New clues to an old mystery?

MXB 1730-335, also known as “the rapid burster”, is a neutron star that is located in the Galactic globular cluster Liller 1 and swallows gas from a companion star. It is infamous for displaying a peculiar phenomena called type-II X-ray bursts. These brief, bright flashes of X-ray emission are likely caused by a short-lived increase in the amount of gas that falls onto the neutron star, but over 40 years after the discovery the exact nature of these tantalizing X-ray flashes remains unknown. One of the puzzles is that there are only two neutron stars in our entire Galaxy that exhibit these flashes; the other is “the bursting pulsar” GRO J1744-28.

The rapid burster exhibits accretion outbursts that lasts a few weeks and recur about every 100 days. It so happened that in 2015 October an outburst was anticipated at a time that both NuSTAR and XMM-Newton could observe the object. This provided the unique opportunity to leverage the strengths of both instruments — high sensitivity at soft photon energies (0.3-3 keV) for XMM-Newton and high sensitivity to reflection features for NuSTAR — to study this peculiar neutron star. To this end, rapid-burster expert and former Amsterdam/SRON PhD student Tullio Bagnoli designed a novel observing campaign with Swift to catch a new outburst and trigger observations with NuSTAR and XMM-Newton accordingly. Jakob analyzed these data and may have found new clues to the old, unsolved mystery of the origin of type-II X-ray bursts.

Accretion disks normally extend close to the surface of the neutron star. However, analysis of reflected X-ray light in the rapid burster reveals that the inner accretion disk is strongly truncated; it lies about a factor of 5 further away from the neutron star than is typically seen in other objects. A plausible explanation for this finding is that the rapid burster has magnetic field strong enough to prevent the accretion disk from coming closer to the neutron star. Since we obtained a similar result for GRO J1744-28, this could indicate that the type-II phenomenon is related to the magnetic field of the neutron stars.

Paper link: ADS
Press release: ESA
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Near-infrared (J,K) images of the Galactic globular cluster Liller 1 obtained with the GeMS camera mounted on the 8-m Gemini telescope in Chile. The inset shows a zoom of the core of the cluster, spanning 1.9 light year across. Image credit: F.R. Ferraro/E. Dalessandro (Cosmic-Lab / University of Bologna, Italy)

A very cool neutron star

HETE J1900.1-2455 is a neutron star that swallows material from a small companion star that a mass of only about 10% of our Sun. It was discovered in 2006 with NASA’s Rossi X-ray Timing Explorer and exhibits some exceptional properties.

Firstly, HETE J1900.1-2455 showed pulses of X-rays every 2.65 millisecond. This shows that the magnetic field of the neutron star is channeling plasma to its magnetic poles which are then heated and lighting up in X-rays. As the neutron star rotates around its own axis a dazzling 377 times per second, this bundle of X-rays sweeps across our line of sight like a light house. Approximately 10% of all neutron stars with low-mass companions show such X-ray pulsations. Secondly, unlike most neutron stars that are eating for only a few weeks at a time, HETE J1900.1-2455 continued to be active for over a decade. Until 2015…

In 2015 November, HETE J1900.1-2455 suddenly dropped off the radar of the Japanese X-ray detector MAXI, which is mounted on the International Space Station and continuously scans the X-ray sky. The sudden drop of X-ray emission indicated that this neutron star had finally stopped eating. To test this, we observed this neutron star with two X-ray satellites that are more sensitive than MAXI and can thus detected much fainter X-ray light, Chandra and Swift. Our observations were carried out a few months after it had disappeared from the daily MAXI scans.

We found that the neutron star had indeed peacefully gone back to sleep. The X-rays observed during quiescent episodes are usually due to heat that radiated by the neutron star. Somewhat surprisingly, we found that HETE J1900.1-2455 was much colder, about 600 000 degrees Celsius, than we typically see for neutron stars after they have been active for many years (>1 million degrees Celsius). The reason that neutron stars are so hot after long meals is because consuming gas generates energy that heats their interior.

The fact that our temperature measurement of HETE J1900.1-2455 was so low, despite 10 years of activity, places exciting constraints on its interior properties. In particular, it requires that the central, liquid part of the neutron star is strongly superfluid. A superfluid is very peculiar liquid that has zero viscosity and freely moves without experiencing any friction. In laboratory experiments on Earth, liquid helium can be made superfluid when it is cooled down to nearly zero temperature. It is quite amazing that in neutron stars superfluidity can be achieved at temperatures of nearly a million degrees Celsius.

The constraints on the intriguing interior properties of HETE J1900.1-2455 will become even stronger if the neutron star cools further down now that it has stopped eating. We therefore plan further temperature measurements of this neutron star in the future.

Paper: ADS link


Schematic representation of the structure of a neutron star.

Staring at a very dim X-ray binary

Some neutron stars that consume gas from a companion star generate much dimmer X-rays than the general population of X-ray binaries. Since the brightness scales with the amount of mass that is being devoured, it seems that in these sub-luminous X-ray binaries the neutron star is not eating much. There are two leading theories to explain why this may be happening, which breaks down to a supply or demand problem.

One obvious explanation might be that some neutron stars have old, small companion stars that are deprived of hydrogen (the most abundant element in the Universe) and supply only a limited amount of gas. Indeed, attempts to study the donor stars suggest that some of these neutron stars are being starved. However, there are also a few cases where the optical emission is bright and the transferred gas clearly contains hydrogen, implying that there shouldn’t be a supply problem.

An alternative explanation is that some neutron stars have little appetite and consume only a small amount of the gas that is offered to them. In particular, these neutron stars may have a relatively strong magnetic fields that is able to stop the gas supplied by the donor from falling on to the neutron star or perhaps spit much of it out. To test this idea, we performed an in-depth study of the sub-luminous X-ray binary IGR J17062-6143.

First, studying its X-ray reflection using the NuSTAR and Swift satellites, we found that the gas stripped from the donor star does not reach as close to the neutron star as  normally the case in X-ray binaries. Second, using the Chandra satellite we found hints of narrow X-ray emission and absorption lines that could indicate that gas is blown away from the neutron star. The picture that appears to emerge from our study is that the neutron star in IGR J17062-6143 has a relatively strong magnetic field that pushes the in-falling gas away. Moreover, as the neutron star is (rapidly) rotating, its magnetic field may blow a large portion of the in-falling gas away, much like the propellor blades of a chopper do.

We are going to carry out further tests of this scenario. In particular, we recently obtained sensitive radio observations with the Australia Telescope Compact Array to search for  additional evidence that this neutron star is spitting out a lot of gas. Moreover, we recently obtained an optical spectrum with the large (8-m diameter) Gemini South telescope in Chili, to test if this neutron star has a normal, hydrogen-containing companion star. We are eager to see what comes out of those new observations and if those allow us to conclusively solve the “supply or demand problem” for the neutron star in IGR J17062-6143.

Paper link: ADS


Artist impression of an X-ray binary with a neutron star that has a relatively strong magnetic field. Image credit: NASA.

Characterizing a new neutron star

Since the dawn of X-ray astronomy over 50 years ago, more than 150 neutron stars swallowing gas from a nearby (Sun-like) companion star have been identified in our Galaxy. Still, every year a few new neutron stars are discovered when they suddenly start to devour their unfortunate neighbours. Different telescopes and satellites are then used to characterize such a previously unknown X-ray binary.

In February 2015, the X-ray emission of an object named 1RXS J180408.9-342058 was suddenly found to have brightened by more than 3 orders of magnitude. It was known to be an X-ray binary since 2012 when it displayed a thermonuclear X-ray burst; a devastating burp from a dining neutron star. At the time, however, it seemed that the neutron star was only taking a mid-night snack and had gone back to sleep before we could point our telescopes to investigate its eating patterns. Luckily, when it awoke in 2015 the neutron star clearly had much more appetite and kept swallowing gas from its companion for several months. This provided ample opportunity to study it in high detail.

We used three different X-ray satellites, namely NuSTAR, Chandra and Swift, to chart the geometry of this X-ray binary and the table manners of its neutron star. NuSTAR is a particularly powerful tool to study X-rays reflecting off the gaseous disk that surrounds and feeds the neutron star. Leveraging this, we determined that we view the binary at an angle of about 30 degrees, and that the gas disk was extending very close to the neutron star. In turn, this shows that the neutron star’s magnetic field is relatively weak and not able to keep the accretion flow at a distance. With Swift and Chandra we collected X-ray data when 1RXS J180408.9-342058 was at its brightest, and this suggested that the neutron star was eating rather messy; hints of narrow absorption lines suggest that part of the gas flowing towards the neutron star was blown away in a disk wind.

Paper link: ADS


Artist impression of an X-ray binary. Image credit: NASA.

Accreting or cooling?

Some brave neutron stars are consuming gas from a big companion star that is about ten times as massive as the tiny neutron star itself. In these so-called Be X-ray binaries, the neutron star is typically in a very wide, eccentric orbit and only able to swallow gas from its companion when it’s making its closest approach. Usually this results in modest accretion outbursts. Occasionally, however, very bright and powerful “giant outbursts” are observed during which the neutron star has a much bigger banquet. X-ray binaries that harbor massive companion stars are younger than those with small companion stars (millions of years compared to billions of years). Furthermore, neutron stars accompanied by a massive star are more strongly magnetized (by a factor of 1000 or so) and rotate much slower (seconds versus milliseconds) than when they are fed by a small companion.

When neutron stars feed off a small companion, their 1-km thick crust is heated and cools on a timescale of years after a meal. This heating and cooling sequence provides very valuable information about the tantalizing interior of neutron stars, but there are many unanswered questions about these processes. For instance, it is not yet established how the spin rate and magnetic field strength of a neutron star affects its temperature changes. Given their widely different properties compared to neutron stars with small companions, searching for heating and cooling in Be X-ray binaries can potentially shed new light on these open questions. Giant outbursts should then provide the best opportunity for this quest, as we might expect a neutron star to become more severely heated (and hence more clearly cooling) when more matter is consumed.

Using the X-ray telescope on board of NASA’s Swift satellite, we followed the behavior two Be X-ray binaries, V0332+53 and 4U 0115+63, after their giant accretion outbursts. Interestingly, we discovered that the X-ray brightness of these neutron stars was higher in the months after they finished their meals than it was before they started eating. This could indeed be a sign of a heated crust! However, neutron stars with massive companions are generally more messy eaters than those with small donors; the quality of our data did not allow us to exclude the possibility that their X-ray emission was elevated because these neutron stars were still scraping for food after finishing their main course. Such behavior is in itself relatively unexplored and interesting to study. Our pilot project therefore warrants follow-up investigation to explore either of these exciting possibilities.

Paper link: ADS


Artist impression of a Be X-ray binary; a neutron star in a wide, eccentric orbit around a massive companion. Image credit: Walt Feimer, NASA/Goddard Space Flight Center.


A mathematical tool to study X-ray bursts

Neutron stars can rip off gas from a nearby companion star and pull this material towards them; a process called accretion. The material that is accreted on to the neutron star undergoes nuclear reactions that can cause detonations generating more energy than an atomic bomb. Such thermonuclear bursts are observed as brief, bright flashes of X-ray emission that last for seconds to hours and repeat on a timescale of hours to months.

Not surprisingly, these violent explosions can be destructive to the surroundings of a neutron star. Indeed, evidence has been accumulating that X-ray bursts have a profound effect on the  accretion flow that transports material toward the neutron star. For example, the explosions can cool, blow away, or accelerate the in-falling flow of material. One way to investigate this is by studying how the X-ray energy spectrum of the accretion flow changes during an X-ray burst. This is not an easy task, however, because the X-ray burst emission typically outshines that of the accretion flow. Small changes in the weak accretion emission are therefore swamped by the bright burst emission.

Mathematical techniques that involve decomposing complex data into matrices (e.g., “non-negative matrix factorization”) have been previously applied to reveal subtle changes in the X-ray emission of super-massive and stellar-mass black holes. Motivated by those successes, we investigated the applicability of such techniques to study changes in the accretion emission induced by X-ray bursts.

For this purpose we used high-quality data obtained with the NuSTAR satellite of a well-known neutron star X-ray binary and thermonuclear burster 4U 1608-52. Our case study revealed that these mathematical techniques can be a very powerful tool to reveal changes in the accretion emission that remain hidden in conventional spectral analysis. Applying these techniques to NuSTAR data is particularly promising, as this instrument can provide valuable information on the  accretion geometry, that helps interpret the results from the X-ray burst analysis.

Paper link: ADS


Artist impression of NASA’s NuSTAR mission (launched in 2012). Image credits: NASA.

Breaking model degeneracies

Neutron stars are sometimes referred to as “dead stars”, because they cannot burn nuclear fuel in their interior as other stars do. Nevertheless, neutron stars can generate energy by accreting gas from a companion star in an X-ray binary.

When a neutron star accretes material, its ~1 km thick crust is compressed. This compression induces nuclear reactions such as atomic nuclei capturing electrons and nuclear fusion reactions. Theoretical calculations and accelerator experiments (e.g., such as done at the nuclear superconducting cyclotron laboratory at Michigan State University), provide a handle on how much heat should be generated inside neutron stars as a result of accretion. These results can then be compared with astrophysical observations.

Neutron stars often feast on their companion only for a few weeks/months (in exceptional cases years), after which they typically sag for years/decades before becoming active again. It is during these periods of quiescence that heat radiation from the neutron star (which have a temperature of millions of degrees Celsius) can be detected with sensitive X-ray satellites such as Chandra, XMM-Newton, or Swift. Since neutron stars do not generate energy when they are not accreting, they will gradually cool as they radiate their heat away as X-rays. Excitingly, dedicated efforts have allowed to detect the cooling trajectories of 7 neutron stars by now.

In this area of research there is thus a direct interplay between astrophysical observations of neutron stars, theoretical calculations about their interior, and nuclear accelerator experiments that mimic the nuclear reactions that take place in their outer layers. Interestingly, our X-ray observations seem to suggest that neutron stars are heated to a larger extend than is currently accounted for by nuclear heating models. It appears that there is a puzzling source of extra energy generation in the outermost layers of the neutron star, referred to as a “shallow heat source“, the origin of which remains to be established.

So far, it appears that the amount of extra heating differs between sources. It is not clear, however, if this is due to their different outburst properties (e.g., length, duration, total accreted mass, accretion geometry), or relates to different neutron star parameters (e.g., rotation period, age, mass, radius). A powerful way to break these degeneracies would be to observe cooling curves for one particular source after different types of outbursts. If the shallow heating is different for each outburst, it is likely related to the accretion properties — else we need to seek its origin in the properties of the neutron star itself.

We took up this challenge by studying the prolific neutron star X-ray binary Aql X-1, which is active approximately once every year and exhibits a wide range of outburst properties. In particular, we exploited the unique flexibility and good sensitivity of the Swift satellite to study the temperature evolution of the neutron star after different outbursts. The first test was to prove whether cooling is actually observable for this particular neutron star, which definitely appears to be the case. Future observations of Aql X-1 aiming to study the cooling trajectories after different outbursts are therefore a very promising avenue to elucidate the origin of the shallow heat generation in neutron star crusts that puzzles astrophysical observers, physicists and nuclear experimentalists.

Paper link: ADS


Long-term activity of Aql X-1 as seen through daily monitoring observations with Swift/BAT (launched in 2005) and MAXI (launched in 2009). This illustrates the frequent activity of this neutron star X-ray binary. Figure from Waterhouse et al. 2016.