular variation in the X-ray brightness of the source, with a period of about six and a half hours, presented a puzzle. All proposed models had problems explaining this slow periodicity, but the main ideas were of either a spinning neutron star that is rotating extremely slowly because of an unexplained slow-down mechanism, or a faster-spinning neutron star that is in orbit with a normal star in a binary system.

On June 22, 2016, an instrument aboard NASA's Swift telescope captured the release of a short burst of X-rays from 1E 1613. The Swift detection caught astronomers' attention because the source exhibited intense, extremely rapid fluctuations on a time scale of milliseconds, similar to other known magnetars. These exotic objects possess the most powerful magnetic fields in the Universe -trillions of times that observed on the Sun - and can erupt with enormous amounts of energy.

Seeking to investigate further, a team of astronomers led by Nanda Rea of the University of Amsterdam quickly asked two other orbiting telescopes - NASA's Chandra X-ray Observatory and Nuclear Spectroscopic Telescope Array, or NuSTAR - to follow up with observations.

New data from this trio of high-energy telescopes, and archival data from Chandra, Swift and ESA's XMM-Newton confirmed that 1E 1613 has the properties of a magnetar, making it only the 30th known. These properties include the relative amounts of X-rays produced at different energies and the way the neutron star cooled after the 2016 burst and another burst seen in 1999. The binary explanation is considered unlikely because the new data show that the strength of the periodic variation in X-rays changes dramatically both with the energy of the X-rays and with time. However, this behavior is typical for magnetars.

But the mystery of the slow spin remained. The source is rotating once every 24,000 seconds (6.67 hours), much slower than the slowest magnetars known until now, which spin around once every 10 seconds. This would make it the slowest spinning neutron star ever detected.

Astronomers expect that a single neutron star will be spinning quickly after its birth in the supernova explosion and will then slow down over time as it loses energy. However, the researchers estimate that the magnetar within RCW 103 is about 2,000 years old, which is not enough time for the pulsar to slow down to a period of 24,000 seconds by conventional means.

While it is still unclear why 1E 1613 is spinning so slowly, scientists do have some ideas. One leading scenario is that debris from the exploded star has fallen back onto magnetic field lines around the spinning neutron star, causing it to spin more slowly with time. Searches are currently being made for other very slowly spinning magnetars to study this idea in more detail.

Another group, led by Antonino D'Aì at the National Institute of Astrophysics (INAF) in Palermo, Italy, monitored 1E 1613 in X-rays using Swift and in the near-infrared and visible light using the 2.2-meter telescope at the European Southern Observatory at La Silla, Chile, to search for any low-energy counterpart to the X-ray burst. They also conclude that 1E 1613 is a magnetar with a very slow spin period.

A paper describing the findings of Rea's team appears in the September 2, 2016, issue of The Astrophysical Journal Letters and is available online. The authors of that paper are Nanda Rea (University of Amsterdam and IEEC-CSIC, Spain), A. Borghese (Univ. of Amsterdam), P. Esposito (Univ. of Amsterdam), F. Coti Zelati (Univ. of Amsterdam, INAF, Insubria), M. Bachetti (INAF), G. L. Israel (INAF), A. De Luca (INAF).

A paper describing the findings of D'Aì's team has been accepted for publication by Monthly Notices of the Royal Astronomical Society and is also available online.

NuSTAR is a Small Explorer mission led by the California Institute of Technology in Pasadena and managed by NASA's Jet Propulsion Laboratory, also in Pasadena, for NASA's Science Mission Directorate in Washington.

NASA's Swift satellite was launched in November 2004 and is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

Fast Facts for RCW 103:

Scale: Image is 24 arcmin across (about 75 light years)
Category: Supernovas & Supernova Remnants, Neutron Stars/X-ray Binaries
Coordinates (J2000): RA 16h 17m 28.80s | Dec -51° 01' 48.00"
Constellation: Norma
Observation Date: 22 pointings between Feb 2000 and June 2006
Observation Time: 99 hours 43 min (4 days 3 hours 43 min).
Obs. ID: 123, 970, 1040, 2314-2318, 2759, 3514-3517, 4596-4599, 5592-5595, 11823, 12224, 17460, 18878
Instrument: ACIS
References: Rea, N. et al, 2016, ApJL (accepted); arXiv:1607.04107
Color Code: X-ray (Red, Green, Blue); Optical (Red, Green, Blue)
Distance Estimate: About 10,700 light years

Source: NASA’s Chandra X-ray Observatory

The Realm of Buried Giants

PR Image eso1607a

The sky around the star formation region RCW 106

PR Image eso1607b

The sky around the star formation region RCW 106 (wide-field view) 

PR Image eso1607c

The sky around the star formation region RCW 106 (annotated)

PR Image eso1607d

The star formation region RCW 106 in the constellation of Norma

Videos

 

PR Video eso1607a

A close look at the sky around the star formation region RCW 106

In this huge new image clouds of crimson gas are illuminated by rare, massive stars that have only recently ignited and are still buried deep in thick dust clouds. These scorching-hot, very young stars are only fleeting characters on the cosmic stage and their origins remain mysterious. The vast nebula where these giants were born, along with its rich and fascinating surroundings, are captured here in fine detail by ESO’s VLT Survey Telescope (VST) at the Paranal Observatory in Chile.

RCW 106 is a sprawling cloud of gas and dust located about 12 000 light-years away in the southern constellation of Norma (The Carpenter’s Square). The region gets its name from being the 106th entry in a catalogue of H II regions in the southern Milky Way [1]. H II regions like RCW 106 are clouds of hydrogen gas that are being ionised by the intense starlight of scorching-hot, young stars, causing them to glow and display weird and wonderful shapes.

RCW 106 itself is the red cloud above centre in this new image, although much of this huge H II region is hidden by dust and it is much more extensive than the visible part. Many other unrelated objects are also visible in this wide-field VST image. For example, the filaments to the right of the image are the remnants of an ancient supernova, and the glowing red filaments at the lower left surround an unusual and very hot star [2]. Patches of dark obscuring dust are also visible across the entire cosmic landscape.

Astronomers have been studying RCW 106 for some time, although it is not the crimson clouds that draw their attention, but rather the mysterious origin of the massive and powerful stars buried within. Although they are very bright, these stars cannot be seen in visible-light images such as this one as the surrounding dust is too thick, but they make their presence clear in images of the region at longer wavelengths.

For less massive stars like the Sun the process that brings them into existence is quite well understood — as clouds of gas are pulled together under gravity, density and temperature increase, and nuclear fusion begins — but for the most massive stars buried in regions like RCW 106 this explanation does not seem to be fully adequate. These stars — known to astronomers as O-type stars — may have masses many dozens of times the mass of the Sun and it is not clear how they manage to gather, and keep together, enough gas to form.

O-type stars likely form from the densest parts of the nebular clouds like RCW 106 and they are notoriously difficult to study. Apart from obscuration by dust, another challenge is the brevity of an O-type star’s life. 

They burn through their nuclear fuel in mere tens of millions of years, while the lightest stars have lifetimes that span many tens of billions of years. The difficulty of forming a star of this mass, and the shortness of their lifetimes, means that they are very rare — only one in every three million stars in our cosmic neighbourhood is an O-type star. None of those that do exist are close enough for detailed investigation and so the formation of these fleeting stellar giants remains mysterious, although their outsized influence is unmistakeable in glowing H II regions like this one.

Notes

[1] The catalogue was compiled in 1960 by three astronomers from the Mount Stromlo Observatory in Australia whose surnames were Rodgers, Campbell and Whiteoak, hence the prefix RCW.

[2] The supernova remnant is SNR G332.4-00.4, also known as RCW 103. It is about 2000 years old. The lower filaments are RCW 104, surrounding the Wolf–Rayet star WR 75. Although these objects bear RCW numbers, detailed later investigations revealed that neither of them were HII regions.

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