Arto Oksanen Interview

Post date: 23-May-2009 17:22:47

Arto Oksanen is a Finnish amateur astronomer interested in observing transient objects like gamma-ray burst afterglows, supernovae, novae and cataclysmic variables. He also observes exoplanet transits, and was the first amateur to observe the transit of HD 209458b.

In 2004, Oksanen received the AAVSO Directors Award for his work in variable star research. In October 2007, Oksanen was the first to find optical afterglow of GRB 071010B, which had been detected by the Swift satellite only 17 minutes earlier.He has also discovered two minor planets (22978 Nyrola and 103422 Laurisiren). Arto Oksanen is an Internet technology consultant by profession. He lives in Muurame, Finland with his wife Minna and their son Atte. Recently, Arto has been observing a very interesting eclipsing polar (a highly magnetic cataclysmic variable). We had a chance to talk about just what it is that is so interesting about this star and what his observations may contribute to the knowledge of this system and magnetic CVs in general.Mike: Hi, Arto. In recent weeks you have been following the very interesting eclipsing polar CSS 081231:071126+440405. How many eclipse timings over how many nights have you now amassed?Arto: Yes, I have been following it practically every clear night since the outburst, or brightening, was discovered by the Catalina Real-time Transient Survey on the last day of 2008. Since that I have observed a total of 48 eclipses during 19 nights.Mike: What telescope or telescopes are you using to obtain the data?

Arto: Mostly the 40 cm RCOS telescope of Hankasalmi observatory. It is a very nice telescope on Paramount ME and with a SBIG STL-1001E CCD. Luckily I have got enough observing time for this project. I used the 40 cm Meade LX200 of the Nyrölä observatory for one night, observing simultaneously with the Hankasalmi telescope. Both telescopes are owned by the local astronomy club. I am the president so that helps a bit.

Mike: Are you manning the telescopes in real time, observing remotely or scripting the runs and then going to bed?

Arto: For the Hankasalmi telescope I have been observing remotely. Basically starting the same script every night and the observatory automation has taken care of observing and parking the telescope and closing the dome the following morning. Photometry is also performed remotely, by a self-written script, and the result is written in the new AAVSO format that can be uploaded by a few clicks. Observing the same object night after night is very effortless. At Nyrölä the dome is manual, so the observer has to stay there to keep the dome slit aligned with the telescope.

Mike: Can you give us an update? Is the outburst over, have you been clouded out, or are you still collecting data?

Arto: I had to stop observing at the beginning of May. Our skies got too bright for observing then. The outburst seems to continue so, I hope other observers with more southern locations will follow it. OT_J0711+44 will be in conjunction in July so the observing season is soon over for everyone, but hopefully it will remain active for fall when it will be on the morning sky.

Mike: From your location in Finland, how many hours of darkness do you get this time of year? When do you lose nighttime completely, and when does it return for you?

Arto: At this time of year (mid May) we here at 62N latitude don't get any dark hours, just a short twilight that allows us to observe bright targets on southern half of the sky. The observing season starts again in the beginning of August or so.

Mike: Are you collaborating with other astronomers to do a paper on this star? If so, who?

Arto: Yes, there has been lots of interest by professional astronomers. I am collaborating with three astronomers: Pasi Hakala from Finland, Boris Gänsicke from England and Ivan Andronov from Ukraine. Each of them is preparing a paper of this star.

Mike: Can you explain how the light curve gives clues to the geometry of this system?

Arto: OK, I will try. It is obvious that this is an eclipsing system so there are two stars and that the orbit is aligned so that the stars eclipse each other. The eclipse is very deep and very fast so the eclipsed body is much brighter and very small in size. It was found very soon that the system is a polar variable, a cataclysmic variable with a very magnetic white dwarf. The strong magnetic field does not allow the accretion disk to form but directs the accretion stream to the magnetic poles of the white dwarf. The eclipse ingress and egress are extremely fast, too fast to resolve even with 5 second exposures so the light emitting region on the white dwarf is very tiny.

Mike: What do you think is happening to the accretion stream as the outburst evolves?

Arto: The stream is like a light switch to the system: when the stream is on the system is bright (high state) and when the stream is off the system is several magnitudes fainter (low state). The star seems to be around mag 18 in low state and mag 15 on high state. The light curve shows a curious dip just before the main eclipse. This is caused by the accretion stream that eclipses the white dwarf. The pre-eclipse dip varies a lot from eclipse to eclipse and is not visible at all when the system is in low state. The bright stream shows itself also on the main eclipses as the eclipse bottom is not flat but fades two more magnitudes after the sudden 2 mag drop during the 7 minute eclipse . I think the accretion is still increasing, the pre-eclipse dips are getting deeper and wider.

Mike: What new science do you think may come from exploring the characteristics of this outburst?

Arto: Probably the most interesting feature is the pre-eclipse dips that gives the (first ever?) opportunity to directly probe the accretion stream. But it needs more observations to model the system properly and making sure of the geometry. The new science is of the accretion stream for sure and probably some more knowledge of the polars as there are not too many eclipsing systems out there.

Mike: Are there any new ideas or conclusions you can share with us, or do we have to wait for the paper?

Arto: From my observations the orbital period is 117 min 10.9 sec and the main eclipse lasts 7 min 15 sec. The eclipse is 4 magnitudes deep. The ingress and egress are shorter than 5 seconds. The eclipse bottom is V (or semi V?) shaped when the star is in high state and flat bottomed in low state. The pre-dip varies a lot from eclipse to eclipse and is visible only when the system is in high state. More detailed analysis will be on the upcoming papers.

Mike: What other objects are you observing right now?

Arto: During this spring I concentrated this star, but managed to observe some other cataclysmic variables (AM CVn, QZ Vir, CP Dra, a blazar (0716+714), a few Gamma-ray bursts and confirmed a supernova.