Credit: X-ray: NASA/CXC/Univ. of Amsterdam/N.Degenaar, et al.; Optical: NASA, ESA
A double star system has been flipping between two alter egos, according to observations with NASA's Chandra X-ray Observatory
and the National Science Foundation's Karl F. Jansky Very Large Array
(VLA). Using nearly a decade and a half worth of Chandra data,
researchers noticed that a stellar duo behaved like one type of object
before switching its identity, and then returning to its original state
after a few years. This is a rare example of a star system changing its
behavior in this way.
Astronomers found this volatile double, or binary, system in a dense collection of stars, the globular cluster Terzan 5, which is located about 19,000 light years from Earth in the Milky Way galaxy. This stellar duo, known as Terzan 5 CX1, has a neutron star (the extremely dense remnant left behind by a supernova explosion) in close orbit around a star similar to the Sun, but with less mass.
In this new image of Terzan 5 (right), low, medium and high-energy X-rays
detected by Chandra are colored red, green and blue respectively. On
the left, an image from the Hubble Space Telescope shows the same field
of view in optical light. Terzan 5 CX1 is labeled as CX1 in the Chandra
image.
In binary systems like Terzan 5 CX1, the heavier neutron star pulls material from the lower-mass companion into a surrounding disk. Astronomers can detect these so-called accretion disks by their bright X-ray light, and refer to these objects as "low-mass X-ray binaries."
Spinning material in the disk falls onto the surface of the neutron
star, increasing its rotation rate. The neutron star can spin faster and
faster until the roughly 10-mile-wide sphere, packed with more mass
than the Sun, is rotating hundreds of times per second. Eventually, the
transfer of matter slows down and the remaining material is swept away
by the whirling magnetic field of the neutron star, which becomes a millisecond pulsar. Astronomers detect pulses of radio waves from these millisecond pulsars as the neutron star's beam of radio emission sweeps over the Earth during each rotation.
While scientists expect the complete evolution of a low-mass X-ray
binary into a millisecond pulsar should happen over several billion
years, there is a period of time when the system can switch rapidly
between these two states. Chandra observations of Terzan 5 CX1 show that
it was acting like a low-mass X-ray binary in 2003, because it was
brighter in X-rays than any of the dozens of other sources in the
globular cluster. This was a sign that the neutron star was likely
accumulating matter.
Credit: NASA/CXC/Univ. of Amsterdam/N.Degenaar, et al.
In Chandra data taken from 2009 to 2014, Terzan 5 CX1 had become
about ten times fainter in X-rays. Astronomers also detected it as a
radio source with the VLA in 2012 and 2014. The amount of radio and
X-ray emission and the corresponding spectra (the amount of emission at
different wavelengths) agree with expectations for a millisecond pulsar.
Although the radio data used did not allow a search for millisecond
pulses, these results imply that Terzan 5 CX1 underwent a transformation
into behaving like a millisecond pulsar and was blowing material
outwards. By the time Chandra had observed Terzan 5 CX1 again in 2016,
it had become brighter in X-rays and changed back to acting like a
low-mass X-ray binary again.
To confirm this pattern of "Jekyll and Hyde" behavior, astronomers
need to detect radio pulses while Terzan 5 CX1 is faint in X-rays. More
radio and X-ray observations are planned to search for this behavior,
along with sensitive searches for pulses in existing data. Only three
confirmed examples of these identity-changing systems are known, with
the first discovered in 2013 using Chandra and several other X-ray and radio telescopes.
The study of this binary was led by Arash Bahramian of the
International Centre for Radio Astronomy Research (ICRAR), Australia and
was published in the September 1st, 2018 issue of The Astrophysical
Journal. A preprint is available here.
Two other recent studies have used Chandra observations of Terzan 5
to study how neutron stars in two different low-mass X-ray binaries
recover after having had large amounts of material dumped on their
surface by a companion star. Such studies are important for
understanding the structure of a neutron star's outer layer, known as
its crust.
In one of these studies, of the low-mass X-ray binary Swift
J174805.3–244637 (T5 X-3 for short), material dumped onto the neutron
star during an X-ray outburst detected by Chandra in 2012 heated up the
star's crust. The crust of the neutron star then cooled down, taking
about a hundred days to fall back to the temperature seen before the
outburst. The rate of cooling agrees with a computer model for such a
process.
In a separate Chandra study of a different low-mass X-ray binary in
Terzan 5, IGR J17480–2446 (T5 X-2 for short) the neutron star was still
cooling when its temperature was taken five and a half years after it
was known to have an outburst. These results show this neutron star's
crust ability to transfer, or conduct, heat may be lower than what
astronomers have found in other cooling neutron stars in low-mass X-ray
binaries. This difference in the ability to conduct heat may be related
to T5 X-2 having a higher magnetic field compared to other cooling
neutron stars, or being much younger than T5 X-3.
Both T5 X-3 and T5 X-2 are labeled in the image.
The work on the rapidly cooling neutron star, led by Nathalie
Degenaar of the University of Amsterdam in the Netherlands, was
published in the June 2015 issue of the Monthly Notices of the Royal
Astronomical Society and a preprint is available here.
The study of the slowly cooling neutron star, led by Laura Ootes, then
of the University of Amsterdam, was published in the July 2019 issue of
the Monthly Notices of the Royal Astronomical Society and a preprint is available here.
NASA's Marshall Space Flight Center manages the Chandra program. The
Smithsonian Astrophysical Observatory's Chandra X-ray Center controls
science and flight operations from Cambridge and Burlington,
Massachusetts. Source: NASA’s Chandra X-ray Observatory
Fast Facts for Terzan 5:
Scale: Each panel is about 3.1 arcmin (17 light years) across.
Category: Normal Stars & Star Clusters
Coordinates: (J2000): RA 17h 48m 4.8s | Dec -24° 46´ 45"
Constellation: Sagittarius
Observation Date: 17 observations from July 13, 2003 through July 20, 2014
Observation Time: 171 hours 37 minutes (7 days 3 hours 37 minutes)
Obs. ID: 3798, 10059, 12454, 13225, 13252, 13705, 13706, 14339, 14475-14479, 14625, 15615, 15750, 16638
Instrument: ACIS
References: Bahramian, A. et al., 2018, ApJ, 864, 28; arXiv:1807.11589v Degenaar, N. et al., 2015, MNRAS, 451, 2071; arXiv:1505.01862. Ootes, L.S. et al., 2019, MNRAS, 487, 1447; arXiv:1805.00610
Color Code: X-ray: Red: 0.3-1.2 keV Green: 1.2-2.0 keV Blue: 2.0-6.0 keV
Source: NASA’s handra X-ray Observatory
Fast Facts for Terzan 5:
Scale: Each panel is about 3.1 arcmin (17 light years) across.
Category: Normal Stars & Star Clusters
Coordinates: (J2000): RA 17h 48m 4.8s | Dec -24° 46´ 45"
Constellation: Sagittarius
Observation Date: 17 observations from July 13, 2003 through July 20, 2014
Observation Time: 171 hours 37 minutes (7 days 3 hours 37 minutes)
Obs. ID: 3798, 10059, 12454, 13225, 13252, 13705, 13706, 14339, 14475-14479, 14625, 15615, 15750, 16638
Instrument: ACIS
References: Bahramian, A. et al., 2018, ApJ, 864, 28; arXiv:1807.11589v Degenaar, N. et al., 2015, MNRAS, 451, 2071; arXiv:1505.01862. Ootes, L.S. et al., 2019, MNRAS, 487, 1447; arXiv:1805.00610
Color Code: X-ray: Red: 0.3-1.2 keV Green: 1.2-2.0 keV Blue: 2.0-6.0 keV
Distance Estimate: About 19,000 light years
