25 March 2015. Santiago de Chile
Yesterday I gave a quick 10 minute discussion of a letter Einstein Fellow Dr. Slavko Bogdanov and my PhD advisor Prof. Jules Halpern wrote to Astrophysical Journal Letters. In astronomy, a “letter” is a short little jaunt to explain a new idea or discovery. Many letters go on to be some of the most cited and celebrated work in science.
This letter is about an object that is identified as 1RXS J154439.4–112820. What’s all those nonsense letter and numbers? The “1RXS” stands for the “First ROSAT X-ray Survey” a catalog of things that a now switched-off x-ray telescope called “ROSAT” saw (ROSAT is short for the German word Röntgensatellit a satellite named after the Nobel Laureate who discovered x-rays, Wilhelm Conrad Röntgen). The “J” in the name stands for “J2000” which is to say “Julian year 2000”, a means to measure time that astronomers invented to stop getting so confused. “J2000” means that we are going to identify where on the celestial sphere the object was located at 12 noon on 1 January 2000. Objects slowly change position on the celestial sphere due to the precession of the Earth’s axis which has the effect of moving the direction the North Pole points away from the North Star (aka Polaris). I don’t intend to explain the full details here, so here is a nice litte blog post about it for you to read if you’re curious. The final numbers indicate where ROSAT said this lightbulb of x-rays was located according to celestial coordinates. The 15 means a “right ascension” of 15 hours, or about 3 pm on the 24 hour clock that is our sky if 00 is what is directly overhead at the Prime Meridian at the vernal equinox. The rest is the decimal change you’re given to find the precise location. The -11 and the following numbers stands for eleven degrees and change of “declination” that the object is below the celestial equator. The catalog number, then, tells me exactly where to find this thing.
What did Jules and Slavko find? They were curious that this location was coincident with a source of gamma-rays, according to the Fermi Space Telescope. Others had noticed this too, but had decided that the x-ray source and the gamma-ray source were just coincidentally located close by because they were convinced that the x-ray source was actually a cataclysmic variable star and such things are not supposed to give off a lot of gamma-rays. However, in the past a few binary millisecond pulsars were falsely accused of by cataclysmic variables including a system named J1023+0038 and another named XSS J12270–4859 (the XSS stands for “RXTE All-Sky Slew Survey Catalog” while RXTE stands for “Rossi X-Ray Timing Explorer”, another intrepid x-ray telescope that was named after Bruno Rossi, an Italian physicist who was a pioneer in cosmic ray and x-ray astronomy). A few observations of the way x-rays are emitted and the way the optical light from the object varies indicates to them that this object is likely one of these “transitioning” systems.
Transitioning between what? There is a kind of system called a “low-mass x-ray binary” which is a neutron star bound to a companion that is the size of the Sun or smaller. The companion to the neutron star has parts of it pulled onto the neutron star (a process known as “accretion”) and tremendous amounts of x-rays are emitted in the process. However, at some point, the accretion ends and we’re left with a system called either a “redback” or a “black widow” (named after poisonous spiders because the neutron stars are killing and “eating” their companions) depending on how large the remaining companion is. The redbacks, in particular, are of interest to us because they seem to transition back and forth between the accreting state and the non-accreting state. So far, there are known only three (and with this letter, I guess four!) such systems that are transitioning back-and-forth. For those keeping track, the fourth one I haven’t mentioned yet is known as J1824–2452I.
I’ll be studying these things a lot.