- A significant property of these systems is the presence in their spectra of strong CA II, H and K emission lines of variable intensity, indicating increased chromo-spheric activity of the solar type. These systems are also characterized by the presence of radio and X-ray emission. Some have light curves that exhibit quasi-sine waves outside eclipses, with amplitudes and positions changing slowly with time. The presence of this wave (often called a distortion wave) is explained by differential rotation of the star, its surface being covered with groups of spots; the period of the rotation of a spot group is usually close to the period of orbital motion (period of eclipses) but still differs from it, which is the reason for the slow change (migration) of the phases of the distortion wave minimum and maximum in the mean light curve. The variability of the wave's amplitude (which may be up to 0.2 mag in V) is explained by the existence of a long-period stellar activity cycle similar to the 11-year solar activity cycle, during which the number and total area of spots on the star's surface vary. GCVS
As stated earlier, the classification RS appears in the GCVS twice. You saw it earlier as one type of eruptive variable star. Here, the RS Canum Venaticorum system is defined as consisting of binaries in which the hotter of the two is F or G. Other distinctions are made but for our purposes, this will suffice. Other distinctions, not usually considered within the definition, are that they generally have at least one star evolved off the main sequence not yet filling its Roche lobe, they emit intense coronal X-ray and radio radiation, have strong emission lines in the far ultraviolet, lose mass in an enhanced wind, have variable orbital periods, show a starspot wave, and undergo more gradual changes in the mean brightness.
The starspot wave, usually the principal cause of variation within RS variables, is usually sinusoidally shaped.
Discovered in the 1930s, the first non-eclipsing RS CVn binary system that varied in brightness as a result of only starspots was y Andromedae. The GCVS did not officially classify it as an RS-type star until 1985.
Mixed siat5 Small ampliu,,^ Mixed periods CCD or PEP
Optically Variable Close Binary Sources of Strong, Variable X-Ray Radiation (X-Ray Sources)
Close binary systems that are sources of strong, variable X-ray emission and which do not belong to or are not yet attributed to any of the other types of variable stars. One of the components of the system is a hot compact object (white dwarf, neutron star, or possibly a black hole). X-ray emission originates from the infall of matter onto the compact object or onto an accretion disk surrounding the compact object. In turn, the X-ray emission is incident upon the atmosphere of the cooler companion of the compact object and is reradiated in the form of optical high-temperature radiation (reflection effect), thus making the area of the cooler companion's surface an earlier spectral type. These effects lead to quite a peculiar complex character of optical variability in such systems.
X-ray variable stars! Surely, you jest?
In fact, many X-ray variable stars have an optical component that is within the observational reach of visual observers. Without a doubt, you will fail to detect the X-ray portion of these star's electromagnetic energy spectrum but that's not what you want anyway. And if you're not new to variable star observing, you have probably already viewed stars similar to X-ray variables. Both low-mass X-ray binaries (LMXB) and cataclysmic variables (CV) have similar orbital periods, low-mass donor stars, mass-transfer rates, variability, and eruptions. The essential difference between CVs and LMXBs is that the accretion is onto a white dwarf for cataclysmic variables and in the case of low-mass X-ray binaries, it is onto a more compact neutron star, or possibly a black hole. In a sense, X-ray variables may provide you with an opportunity to study, in an indirect manner of course, neutron stars and perhaps black holes (Figure 8.1).
With all of that said, I must also caution that as a group this class of variable stars is not a good starting point for beginning amateur astronomers. The great majority of these stars are fainter than 14T0 and many are as faint as 18™0 and 19T0 and those displaying a reasonable brightness are variable for reasons other than X-ray radiation, such as being y Cas, novae, or emission type variable stars. As you will discover, many variable stars found within the other five major classes
Figure 8.1. Artist's conception of an X-ray-type variable showing the high-energy produced by the accreting material. Copyright: Gerry A Gooi of variable stars are also X-ray sources. Still, the intrigue regarding X-ray binaries exists and their study is not beyond the capabilities of serious amateur astronomers. A few years ago who would have believed you could detect planetary transits with a telescope in your backyard? Tough to do, yes! Impossible, no!
X-ray radiation is electromagnetic radiation, similar to visible light but possessing much higher energies. As a result, X-rays exist in the upper energy levels of the spectrum, far beyond the visible, and can only be detected by special X-ray detectors that are usually placed in an orbiting satellite. X-rays find it difficult to penetrate the Earth's atmosphere because they are absorbed by the atmosphere. They indicate high temperatures; very high temperatures and high temperatures reveal high energies. Anything producing a lot of energy, far in excess of what astrophysicists consider normal, is going to be interesting because it's relatively rare or because it will provide an opportunity to observe something that cannot be produced in a laboratory. It's tough to sustain temperatures in the neighborhood of ten million degrees or to produce gravity fields millions of times greater than the Earth's within even the best equipped research facilities on Earth. Perhaps one day.
With all of that said, you can continue your exploration of the Universe through the observation and study of these exotic stars with a little preparation, a lot of patience and a pinch of luck. You won't need your X-ray goggles either. However, there exists several types of X-ray variable stars so you have more decisions to make. The GCVS defines the various types of X-ray variables as:
XB - X-ray bursters are close binary systems showing X-ray and optical bursts, their duration being from several seconds to ten minutes, with amplitudes of about 0".'l in V.
Examples of XB-type variables are V801 Ara and V926 Sco. The star V801 Ara shines at 16T0 when brightest and V926 Sco at 17"?4. As you can see, these are pretty faint stars and it will require a large telescope to observe stars like this. Fortunately, there is a place that you can visit to better understand X-ray variables when you can't actually observe them yourself. It's the High Energy Astrophysics Science Archive Research Center (HEASARC) (http://heasarc.gsfc.nasa.gov/).
You can find gamma-ray, X-ray, and extreme ultraviolet observations of cosmic, non-solar system sources at the HEASARC Web site along with archival data, associated analysis software, documentation and guidance in how to use it all, as well as educational and outreach material. This site also provides astronomical tools so that you can obtain multiwaveband images of the sky and conduct astronomical catalog searches. You can also study images, spectra, and light curves from celestial high-energy sources including cataclysmic variables, X-ray binaries, supernova remnants, pulsars and gamma-ray bursts. Another must-visit site!
Now, let's finish looking at the types of X-ray variables.
XF - Fluctuating X-ray systems showing rapid variations of X-ray and optical radiation on time-scales of dozens of milliseconds.
Because of the rapid variations, large telescopes with instruments are required to collect valuable data on fluctuating X-ray systems.
XI - X-ray irregulars are close binary systems consisting of a hot compact object surrounded by an accretion disk and a dA-dM-type dwarf. These display irregular light changes on time scales of minutes and hours, and amplitudes of about 1 magnitude in V. Superposition of a periodic variation because of orbital motion is possible.
The star V818 Sco may give you a good opportunity to observe this type of X-ray variable. Its brightness is about 12T0, perhaps within the reach of good quality binoculars.
XJ - X-ray binaries characterized by the presence of relativistic jets evident at X-ray and radio wavelengths, as well as in the optical spectrum in the form of emission components showing periodic displacements with relativistic velocities.
The prototype star for this group of X-ray variables is VI343 Aql. This star is also an eclipsing binary with a period of 13.0848 days. At its brightest, it is 13™0, within the capabilities of medium-sized telescopes.
XND - X-ray, novalike (transient) systems containing, along with a hot compact object, a dwarf or subgiant of G-M spectral type. These systems occasionally rapidly increase in brightness by 4-9 magnitudes in V simultaneously with the X-ray range, with no envelope ejected. The duration of the outburst may be up to several months.
V616 Mon is the prototype star of this group of X-ray variables and is also an elliptical variable. At its brightest, this star is within the range of a good pair of binoculars at lln.'26 but it fades to 20T2.
XNG - X-ray, novalike (transient) systems with an early-type supergiant or giant primary component and a hot compact object as a companion. Following the main component's outburst, the material ejected by it falls onto the compact object and causes, with a significant delay, the appearance of X-rays. The amplitudes are about 1-2 magnitudes in V.
An 09.7IIIe-type star, the prototype for this type of system is V725 Tau and shines at a bright 9™4 at its brightest. It will fade to 10n.'l, still within the capabilities of a good pair of binoculars.
XP - X-ray pulsar systems. The primary component is usually an ellipsoidal early-type supergiant. The reflection effect is very small and light variability is mainly caused by the ellipsoidal primary component's rotation. Periods of light changes are between 1 and 10 days; the period of the pulsar in the system is from 1 s to 100 min. Light amplitudes usually do not exceed several tenths of a magnitude.
GP Vel, the prototype star for this type of system, is also known as Vela X-l. This system, listed as a B0.5Iaeq-type star, shines within a range between 6".176 and 6T99, well within the range of binoculars and small telescopes.
XPR - X-ray pulsar systems featuring the presence of the reflection effect. They consist of a dB-dF-type primary and an X-ray pulsar, which may also be an optical pulsar. The mean light of the system is brightest when the primary component is irradiated by X-rays; it is faintest during a low state of X-ray source. The total light amplitude may reach 2-3 magnitudes in V.
The prototype star for this group of X-ray variables is HZ Her, a B0Ve-F5e-type system. As you might suspect, it is an eclipsing binary pair ranging in brightness from 12I?8 to 15T12 with a period of I"? 700175. All of the X-ray variables consist of a binary pair so there is a non-zero probability that any particular system is an eclipsing binary.
XPRM - X-ray systems consisting of a late-type dwarf (dK-dM) and a pulsar with a strong magnetic field.
Matter accretion on the compact object's magnetic poles is accompanied by the appearance of variable linear and circular polarization; hence, these systems are sometimes known as "polars." The amplitudes of the light changes are usually about 1 magnitude in V but, provided that the primary component is irradiated by X-rays, the mean brightness of a system may increase by 3 magnitudes in V. The total light amplitude may reach 4-5 magnitudes in V.
Two stars, AM Her and AN UMa, are listed within the GCVS as examples of this type of system. You probably recognize AM Her from the chapter on cataclysmic variables. It's listed there as a novalike cataclysmic variable and here it's listed as an XPRM. The second example, AN UMa, is a faint 15n.'0 star, at its brightest.
Now is a good time to point out something but I don't want it to be taken in the wrong context. Within the last few chapters, we've been examining the various classes of variable stars; their characteristics, how they're classified, where we can find catalog data and how to observe them. As I stated earlier, the classification of variable stars cannot be taken lightly. It's difficult and the compilation of the various catalogs and databases requires an enormous effort by dedicated people. It's inevitable that some ambiguous characteristics will cause you some confusion.
The cataclysmic variable star AM Her has just been identified as an example of the XPRM type of X-ray binary system. We also know that this star is recognized as a cataclysmic variable, specifically a novalike subclass known as a "polar." As you can see, a little confusion is beginning to emerge here. If you were to check the SIMBAD database, a great resource with a huge amount of information regarding stars, you will find that AM Her is listed as a S Scuti type variable, spectral type M4.5, and AN UMa is listed as a cataclysmic variable with a spectral type of CV (at least, at the time of the writing of this book).
Obviously, something is wrong but you should be able to recognize the problem without much effort. Here is an example of both classification ambiguity and of human error; nothing spectacular and it certainly doesn't need to become a subject of profound confusion but it is typical of the type of puzzlement that you will find. When detected, errors should be reported so that the problem can be remedied.
Let us apply a little logic and sort this problem out. In this case, we know that AM Her stars and all cataclysmic variables in no way resemble fast pulsating stars such as 8 Scuti variables other than that they are all stars. Regardless of the differences in the underlying physics, simply looking at a light curve will bear this out. Also, the spectral type listed for AM Her within the SIMBAD database, M4.5, is well outside of the boundary in which we find the 8 Scuti stars. If you look carefully at the description of the XPRM stars you will see that they consist of a late-type dwarf (dK-dM) and a pulsar. Since we know that accretion disks, such as are found around these types of stars, cause a peculiar spectral type (pec) and that these systems are binary, the simple answer is that the spectral type should be listed as pec+M4.5 and that the 8 Scuti classification is simply a human error. In the case of AN UMa and the SIMBAD database, human error is responsible too. Again, all of this should be apparent when you see it and it shouldn't really cause you much of a problem. What will cause you problems is when an error is more subtle, for example when a star is misclassified as a variable that exhibits the characteristics of several unrelated classes or when the spectral type is slightly in error. Certainly you would have no problem detecting an error if a Mira type variable was listed as having a spectral type of B9V.
The point of all this? Errors exist within the catalogs and databases that you are going to use. Sometimes the error will be small and sometimes it will be large. When you find errors, do a little research and you'll work yourself clear of the confusion. Apply a little logic, don't jump to conclusions and enjoy the intellectual challenge. Don't get annoyed either. The people providing all of these services, catalogs and databases, are doing their best. Just let them know when you find an error.
I've placed gamma ray bursts (GRBs) within the X-ray binaries in this book because of their behavior and the amount of energy they produce. When you first think about GRBs, you may consider the classification of eruptive variables but we really don't see violent processes and flares occurring within a star's chromosphere and corona. In fact, we don't even see a star that can be examined for very long. You might even consider cataclysmic variables but the energy produced within a GRB flash far exceeds the energy produced from accretion material surrounding, or impacting, a white dwarf or even the collapse of a Type II supernova. The energy produced within GRBs indicates that X-ray binary systems is a good place for them to reside right
Time after burst
Maximum visual magnitude Minimum visual magnitude
10 min 30 min
2 hours 4 hours 6 hours
15T6 16T6 17T4 18T5 19?7 20? 3 24T0
now. Perhaps, it may even be that GRBs end up being a new class of variable stars.
Astronomers are struggling to provide a good explanation for these enigmatic bursts of energy. When a GRB is detected, no star is known to be at the position of the burst and the burst fades quickly, sometimes within minutes. After the burst has faded, a check of the area again fails to uncover a star. To give you an example of how fast these events appear and then fade, Table 8.1, provided by Scott Barthelmy (NASA-GSFC) and Jerry Fishman (NASA-MSFC), shows how the brightness of a typical faint GRB afterglow might be expected to diminish with time. Fading fireballs can be fainter than 20™0 just a few hours after the onset of the explosion.
We are not going to spend a lot of time on X-ray variables. I've mentioned them here so as to be as complete as possible. They cannot be recommended for beginning variable star observers nor can they be adequately presented in this book. In almost all cases, instruments such as CCD cameras or photoelectric devices such as photometers must be used to adequately study these stars.
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