A New Puzzle from NuSTAR: most wildly luminous Neutron Star ever discovered, or a very new form of Black Hole?

By Greg Madejski

The concept of a black hole seems to be shrouded in mystery, perhaps partly because of the enigmatic name, but in reality it is a very simple one:  a black hole is an object containing an enormous quantity of mass shrunk down to a tiny volume - so much so that the speed required to escape the pull of this compact object’s gravity would exceed even that of light.  

From Small to Large -- but what about in between?

Most astronomers believe that astrophysical black holes come in two sizes:  the small ones, only weighing up to a few dozen times as much as our Sun, and the big ones, weighing in at a million to a billion times heavier than the Sun. The latter are normally described as, unsurprisingly, “supermassive.”  We know how the small fry are formed:  they are the results of the collapse of massive stars perhaps 30 to a 100 times more massive than the Sun.  On the other hand, the formation history of supermassive black holes is less clear to us, although we do have very clear evidence for their existence.  But what about black holes with masses between these two extremes?  Do they exist?  

Let’s take a closer look at one of these small black holes. The artist’s rendition above shows (in blue and white) a normal star being steadily stripped of its gas by its black hole companion. As the gas falls into an “accretion disk” (shown in red, yellow and white) it heats up to high temperatures, and so emits radiation which we can detect with our X-ray telescopes. About a decade ago, astronomers noted that several nearby galaxies contained some very bright X-ray sources.  For these sources, we know that the higher the rate of matter flowing onto the black hole, the greater the brightness of the source, since the energy is ultimately supplied by the release of the gravitational energy of material.  And knowing the distances to these galaxies, the apparent brightnesses could be directly translated into the total amount of power generated by each source. Their numbers came out so high that they were named “ultra-luminous X-ray sources” (ULXs), and we had a new puzzle: how can the ULXs be so bright?

One limit even Black Holes must obey

The problem is that it’s hard to accrete a lot of gas when you’re shining so brightly. In fact there is a limit to the luminosity of an object powered by accretion, called the “Eddington Limit.” As gas falls in, the accretion disk gets brighter and hotter, and more radiation is emitted - but this radiation exerts an outward pressure on the infalling gas, slowing it down and in turn reducing the luminosity of the disk. The Eddington Limit is the luminosity of the object when the outward radiation pressure just balances the inward gravitational force, and so is normally the maximum power that the object can emit. The Eddington Limit depends on the object’s mass: the higher the mass, the brighter the disk can be, and the higher the Eddington Limit. It was first calculated from basic physical constants about 100 years ago, and turns out to be directly proportional to the mass of the star.  This means that we can place a lower limit on an object’s mass just by measuring its luminosity: the object could be more massive and shining at less than maximum power, but it’s hard for it to be less massive and shine brighter than the Eddington Limit.  The Eddington accretion limit is obeyed by all known neutron stars (which are superdense objects composed primarily of neutrons -- something like macroscopic atomic nuclei -- which result from gravitational collapse of some classes of highly massive stars.)

And so here came the surprise:  the ULXs are generally variable, changing their brightness on timescales of days implying that they are highly compact, like neutron stars or black holes. But to be so luminous, and still obey the Eddington Limit, the masses of the ULXs would have to be not stellar-sized at all, but instead greater than hundreds or in some case thousands that of our Sun. Since by the laws of physics neutron star cannot be more massive than roughly three times the Sun (see e.g. A Mad Ballerina Consumes Her Companion), a compelling ULX candidate might instead be an accreting black hole that is roughly a thousand times more massive than the Sun.  How these “intermediate mass black holes” could ever be formed is in fact even more of a mystery than that of the small and large mass black holes discussed earlier.  

Breaking the law

There are some (perhaps slightly contrived) ways to avoid the Eddington Limit. For example, the radiation could be being emitted from material that is preferentially streaming in our direction, which would mean that Eddington’s assumption of the source’s power being uniformly emitted in all directions was incorrect.  Possibilities like this “beaming” meant that not everyone bought into the intermediate mass black hole picture - and these possible loopholes certainly have to be taken into account when looking at new data.  

Going to the "ultra max" -- the most anomalous source

Discoveries sometimes happen in places where they are least expected.  In the middle of 2014, a star was observed that had exploded 3 million years ago in a ‘nearby’ galaxy known as M82 (or the “Cigar Galaxy”).  The “supernova” explosion, labelled SN2014j, was noted by many astronomers, with the published report by Ariel Goobar from Stockholm University (Astrophysical Journal, Vol. 784, L12, or arxiv version here) using optical telescopes. Supernovae generate millions of times more power during and after the main explosion than an ordinary star does (but with this level of ‘afterglow’ luminosity lasting only for a few months).  Since such explosions in the local Universe are relatively rare, many telescopes with sensitivity in all spectral bands started to observe SN2014j.  One of those was an instrument mounted on the NASA satellite NuSTAR, launched just over two years ago by a team led by Prof. Fiona Harrison at Caltech.  NuSTAR is sensitive in the X-ray band, and was hoping to reveal the properties of the “shrapnel” thrown off by the SN 2014j into the interstellar medium of M82.  

NuSTAR was pointed towards M82 and stared and stared for a whole month, and just barely detected SN 2014j.  In the same field, a tiny bit away from where one would expect the SN 2014j remnant to be, NuSTAR saw another bright source.  This by itself wasn’t surprising:  this object was a previously known as a ULX, producing a total power of at least 100 million times more than our Sun.  The real surprise came from looking at the pattern of change of the X-ray brightness of this source with time.  It appeared to be pulsing at very precise intervals, once every 1.3 seconds!  This discovery was published by the NuSTAR team in early October in the journal Nature (Bachetti et al. 2014, Nature, 514, 202, or Arxiv here).  

To get some sense of how precisely these periods can be measured: the figure above shows detections of the pulse period (black points) of the ULX with a mean period of  1.37252266 +/- 0.0000012 seconds lying on top of a curve indicating a binary orbit with a companion with an orbital modulation period of 2.51784 +/- 0.00006 days.  (The lower panel shows pulsed flux as a fraction of the emission from the region around the ULX, see Fig 1 in the Bachetti paper for more details for this panel).

The strictly periodic pulsations by themselves were not very surprising:  neutron stars like those mentioned earlier often pulsate, at precise intervals.  These “pulsars” are like strongly focused beacons; every time their axis swings towards us, we get a pulse of radiation. The problem is still the brightness of the ULX: even taking the beaming into account, neutron star pulsars can’t be as massive as the Eddington limit for the ULX implies.  It would have to be more than 50 times the mass of the Sun!  

So, if the Eddington limit is to be obeyed, the only option is for the ULX object to be a black hole. But, on the other hand, we know of no obvious mechanism which would make the black hole pulsate with such great precision.  It’s a real puzzle.

Pulsating Black Hole, Ultra-luminous Neutron Star, or something else?!

The NuSTAR observation is a major discovery, and is in tension with our current understanding of neutron stars and stellar-size black holes.  What gives?  Do we have an incomplete understanding of the physics governing neutron stars?  Or is that black holes can act as very precise flywheels, just like pulsars - something that was not at all expected.  Of course, the NuSTAR team must be combing through their observations of other ULX sources to search for periodicity in other sources.  Is the ULX in M82 unique, or is the intermediate mass black hole scenario to explain all ULXs incorrect?  

In this case, SN 2014j provided an excuse for the observation, but the real result was finding the incredible properties of the nearby object.  For sure, it was the advanced technology of the NuSTAR satellite allowing precise timing with great sensitivity over a broad X-ray band, but I would give the real “kudos” to the NuSTAR team for the careful look at their data.  Discoveries such as this one are not common, but their unexpected and sudden appearance is a big part of what continues to make astronomy such an exciting endeavor to be involved in, for the astronomers, and pay attention to, by all!