Tyler Brooks
X-ray bursters are a class of X-ray binary stars that exhibit rapid and periodic increases in luminosity, known as X-ray bursts. These bursts typically exhibit a sharp rise in luminosity followed by a gradual decline and can recur on timescales ranging from hours to days. The behaviour of X-ray bursters is similar to recurrent novae, with the compact object being a neutron star instead of a white dwarf. Due to their relatively predictable luminosities, X-ray bursters, specifically Type I X-ray bursts, have been proposed as potential standard candles for distance measurements. Standard candles are objects with known luminosities, which can be used to determine distances by comparing their observed brightness to their expected brightness. While there is some promise for using X-ray bursters as standard candles, especially those exhibiting photospheric radius expansion, there are also challenges due to deviations and uncertainties in the data. Further research and improvements in X-ray imaging telescopes will help refine our understanding of X-ray bursters and their potential as standard candles.
| Characteristics | Values |
|---|---|
| Definition | X-ray bursters are a class of X-ray binary stars that exhibit rapid and periodic increases in luminosity, known as X-ray bursts. |
| Composition | An accreting neutron star and a main sequence companion 'donor' star. |
| X-ray Burst Types | Type I (caused by thermonuclear runaway) and Type II (arising from the release of gravitational energy through accretion). |
| Burst Periodicity | Most X-ray bursters have irregular burst periods ranging from a few hours to several months, influenced by factors such as stellar masses, distances, accretion rates, and composition of accreted material. |
| Type I Characteristics | Sharp rise followed by a slow luminosity decline. |
| Type II Characteristics | Quick pulse shape with multiple fast bursts separated by minutes. |
| Observed Instances | Type I bursts are more commonly observed, with Type II bursts seen from only two sources. |
| Anomalies | Quasi-periodic oscillations and dips in the burst lightcurve shape have been observed without a definitive explanation. |
| Standard Candle Potential | The average peak luminosity of Type I X-ray bursts has been proposed as a standard candle, aiding in deriving distances and radii of neutron stars. |
| Standard Candle Value | A "standard candle" value of erg s-1 was determined for Type I bursts, with deviations of up to 15% observed. |
| Critical Luminosity | The maximum peak luminosities during photospheric radius expansion bursts reached a critical value of (3.79+/-0.15)x10^38 erg/s, suggesting these bursts as empirical standard candles. |
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What You'll Learn
X-ray bursters are a class of binary stars
However, as accretion continues, a degenerate shell forms, causing a rise in temperature without a relief of thermodynamic conditions. This shift favours the triple-α cycle, resulting in a helium flash that provides additional energy for the CNO burning to transition into thermonuclear runaway. The early phase of the burst is fuelled by the alpha-p process, which then gives way to the rp-process. Nucleosynthesis can progress to high mass numbers, but it terminates at tellurium isotopes that undergo alpha decay.
There are two types of X-ray bursts: Type I and Type II. Type I bursts are caused by thermonuclear runaway, as described above, while Type II bursts arise from the release of gravitational potential energy through accretion. Type I bursts typically exhibit a sharp rise in luminosity followed by a gradual decline, whereas Type II bursts display a rapid pulse shape with multiple fast bursts separated by minutes. Most observed bursts are Type I, as Type II bursts have only been observed from two sources.
X-ray bursters play a role in the concept of "standard candles," which are objects used for distance measurements due to their predictable luminosities. While Type Ia supernovae have traditionally served as standard candles, the peak luminosities of Type I X-ray bursts have also been proposed as potential standard candles. This idea was introduced by van Paradijs in 1978, suggesting that the average peak luminosity of Type I bursts could be used to estimate neutron star radii and distances. Subsequent studies by Verbunt et al. (1984) and others expanded upon this concept, finding that the peak luminosities of X-ray bursters in globular clusters agreed with the average peak luminosity of Galactic centre X-ray bursters.
However, it is important to note that deviations do occur, and the standard candle based on Type I X-ray bursts is only accurate to within 15%. This accuracy limitation was recognised in studies examining the maximum bolometric peak luminosities during Type I bursts from globular clusters. While these bursts typically reach a critical luminosity, significant deviations were observed, impacting the accuracy of the standard candle.
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Type I and Type II bursts
X-ray bursters are a class of X-ray binary stars that exhibit X-ray bursts, characterised by rapid and periodic increases in luminosity (typically a factor of 10 or greater) that peak in the X-ray region of the electromagnetic spectrum. These systems are composed of an accreting neutron star and a main-sequence companion donor star.
There are two types of X-ray bursts: Type I and Type II. Type I bursts are caused by thermonuclear runaway, while Type II bursts arise from the release of gravitational potential energy through accretion. Type I bursts occur when the mass transferred from the donor star accumulates on the surface of the neutron star, causing thermonuclear bursts. This process begins with the hot CNO cycle, but continued accretion leads to a degenerate shell of matter, causing a rise in temperature and triggering the triple-alpha cycle, resulting in a helium flash. The early phase of the burst is powered by the alpha-p process, transitioning to the rp-process. Nucleosynthesis can reach as high as mass number 100 but ends at isotopes of tellurium that undergo alpha decay.
Type I bursts have a sharp rise followed by a slow decline in luminosity. They are Eddington-limited in flux, with a larger amount of energy in the bursts than in persistent emission. Type I bursts span a wide range of accretion luminosities, from about 45% of the Eddington luminosity down to almost quiescence.
Type II bursts, on the other hand, exhibit a quick pulse shape with multiple fast bursts separated by minutes. They are thought to arise from instabilities in the accretion flow onto a neutron star. Type II bursts are shorter than Type I bursts and can be as brief as 0.130 seconds. They only appear below a critical luminosity of about 10% of the Eddington luminosity. While Type I bursts are common, Type II bursts have only been observed from two sources.
X-ray bursts typically exhibit sharp rise times, ranging from 1 to 10 seconds, followed by spectral softening, and have integrated fluxes of 10^32-10^33 joules. The bursts recur on timescales ranging from hours to days, with some systems exhibiting longer recurrence times, and weak bursts recurring between 5 and 20 minutes.
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Type I bursts are caused by thermonuclear runaway
X-ray bursters are a class of X-ray binary stars that exhibit X-ray bursts, characterised by rapid increases in luminosity, typically by a factor of 10 or more. These binary systems are composed of an accreting neutron star and a main-sequence companion donor star. There are two types of X-ray bursts: Type I and Type II. Type I bursts, or thermonuclear bursts, are caused by a process known as thermonuclear runaway.
Type I bursts occur when material from the donor star accumulates on the surface of the neutron star. This accumulation leads to gravitational compression, initiating nuclear fusion in the form of the hot CNO (carbon-nitrogen-oxygen) cycle. As more material is accreted, a degenerate shell of matter forms, causing a rise in temperature without a corresponding relief in thermodynamic conditions. This temperature increase favours the triple-alpha cycle, resulting in a helium flash that provides additional energy. The extra energy enables the CNO burning to transition into a thermonuclear runaway state, marking the onset of the burst.
The initial phase of the burst is fuelled by the alpha-p process, which swiftly transitions into the rp-process. Nucleosynthesis can advance to mass numbers as high as 100 but terminates at isotopes of tellurium, such as 107Te, which undergo alpha decay. Type I bursts are analogous to the behaviour of recurrent novae, where the compact object is a white dwarf that undergoes explosive hydrogen burning.
The identification and characterisation of rapid bursts in accreting white dwarfs have revealed similarities to Type-I X-ray bursts. These magnetically confined thermonuclear runaways occur in the surface layers of white dwarf atmospheres, resembling the behaviour observed in Type-I bursts. Furthermore, the thermonuclear runaway process in Type I bursts is comparable to the central carbon burning in white dwarf stars, which leads to a thermonuclear runaway and subsequent disruption of the star.
In summary, Type I X-ray bursts in X-ray bursters are caused by thermonuclear runaway. This process involves the accumulation of material from the donor star on the neutron star's surface, initiating nuclear fusion and eventually leading to a thermonuclear explosion. Similar thermonuclear runaway behaviour has been observed in accreting white dwarfs and carbon-burning white dwarf stars, reinforcing the understanding of Type I bursts in X-ray bursters.
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Type II bursts arise from gravitational energy
X-ray bursters are a class of X-ray binary stars that exhibit X-ray bursts, characterised by rapid and periodic increases in luminosity that peak in the X-ray region of the electromagnetic spectrum. These systems consist of an accreting neutron star and a main-sequence companion donor star. There are two types of X-ray bursts: Type I and Type II. While Type I bursts are caused by thermonuclear runaway, Type II bursts arise from the release of gravitational potential energy through accretion.
Type II X-ray bursts, also known as Eddington-limited bursts, are believed to originate from instabilities in the accretion flow onto a neutron star in an X-ray binary system. These instabilities can be thermal or viscous in nature. Thermal instabilities occur when there is a failure to maintain thermal balance, while viscous instabilities arise due to the inverse relationship between viscous stress and surface density, leading to the breakup of the inner disc. The analytical criteria for these instabilities were initially established by Taam and Lin in 1984, who confirmed the presence of unstable solutions through global, time-dependent simulations.
The release of gravitational potential energy in Type II bursts is a result of the accretion process. In an X-ray burster, material from the donor star accumulates onto the surface of the neutron star, forming a dense layer. This accumulation occurs over a few hours, accompanied by gravitational compression. The accreted material, rich in hydrogen and helium, originates from the surface layers of the donor star and streams towards the accretor through the intersection of their Roche lobes. Due to the high gravitational fields of compact stars, the material falls with high velocity and angular momentum, preventing it from immediately joining the surface of the neutron star. Instead, it forms an accretion disk, orbiting the accretor in the orbital plane.
As the accretion process continues, nuclear fusion initiates in the accumulated matter, marking the beginning of the hot CNO cycle. However, sustained accretion leads to the formation of a degenerate shell of matter, where the temperature rises beyond 109 Kelvin. This elevated temperature favours the triple-α cycle, resulting in a helium flash. The additional energy from the flash enables the CNO burning to transition into thermonuclear runaway. The burst's early phase is fuelled by the alpha-p process, which then gives way to the rp-process. Nucleosynthesis can progress to high mass numbers, but it concludes with isotopes of tellurium undergoing alpha decay.
Type II X-ray bursts exhibit distinct observational characteristics. They are characterised by a quick pulse shape and can encompass multiple fast bursts separated by intervals of a few minutes. These bursts are relatively short, with durations as low as 0.130 seconds, and their recurrence times can be as short as 15-18 seconds. Type II bursts have only been confidently detected from two sources: the Rapid Burster (MXB 1730-335) and the Bursting Pulsar, an accretion-powered pulsar. The behaviour of Type II bursts contrasts with Type I bursts, which display a sharp rise followed by a slow and gradual decline in luminosity.
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X-ray bursts have a sharp rise time
X-ray bursts are characterised by a short rise time, typically between 1 and 10 seconds, followed by spectral softening, a property of cooling black bodies. The burst energetics are characterised by an integrated flux of 10^32–10^33 joules, compared to the steady luminosity of 10^30 W for steady accretion onto a neutron star. The ratio of the burst flux to the persistent flux (α) ranges from 10 to 1000 but is usually around 100.
X-ray bursts are rapid increases in luminosity, typically by a factor of 10 or more, that peak in the X-ray region of the electromagnetic spectrum. They occur in X-ray binary systems, composed of an accreting neutron star and a main-sequence companion donor star. The donor star transfers mass to the neutron star, which accumulates on its surface until it ignites and fuses in a burst, producing X-rays. This process is known as thermonuclear runaway and results in a sudden and significant increase in luminosity.
The sharp rise time of X-ray bursts is due to the rapid accumulation and ignition of material on the surface of the neutron star. Within seconds, most of the accreted material is burned, powering a bright X-ray flash observable with X-ray or gamma-ray telescopes. The burst is further characterised by the spectral softening phase, where the luminosity gradually declines.
Type I X-ray bursts, caused by thermonuclear runaway, exhibit a sharp rise followed by a slow and gradual decline in luminosity. These bursts are more commonly observed than Type II bursts, which exhibit a quick pulse shape with multiple fast bursts separated by minutes. Type I bursts also show spectral softening during the burst tail, indicating a decrease in temperature as the burst cools.
The sharp rise time of X-ray bursts is a distinctive feature that allows for their identification and characterisation. By studying the behaviour of X-ray bursts, astronomers can gain insights into the nature of these explosive events and the underlying mechanisms that power them. The short rise time, high luminosity, and subsequent spectral softening make X-ray bursts a unique phenomenon in the universe.
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Frequently asked questions
X-ray bursters are a class of X-ray binary stars that exhibit rapid and periodic increases in luminosity, known as X-ray bursts. These systems are composed of a neutron star and a companion 'donor' star.
Standard candles are objects used for distance measurements due to their predictable luminosities.
It has been proposed that the average peak luminosity of Type I X-ray bursts could serve as a standard candle. However, there are deviations, and the strongest possible X-ray bursts may not have been observed yet.
Gamma-ray bursts (GRBs) are among the most energetic events in the universe. While their exact mechanism is unknown, there is a correlation between the brightness and shape of a GRB light curve. This suggests GRBs may have potential as standard candles, but more research is needed.
