E 0102 : A mysterious isolated neutron star

E 0102 : A mysterious isolated neutron star

Reading Time: 11 minutes

Neutron stars are exotic remnants of supernova explosions of massive stars. Most rotating neutron stars have extraordinarily high magnetic fields. These pulsars very often have a binary companion star from which they steal stellar matter. Recently, a neutron star in a neighbouring galaxy was found to have neither a binary companion nor a strong magnetic field. Curiously, the position of the supernova remnant with respect to the gases thrown off at the time of supernova does not quite tally. The Nerd Druid investigates E 0102, the lonely offset neutron star.

Neutron stars

Neutron stars are supernova remnants that are not quite massive enough to form black holes1. They are extremely dense objects and have phenomenally strong gravitational fields. The pressure in the interior of a neutron star one of the highest found naturally in the universe, second only to the pressure inside protons. Temperatures within neutron stars right after formation can be as high as hundreds of billions of degrees, but escaping neutrinos cool the star down.

Schematic of pulsar, showing the rotation axis, the magnetic axis, and the twin beams.
Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines, the protruding cones represent the emission beams and the green line represents the axis on which the star rotates. Image and caption from Wikipedia.

Pulsars

Rotating neutrons stars that also have high magnetic field emit synchrotron radiation2. This radiation forms a beam along the magnetic axis of the neutron star; as the star rotates about its rotational axis–which may or may not coincide with the magnetic axis–these beams are observed at precise regular intervals. These are pulsars, celestial timekeepers that are highly accurate. Thanks to the beams, pulsars are easy to detect.

Optical mosaic image of the Crab nebula, thanks to the Hubble Space Telescope. The bluish glow from the central region is due to synchrotron radiation.
Hubble Space Telescope mosaic optical image of the Crab Nebula. The bluish glow from the central region of the nebula is due to synchrotron radiation.

Pulsars are almost always binary stars; their stellar companions are often main-sequence or giant stars that have not yet reached supernova, or, due to lack of mass, never will. Most pulsars are found in the Milky Way galaxy and its neighbourhood.

So, basically, pulsar neutron stars are social beings that have magnetic personalities, but you’re likely to get burnt if you venture too close. Clearly, they have issues.

Composite optical and X-ray image of the inner Crab nebula. The central part shows the pulsar wind nebula, with the red star in the centre being the Crab pulsar.
Composite optical/X-ray image of the Crab Nebula, showing synchrotron emission in the surrounding pulsar wind nebula, powered by injection of magnetic fields and particles from the central pulsar. Image and caption from Wikipedia.

Central Compact Objects (CCOs)

However, not all neutron stars are pulsars. There are some that have very low magnetic fields compared to their pulsar cousins. Called central compact objects (CCOs), these supernova remnants lack the active wind nebulae3 that are a telltale signature of pulsars. The pulsars PuppisA and CassiopeiaA (CasA), both located within the Milky Way, are excellent examples of CCOs.

Similarly, not all neutron stars have a binary companion. The Magnificent Seven, named after the spaghetti Western, are a group of seven isolated neutron stars. A decade ago, an eighth such star was found, and was promptly named Calvera, after the villain of the movie. Going by that logic, a ninth star might well be named Gabbar4.

It is quite rare to find neutron stars that both CCOs and isolated. Vogt et al have recently found one that is located outside the Milky Way. However, in doing so, they have uncovered a rather puzzling mystery.

E 0102

The object of interest is the supernova remnant 1E 0102.2-7219. E 0102 for short, it is one of the few oxygen-rich young supernova remnants (O-rich SNR) in the Magellanic clouds5, and the only low magnetic field isolated neutron star outside the Milky Way. E 0102 is a little more than 200,000 light years away from Earth, and is only about two thousand years old.

Chandra/ACIS X-ray image of E 0102. The blue and the purple are false-coloured fast-moving highly ionized gases. This forms a lightyears-wide shell around what is thought to be the origin of the supernova. Note the small blue dot south-south-west of the centre, and about midway to the outer ring.
Chandra/ACIS X-ray image of E 0102. The blue and the purple are false-coloured fast-moving highly ionized gases. This forms a lightyears-wide shell around what is thought to be the origin of the supernova. Note the small blue dot south-south-west of the centre, and about midway to the outer ring.

Observing E 0102 with Chandra and Hubble

Unlike pulsars, CCOs such as E 0102 cannot be detected by looking for synchrotron radiation. Instead, one has to rely on blackbody radiation from both the CCO and the supernova debris surrounding the CCO. Images from NASA’s Chandra X-ray Observatory (CXO, named after Indian astrophysicist S. Chandrasekhar) show that E 0102 is dominated by a fast-moving large ring-shaped structure (blue and purple in the X-ray image). This is, quite possibly, the forward envelope of the supernova blast wave. Ejected at supernova at speeds of millions of kilometres per hour, this shell is now, two thousand years later, approximately 12 light years wide. Older optical images from the Hubble Space Telescope show beautiful filaments of superheated highly ionized oxygen filaments (green in the optical image) within the shell.

Hubble Space Telescope (HST) and VLT/MUSE optical image of E 0102. The green filaments (HST) are highly-ionized oxygen-rich filaments of supernova ejecta. The bright red ring (MUSE) is slow-moving low-ionization gas surrounding an optical dark spot.
Hubble Space Telescope (HST) and VLT/MUSE optical image of E 0102. The green filaments (HST) are highly ionized oxygen-rich filaments of supernova ejecta. The bright red ring (MUSE) is slow-moving low-ionization gas surrounding an optical dark spot.

Position of E 0102 supernova remnant

Usually, one would expect, given the data available, that the supernova remnant should be right in the middle of the ring that Chandra observed. Unfortunately, as you can see from the X-ray image above, there is no corresponding X-ray source at the centre of the ring. There is, however, an X-ray point source slightly offset from the centre of this large ring, a bit towards the south-west. Given available data, that cannot be the remnant.

Vogt et al disagree. And they evidence, persuasive evidence. And to do that, they have moved beyond what NASA has had to offer.

ESO, VLT, & MUSE

The European Southern Observatory is an international collaborative effort at observing the southern skies. Established in 1962 with 5 members, ESO now boasts a roster of 15 member states that sponsor the installation, operation and maintenance of state-of-the-art telescopes, all located in Chile. Of them, the imaginatively named Very Large Telescope (VLT) is probably one of the most advanced optical telescopes available for research now.

The Multi Unit Spectroscopic Explorer (MUSE) is an advanced spectrograph attached to the VLT. Unlike older devices, MUSE provides high-end resolution in both spatial and spectral domains. Which basically means that it produces very clear pictures with sharp colours.

Observing E 0102 with MUSE

Instead of relying on old Hubble and Chandra data, Vogt et al pointed the VLT towards E 0102. New optical data from MUSE showed that, in addition to the highly ionized fast-moving large gaseous ring, E 0102 also shows a smaller low-ionization slow-moving gas ring (bright red in the optical image) that is about a sixth of the former’s size. The origin of this new ring cannot be explained from the optical picture alone.

Animation of the optical and X-ray channels of E 0102. Overlaying one on the other strongly indicates that the blue off-centre dot in the X-ray image is indeed the CCO.
Superimposing the Chandra X-ray image on top of the HST+MUSE optical one shows that the blue off-centre X-ray dot does indeed fall within the red optical gas ring, and that it is indeed the CCO.

The fun begins when you put the optical and X-ray images on top of each other. It is immediately obvious that the smaller red optical ring neatly encircles the small blue X-ray dot. This perfect coincidence convinced Vogt et al that the small blue dot was indeed the hitherto undiscovered supernova remnant they had been looking for.

Is that really the supernova remnant?

Apparent and absolute magnitudes

Well, sort of. Stargazing is a funny job. Since the stars are so far away, the night sky looks more or less like an upturned bowl, with the stars stuck to its underside. Take Venus and Sirius, for instance. Venus is the brightest point object in the night sky, while Sirius is the brightest star in the night sky. Naturally, the Sun and the Moon are disq-ualified.6 Now, going by apparent brightness, Venus outshines Sirius by a factor of 23.337. Actually, of course, Venus is a planet, while Sirius is a really bright star. This is evident in their actual luminosities : Sirius is 22.5 times as luminous as the Sun, which in turn is 550 million times brighter than Venus.

Constellations and distances

The same confusion holds for distances. Constellations are groupings of stars that are close together. For instance, Orion (Kalpurush), one of the most distinctive and conspicuous constellations in the night sky, has seven major stars. Of these, the two brightest are Rigel, a blue-white supergiant, and Betelgeuse, a red supergiant. The most distinctive feature of the constellation is the belt; Alnitak, Alnilam and Mintaka are the three stars that make up Orion’s belt. All these stars, being part of the same constellation, seem to lie close to each other on the same plane.

Photograph of Orion's belt. On the northeast (top right) is Mintaka, on the southwest is Alnitak, and at the centre is Alnilam, the brightest of the trio.
Orion’s belt. On the northeast (top right) is Mintaka, on the southwest is Alnitak, and at the centre is Alnilam, the brightest of the trio. Image credit : Davide De Martin (http://www.skyfactory.org); Digitized Sky Survey, ESA/ESO/NASA FITS Liberator.

The operative word is, naturally, seem. Here is how far the stars actually are from Earth (lyr = lightyear) :

  1. Betelgeuse = 640 lyr
  2. Rigel = 863 lyr
  3. Alnitak = 800 lyr
  4. Alnilam = 1340 lyr
  5. Mintaka = 915 lyr

Clearly then, if we move away from Earth, the constellations will not retain their familiar shapes.

So, is it the CCO or not?

The same issue is relevant when it comes to E 0102’s CCO. Wouldn’t it be possible that the blue X-ray point source simply lies in the background of the red low-ionization ring of gas. Is it impossible to imagine that the extraordinary evidence provided by MUSE might simply be chance alignment?

Well, after four paragraphs and 300 words, you would be entitled to a detailed explanation. Sadly, Vogt et al simply say that the area of the structure seen by MUSE is too small. They calculate that there might be at most 8 × 10-5 sources in the background that are as bright or brighter.

Rather compelling, that number, ain’t it.

What’s more, the MUSE data does not provide any evidence for active pulsar wind nebulae, or the presence of a binary companion to the supernova remnant. Vogt et al can very well claim to have found the first truly isolated CCO outside the Milky Way.

Which is when all hell breaks loose.

The E 0102 offset mystery

The problem, of course, is that the E 0102 CCO does not quite lie at the centre of the large gas ring.

A bit of firecrackers

Picture a firecracker going off in the night sky. Imagine one of those expensive ones that, um, look good and are made well. Soon after explosion, the expanding sparkles form a ball, while whatever part did not explode (there’s always some) stays more or less at the centre of this ball. Of course, gravity soon makes all of this come crashing down onto the Earth.

Now imagine you are in charge of firecrackers on a cricket night, and you are a bit short on funds. You buy perhaps one of those fancy firecrackers and scrounge the bottom of the barrel for the rest. Your idea is to impress people with that one good one, the one that looks very symmetric, and then quickly pass the other asymmetric ones under the radar. You succeed, for there are people in the audience that are clearly more interested in the badly made ones, the ones where the explosion remnant doesn’t quite remain in the middle of the ball.

Photograph of fireworks at the Sydney Harbour Bridge, taken on New Year's Day 2017. Image credit : Saeed Khan.
Fireworks at the Sydney Harbour Bridge, taken on New Year’s Day 2017. Image credit : Saeed Khan.

Why does that happen? Well, simply because the explosive mixture in the cracker wasn’t uniform or homogeneous enough. There might have been bumps or hollows, or some parts might have had more or less of one component than necessary for the correct amount of burn. Essentially, the firecracker must have had some asymmetry to begin with.

How a supernova works

A similar specific set of circumstances might have been applicable with E 0102. Here’s how a supernova works : at the end of its life, massive stars8 throw off most their stellar material in a titanic explosion. What remains behind is the superdense core; if massive enough, it forms a black hole; if not, neutron star. The stellar material is thrown off at the rate of thousands of kilometers a second. This translates into a speed of a few lightyears every thousand years. The gaseous material thrown off forms a spherical shell around the supernova remnant, very much like the fast-moving large ring of high-ionization gas in E 0102 as detected by Chandra’s Advanced CCD Imaging Spectrometer (ACIS).

Two scenarios

Had E 0102 behaved like that good high-end expensive firecracker, its supernova remnant should have been right at the centre of this big shell. However, given the large offset, clearly this isn’t the case. This leaves us with two scenarios :

  1. The supernova was indeed centred at the centre of the large X-ray ring, and that, somehow, the supernova remnant has migrated to where it is now.
  2. The supernova was centred at the centre of the smaller optical ring, more or less where the CCO is now, and somehow the larger gas envelope got distorted.

Both scenarios are plausible. Both scenarios are problematic. Which is of course what makes this such a delicious mystery.

Supernova remnant migrated

Optical image (VLT/MUSE) of the centre of large gas shell (white +) and CCO position (white crosshairs) in E 0102. Image taken from Vogt et al, arXiv:1803.01006
Optical image (VLT/MUSE) of the centre of large gas shell (white +) and CCO position (white crosshairs) in E 0102. The black bar in the key equals 3 parsec or 9.78 lightyear. Image taken from Vogt et al, arXiv:1803.01006

For

The greenish blob is optical light from the oxygen-rich gases floating around E 0102. The small white + is where the centre of this expanding mass of gas should lie. The white crosshair marker slightly southwest of the centre marks the position of the CCO. As you can see, that portion does not have any oxygen. The scale below converts between seconds of arc and parsecs (pc); the thick black line is equal to 3 pc or 9.78 lyr. If you take a ruler and measure it, you’ll find that the distance between the centre and the CCO is approximately 1.8 pc or about 6 lyr.

6 lightyear! That is a lot of distance to cover. If our first scenario is correct, and the stellar core was indeed at the white + when the progenitor star went supernova, then it must have given a mighty kick to the CCO for it to have travelled 6 lyr in 2000 yr. However, the radius of the green blob is almost 4.5 pc (= 14.5 lyr). Also, 6 lyr in 2000 yr equates to a speed of about 9000 km/s (about 3% lightspeed), which is about what a supernova is capable of. The idea of a massive kick seems legit.

Against

However, there is a but. Where did the low-ionization slow-moving gas ring come from? The smaller ring is centred on the current position9 of the CCO, which means that somehow, the CCO must have reached the crosshair position, stopped (which is impossible), ejected a whole lot of gas (also impossible), and then waited as that gas very quickly spread to a diameter of a lightyear.

The timeline doesn’t match. Nor do the evidences tally with one another. And the physics is quite impossible. Stellar cores post-supernova do not have any more gas to expend. Yet we have a new gas ring. Optical analysis proves that this gas is slow-moving. Yet the timeline demands that it move ultrafast. Also, how in the world would a stellar core moving at 0.03c brake?

Hmm, trouble. Shall we try the alternate scenario, then?

Large gas ring distorted

Optical image of the inner gas ring (pink) and CCO position (crosshairs) in E 0102. Image taken from Vogt et al, arXiv:1803.01006
Optical image of the inner gas ring (pink) and CCO position (black crosshairs) in E 0102. The black bar in the key equals 3 parsec or 9.78 lightyear. Image taken from Vogt et al, arXiv:1803.01006

For

In the second scenario, the supernova didn’t kick the remnant away, and the CCO has been drifting more or less at its present position, as indicated by the black crosshair. The pink shell of low-ionization gas almost perfectly, but not exactly, encircles the CCO. Vogt et al show that the CCO has drifted only about 0.36 pc (= 1.17 lyr) from the centre of the smaller ring, thereby setting an upper limit for its drift velocity to be 170 kms-1, a far cry from 9000 km-1 in the kick scenario.

The pink inner shell could well have been ejected at supernova. Alternatively, the progenitor star might have been ejected it a few thousand years before supernova. This indicates that E 0102 might have been unstable, a variable star that would shed stellar matter periodically.

Against

Which begs the question : why did the bow shock of the supernova not blow this inner ring away? Also, and this is a bigger problem, the explosion site is now located well away from the centre of the X-ray emissions. In the image below, the white crosshair again marks the position of the CCO, while the bright bands of white, red, blue and some green make up a false-coloured image of the Chandra ACIS X-ray data.

X-ray image (Chandra/ACIS) of the outer gas ring and CCO position (white crosshairs) in E 0102. Image taken from Vogt et al, arXiv:1803.01006
X-ray image (Chandra/ACIS) of the outer gas ring and CCO position (white crosshairs) in E 0102. The black bar in the key equals 3 parsec or 9.78 lightyear. Image taken from Vogt et al, arXiv:1803.01006

According to Vogt et al, this might have been possible due a very specific set of circumstances. For instance, before exploding, stellar wind from the progenitor might have skewed the direction of the shock wave so that it is now off-kilter–a bit like one of those cheap firecrackers. Alternatively, variations in density in interstellar dust and gas might have altered the flow of the ejecta, so that some parts would move faster than the others. Thinking in terms of electricity10, the interstellar medium, though a vacuum, would have enough dust and gas to be inhomogeneous enough to distort the ejecta.

A specific set of circumstances

Which begs the question : how is the outer ring so round and regular?

Vogt et al attribute this to a specific set of circumstances. Essentially, it still is a mystery. However, with increased precision analysis of MUSE data, Vogt et al are confident they will have the answer for us. Until that time, here is a composite image of the Crab Nebula, showing infrared (Spitzer), optical (Hubble), and X-ray (Chandra) bands. You can make out the remnant as a bright blue-white dot near the centre.

Chandra (X-ray), Hubble (optical), and Spitzer (infrared) composite image of the Crab Nebula. Credit : X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz.
Chandra (X-ray), Hubble (optical), and Spitzer (infrared) composite image of the Crab Nebula. Credit : X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz.

 

References

Papers

  1. Vogt et al. : Identification of the Central Compact Object in the young supernova remnant 1E0102.2-7219, arXiv (2018)

Articles

  1. Press Release : Astronomers Spot a Distant and Lonely Neutron Star, Chandra X-ray Observatory (2018)
  2. PTI/Firstpost : NASA discovers rare neutron star outside of Milky Way and releases a stunning image of it (2018)

The Great Telescopes and their instruments

  1. Hubble Space Telescope (NASA)
  2. Chandra X-ray Observatory (NASA)
  3. Very Large Telescope (ESO)
  4.  Spitzer Space Telescope (NASA)

Image sources

  1. Composite, Optical, and X-ray images of E 0102 : Chandra X-ray Observatory
  2. Other images linked directly
Facebook Comments

Footnotes

  1. Stellar cores need to have at least 2.17 times the mass of the sun for them to become supernova. This is the TOV limit.
  2. Synchrotron radiation is that emitted by accelerating charged relativistic particles. Radiation from accelerating non-relativistic charged particles is called cyclotron radiation.
  3. The immense magnetic field of the spinning pulsar sends plasma particles shooting out at relativistic speeds into the surrounding interstellar medium, creating an active pulsar wind nebula. Bit like solar wind, only much stronger.
  4. Sholay
  5. Close neighbours of the Milky Way, the Large and the Small Magellanic Clouds are small irregular galaxies a fraction its size. PuppisA and CasA are E 0102’s O-rich SNR counterparts in the Milky Way.
  6. Sorry :)
  7. The apparent magnitudes of V and S are 4.89 and 1.47 respectively. The difference is 3.42. Thus the factor is 100.4×3.42 ~= 23.33
  8. Stars whose core mass is greater than the Chandrasekhar limit, 1.4 times the mass of the sun
  9. This object is 200,000 lyr away from us, so any light (optical or X-ray) that we receive from it it is 200,000 yr old. The word current is used in that context
  10. Though that is a horrible analogy here

Leave a Reply

Your email address will not be published. Required fields are marked *