What are X ray binaries</head> <body bgcolor="blue" text="white" link="yellow" alink="ff0ff" vlink="ff0000"> <center> <h1><font size=5><b>What are X ray Binaries</h1></b></center> <p><font size=3> X ray binaries are amongst the brightest X ray sources observed in the sky. Galactic X-ray binaries are binary systems containing a compact object (the primary: a black hole, neutron star, or white dwarf) and a normal star (the secondary). The compact primary accretes matter from a stellar wind of the secondary or by direct mass transfer from the surface of the secondary, if the secondary has expanded so far that matter on a section of its surface becomes gravitationally bound to the primary (this is called Roche-lobe overflow). The accreted matter, carrying large amount of angular momentum from the orbital motion, circulates around the compact object, and forms an accretion disk. Due to loss of angular momentum from viscous processes, matter from the companion star spirals in towards the compact object, and its gravitational energy is converted into heat. The temperature of the inner accretion disk can reach more than a million degrees, and X rays are produced.</br> </p> <p> Besides mass accretion, outflows also seem to be quite common in X ray binaries, like in many other astronomical environments. In some cases, these outflows are highly collimated, forming observable jets. The physics of jet formation being yet largely unknown, there have been considerable theoretical efforts directed towards finding mechanisms of accelerating and collimating particles originating in the accretion disk to form jets. Many of the X ray binaries also being strong radio sources, the jets have been directly imaged exclusively at radio wavelength. In some cases, a pair of bi-polar (or twin) jets are observed, while in other cases, only a single jet is visible.</br> </p> <br><table CELLSPACING=5 WIDTH=70% ALIGN=CENTER><tr><td> <img src="binary.gif" > </td><td><font size=3> X Ray Binary - An artist's rendition<br> <font size=2>Credit : www.physics.purdue.edu/~cui/binaries.html </font></td></tr></table><br> <p> Microquasars are generally regarded as counterparts of more powerful extragalactic sources, like quasars, radio galaxies and BL Lac objects, all classified under AGNs. The three common ingredients of microquasars and compact extragalactic sources are the central black hole, accretion disk, and relativistic jets - the only difference is, that microquasars have a stellar mass black hole <i>M~4-10</i> solar masses, whereas in AGNs, it is a supermassive black hole <i>M~10<sup>6-9</sup></i> solar masses. The similarity in the configurations of these two types of systems hint towards the fact that, in spite of their different scales, the underlying physical processes in these systems are similar, and are manifested accordingly in their radiative properties. </p> <center><h1><font size=4><b>Microquasars - A Galactic - extragalactic connection?</h1></b></center> <p><font size=3> Microquasars, and AGNs are both accretion powered, and produce strong X ray emission. The X ray spectra of microquasars in the "soft" state is characterised by a thermal blackbody component (with temp ≈ 1keV), believed to be associated with emission from the inner region of the accretion disk, and a power-law tail (with a photon index <i>Γ</i>≥ 2, that is generally interpreted as comptonised emission arising from hot thermal <i>kT</i>>>1 keV or high speed <i>v</i>∼<i>c</i> electrons very close to the black hole . In the hard state, the spectrum is dominated by a power-law, with a slope flatter than that of the power law in the soft state <i>Γ</i> < 2. The power-law in the hard state is often explained by comptonization models and sometimes as being the high energy tail of a broad synchrotron continuum emitted from the relativistic jets Black body and power law components are also found in the UV/X ray spectra of AGNs, again arising from accreting / outflowing matter, near the central engine. Due to higher black hole masses, the AGNs have their accretion disks truncated at larger inner radii <i>r</i>∝<i>M</i>, leading to lower temperatures for the black body components in their spectra <i>T<sub>disk</sub></i>∝ <i>M</i><sup>-1/4</sup>. </p> <br><table CELLSPACING=5 WIDTH=70% ALIGN=CENTER><tr><td> <img src="fender.gif" > </td><td><font size=3> Generally accepted structure of an X ray Binary system<br> <font size=2>Credit : Rob Fender - Relativistic Outflows from X ray Binaries </font></td></tr></table><br> <p> In the radio, compact extragalactic sources often show power-law spectra at high frequencies ≥ 90 GHz. This, coupled with high levels of linear polarization, provides strong evidence for optically thin synchrotron emission from non-thermal relativistic electrons . More than 30 X-ray binaries have shown detection of radio emission, and more than 10 sources have shown resolved structure in their radio maps, showing the presence of large scale jets / outflows. Some of these are persistent sources with more or less constant radio flux densities over timescales longer than half a decade (eg., GRS1758-258, 1E 1740.7-2942), while some are transients showing radio flaring activity only in the X-ray active states. Some of the transient sources also show superluminal ejection events (eg., GRS 1915+105). Therefore, it follows that the term ``microquasar'' more than simply being an indicator of similar morphologies between jet emitting XRBs (microquasars) and AGNs, might also be indicative of similar physics. Since spectral variation timescales are expected to scale with mass of the accretor, processes which could not be observed in AGNs over a human lifetime may be easily monitored many times over on that timescale from a microquasar. Microquasars, thus, are unique cosmic laboratories that allow a deeper insight into the complex phonomenon of the jets common in powerful extragalactic objects. </p> <b><font size=4><center>Still hungry? Click <a href= "http://www.phy.ohiou.edu/~gupta/mq2.html"> here</a>.</br> </center></b> <hr> <p> Back to my <a href="http://www.phy.ohiou.edu/~gupta">homepage</a>. <br> Back to <a href="http://www.phy.ohiou.edu/~astro"><i>API</i> at O.U</a>. </body> </html>