Tag Archives: UV

A windy surprise

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Binary star systems that contain a neutron star are important for probing fundamental theories of physics and for studying a large variety of astrophysical processes. For instance, the most energetic explosive phenomena seen in the cosmos, such as supernovae, kilonovae, gamma-ray bursts, gravitational wave mergers and fast radio bursts, often involve neutron stars in binary systems. Furthermore, they serve as an important testbed for Einstein’s General Relativity Theory, and binaries containing neutron stars are also excellent laboratories to study the behavior of cold ultra-dense matter. Finally, studying populations of binaries with neutron stars further allow us to several key processes of stellar evolution.

A particularly important phase in the life and evolution of neutron stars in binary systems is when the neutron star accretes mass from its companion star. This is when the system manifests itself as an X-ray binary. However, neutron stars do not only swallow gas, they also blow matter and energy back into space via outflows. These can be observed as highly collimated streams that are called jets and thought to be shot out with velocities of tens to hundreds of thousands of kilometers per second, or dense winds that have a larger opening angle and travel at lower speeds of a few hundreds to thousands of kilometers per second.

As in any astrophysical system where accretion takes place, outflows are ubiquitous among neutron star X-ray binaries. However, two key aspects of jets and winds are not understood yet: how these outflows are actually launched and how much mass can be lost from the binary in this way. Determining the mass loss is important, for instance, for understanding how long it will take for the neutron star to close in on its companion star and eventually collide with it, generating a burst of gravitational waves. The amount of mass contained in a wind is closely related to the mechanism that drive the wind.

Studies of X-ray binaries containing black holes have shown that disk winds are likely driven by thermal processes: X-rays produced in the inner parts of the accretion disk heat the outerparts of the disk, causing these to puff up. If the disk is large enough, the gas may at some point in the disk puff up to such an extent that it’s able to escape the gravitational pull of the black hole and flow away as a disk wind. Based on theoretical knowledge, it is expected that black hole X-ray binaries should have orbital periods of more than 8 hours to be able to have large enough disks to launch thermal winds. So far, this was consistent with observations, since disk winds have almost exclusively been detected in X-ray binaries with orbital periods exceeding 8 hours.

Analysing far-UV spectra of a very small neutron star X-ray binary called UW CrB, with the aim to understand the composition of its accretion disk, we surprisingly discovered features of a wind. Since the orbital period of this binary is only 2 hours, it should not be able to launch a thermal wind. Based on this observational discovery, we performed preliminary simulations and actually found that the X-rays emitted from the surface of the neutron star make it possible to drive a wind from smaller accretion disks than would be possible in black hole X-ray binaries (since black holes to have a surface where they can emit X-rays from). The wind in UW CrB does remain mysterious, since it was detected in only a fraction of the data that we analysed. This suggests that winds can potentially switch on and off on a time scale of hours, which was not previously known.

To establish the nature and time-variability of the wind in UW CrB, we have been granted time on several big observing facilities: the space satellites Hubble Space Telescope, XMM-Newton and Swift, as well as the optical/near-infrared Very Large Telescope (VLT, in Chile) and Grantecan (on La Palma). It was a huge challenge to figure out at what exact time all these telescopes could point to UW CrB at exactly the same time, but this ambitious and exciting observing campaign is happening in mid July. Stay tuned for the outcome!

Fijma, Castro-Segura, Degenaar, Knigge, Higginbottom, Hernandez Santisteban, Maccarone 2023, submitted to MNRAS: A transient ultraviolet outflow in the short-period X-ray binary UW CrB

Paper link: ADS

Hubble Space Telescope far-UV lightcurve (left) and a Zoom of the spectrum (right) around the Si-iv emission line (at 1402 Angstrom). The Si line in the right plot shows a P-Cygni profile, which is the hallmark of an outflowing wind. However, this wind feature was seen in only part of the observation, namely in the time interval colored red in the left plot.

Gone with the wind

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The discovery of a persistent UV outflow from a neutron star.


X-ray binaries consist of a neutron star or a black hole that are accompanied by another star (e.g. one like our Sun, a red giant, or a white dwarf). Neutron stars and black holes are not friendly neighbors, however, and will relentlessly rip gap from their companion and swallow it. This cannibalistic process is called accretion. At the same time, some of the gas inswirling is propelled back into space through dense winds or highly collimated jets.

The most common signatures of outflowing material from astronomical objects are associated with “warm” gas. Despite this, only winds of “hot” or “cold” gas have been observed in X-ray binaries… until now! In this new study, we observed the recent accretion eruption of the X-ray binary known as Swift J1858 with a menagerie of ground-based and space-based observatories, including NASA’s Hubble Space Telescope (HST), the European Space Agency’s XMM-Newton satellite (XMM), the European Southern Observatory Organisation’s Very Large Telescope (VLT) located in Chile and the Spanish Gran Telescopio Canarias (GTC) located at La Palma (Canary Islands).

The results of our campaign, which was a joint effort of a team of researchers from 11 countries and was published in the journal Nature, showed persistent signatures of a warm wind at ultraviolet wavelengths occurring at the same time as signatures of a cold wind at optical wavelengths and hints of a hot wind at X-ray wavelengths. This is the first time that winds from an X-ray binary have been seen across different bands of the electromagnetic spectrum. This new discovery provides key information about the messy eating patterns of these cosmic cookie monsters. It allows us, for instance, to better understand how much gas is blown away in winds and by what mechanism winds are produced.

Designing the an ambitious observing campaign, built around the best telescopes on Earth and in space, was a huge challenge. This is mainly because it requires coordinating different observatories located at different parts of the Earth and space to look at your target all at the same time. So, it is incredibly exciting that all this work has paid off and allowed us to make a key discovery that would not have been possible otherwise.

Some press coverage: Independent

Castro-Segura et al. 2022, Nature 603, 52: A persistent ultraviolet outflow from an accreting neutron star binary transient

Paper link: ADS

Artist’s impression of a wind blown from the inner part of the accretion disk around a neutron star devouring gas from a companion. Image credit: Gabriel Pérez (IAC).

Changing emission mechanisms

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X-ray binaries, in which a neutron star or black hole swallows gas from a companion star, have been known since the dawn of X-ray astronomy in the 1960s. With decades of studies, using many different space-based and ground-based telescopes across the electromagnetic spectrum, we have learned many things about how neutron stars and black holes accrete gas. Nearly all of this knowledge has been assembled for stages during which X-ray binaries are rapidly accreting gas, making them shine bright at all wavelengths and hence can easily be studied. However, most X-ray binaries spend only a fraction of their time being bright and rapidly accreting; the far majority of their time their gas consumption occurs at a very low level. Because X-ray binaries are much dimmer when accreting slowly, it is much more challenging to study them. Our standard  models predict that accretion proceeds very differently at low rates, but we have hardly any observational constraints to test and further develop models of low-level accretion.

There are many different components in an X-ray binary that emit electromagnetic radiation. One of the prime emission components is the accretion disk that, depending on its physical properties like its temperature, radiates at X-ray, UV, optical and near-infrared wavelengths. At low accretion rates, however, theory predicts that this disk evaporates into a more extended hot flow that may emit energy at the same wavelengths as the disk. Apart from the accretion stream, be it a disk or a hot flow, the companion star also emits UV, optical and near-infrared emission  (depending on what type of star it is and whether it is being irradiated by the accretion flow), whereas X-ray binaries also launch jets that can be detected at radio and near-infrared wavelengths, but possibly also in the optical, UV and X-ray bands too. Disentangling the different emission components, and finding which one(s) dominate(s) the total observed emission, can be a powerful way to obtain details about the accretion process. This is not an easy task, however, because for each different wavelength we (generally) need different telescopes and it’s very challenging to coordinate different observatories.

The Swift satellite is a very important observatory to study X-ray binaries. This is for two reasons. Firstly, it is a relatively small satellite that can easily maneuver around, allowing us to take frequent snapshots of sources (which is not possible for bigger satellites). Secondly, Swift carries both an X-ray telescope and a UV/optical telescope, which can observe an astrophysical object at the same time. Combined with its good sensitivity, this makes that Swift is a powerful tool to study how the accretion flow in X-ray binaries changes when it moves (quickly!) from high to low accretion rates. We attempted such a dedicated study for the well-known neutron star X-ray binary Aquila X-1 (aka Aql X-1).

Using Swift data from the NASA archives that covered three different accretion outbursts of Aql X-1, we studied how the X-ray, UV and optical emission changed as the source evolved between high and low accretion rates. We found that the X-ray and UV/optical emission always change together, but that this happens in a different manner when Aql X-1 is bright than when it is fading. This implies that the dominant mechanism producing UV and optical emission changes during the decay of an outburst, as is expected from accretion theory. It might be a hint that an accretion disk is changing into a hot flow, or that the properties of the accretion disk are changing otherwise. Moreover, we found that the UV and optical emission behaves differently during the rise of an outburst than during the decay. This suggests that the accretion flow may have different properties at the start and the end of an outburst. This investigating has exposed some interesting behavior that warrants follow-up by performing similar studies for other X-ray binaries or using additional observatories.

López-Navas, Degenaar, Parikh, Hernández Santisteban, van den Eijnden 2020, MNRAS 493, 940: The connection between the UV/optical and X-ray emission in the neutron star low-mass X-ray binary Aql X-1

Paper link: ADS

14U_fits

The X-ray and UV flux of Aql X-1 over an entire outburst. It can be seen that the emission at the two wavelengths is coupled in a different manner when the source is bright than when it is faint. Moreover, the UV flux is fainter during the decay (yellow/green points) than during the rise (blue/purple points) of the outburst.