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Sep 20th: Monitoring BL Lacertae Objects


Podcaster: Cosmic

Apogee PodcastTitle: The Apogee Podcast – Monitoring BL Lacertae Objects


Original Paper:;

Description:  In this apogee podcast, Cosmic discusses BL Lacertae Objects

Bio: Cosmic is a self- and crowd-funded independent research astronomer and space musician.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2014, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at

Hello! This is Cosmic, and welcome to the Apogee podcast! The format of this podcast is myself describing, discussing, and critiquing an article of my choice from an astronomical journal. Sometimes topics within these articles send me off on temporary tangents, but they’re always relevant to the overall topic. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.

If you have any articles to suggest for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT yahoo DOT com.

The music you hear in the background are my own compositions, which can be downloaded from my soundcloud channel ‘cosmiclettuce’. I hope you enjoy listening to space music as much as I do.

The apogee for this podcast takes place today, 20 Septemer 2014, at 14:23 UTC. The lunar distance at that time is 405568 km, which is 677 km closer than last apogee on the 24 August, and 948 km further away than the next apogee on 18 October.

I’d like to wish you a splendid autumnal equinox, which takes place this year in the early hours of 23 September UTC at 02:29. The equinoxes and solstices are my adopted holidays.

Ok, on with the paper!

This apogee’s paper is titled, “Six-year optical monitoring of BL Lacertae object 1ES 0806+52.4′ by astronomers at Beijing Normal University, the Chinese Academy of Sciences, and Nanjing Normal University. Before I get into the paper, I’m wondering how many of you know how astronomers decide what to name a particular object? I mean, ‘1ES 0806+52.4’ is a pretty odd name don’t you think? Well, astronomers do tend to use one particular forumla fairly often. So whenever you see a name like this, the first part of the name usually designates the name of the survey or telescope. In this case, ‘1ES’ corresponds to the Einstein Slew Survey from the early 1990s. The second part of the name — the one with all the numbers — usually represents the approximate position of the object in the sky. So in this case, this object is at a right ascension of 8 hours 6 minutes, and a declination of 52.4 degrees north. Right ascension and declination is a coordinate system astronomers use to locate the position of objects in the sky, much like longitude and latitude is used to locate the position of objects on a planet.

Back to the paper — These kinds of projects catch my eye because I’m very interested in long-term, high cadence monitoring and measurement of astronomical objects. For too long this type of observing hasn’t taken place, but over the past few years the astronomical community has finally begun to realize that the universe is a much more dynamic place than was thought — everything is changing moment to moment — and that the most interesting stuff is hidden away in the details. As more automated telescopes and all-sky monitoring systems come on-line, astronomers will most surely be able to better understand the incredible and beautiful structures that make up the observable universe.

Among the multitude of oddities facing astronomers, BL Lacertae objects are probably one of the most enigmatic and baffling. The namesake of these, BL Lacerta, was first observed in 1929 and described as an “irregularly variable star”. It wasn’t until 1968 that BL Lacerta was found to have a radio counterpart. Closer examination over the next few years revealed that this object was extra-galactic, its light was highly polarized, had practically no spectral features whatsoever, and was surrounded by a nebulosity that looked like an elliptical “host galaxy”. Because of a lack of spectral lines, it’s very difficult to calculate distances through the usual means of redshift. However, redshifts of the host galaxies can be measured (sometimes) and since it’s assumed that BL Lacertae objects are the cores of these galaxies, astronomers are fairly confident that they have a good handle on distance. Distance and brightness implies a certain intrinsic luminosity, and luminosity implies a certain amount of energy, which further implies a certain amount of mass. Once the mass is known, many other physical properties can be inferred.

By 1976, 33 objects had been designated as BL Lacertae objects, all with the same characteristics listed above. No one had even a guess as to what these objects were. In 1978, a theory proposed that the observed characteristics could be explained if we assume that what was being seen was a relativisitic jet viewed at a small angle. It’s as if we were looking down the barrel of a shotgun with bullets traveling at (from our perspective) superliminal speed. This theory has stuck to this day.

In the intervening years, BL Lacertae objects were found to be members of a larger families of objects. The largest family are the Active Galactic Nuclei (AGN). As the name implies, these energetic objects are in the centers of distant elliptical galaxies and emit light at variable intensities with variable (or irregular) periods. The engines that drive these variable emissions have been theorized to be black holes. Many of these AGN have jets (usually seen only in the radio) eminating from this central black hole. In order for the jet to stay intact over so many thousands of light years and for such a long period of time, strong magnetic fields are suggested. AGN who’s jets are pointing nearly along our line of sight are in a family of objects called ‘Blazars’, which exhibit not only variable emission, but also highly polarized light. BL Lacertae objects are a type of Blazar that show very weak or no emission lines in it’s spectrum. As of 2010, there were about 1400 known BL Lacerta objects.

In a paper that will appear in a future issue of ‘The Astronomy and Astrophysics Review’ entitled “An Optical View of BL Lacertae Objects” (I’ll include a link to this paper in the podcast notes) has an excellent summary of what we currently think BL Lacertae objects are and I encourage you all to take a look at that paper if you want to know more details. In their concluding remarks, the authors of that paper state:

“BLLs are active nuclei of massive elliptical galaxies the emission of which is dominated by relativistic jets. The bright compact radio cores, high luminosities and rapid, large amplitude flux variability at all frequencies and the strong polarization that characterise these objects are well explained in this scenario. Also the observed quasi featureless optical spectra of BLL, that is one of the distinctive properties of the class, find a natural explanation in the above beaming model. At variance with other classes of AGN, like quasars and Seyfert galaxies, the lack of prominent spectral features was, and still in part is, a significant limiting factor for the determination of their redshift and ensuing evaluation of their physical properties. The implication of this model is the existence of a large number of misaligned objects with the same intrinsic properties as BLLs. The most obvious candidates are low-power radiogalaxies.”

The last part of the quote says that there should be a large number of objects that would be BL Lacertae objects if their jets were pointed more towards us, but they’re not sure what they’d exactly look like.

Now while the quote I just read speaks with some surety and confidence, it’s clear from a review of a number of papers on various BL Lacertae observing projects that these objects still have the experts baffled. Do a search for BL Lacertae objects on the or the Astronphysics Data System website and you’ll see what I mean. Multi-year, high cadence observations are needed at as many different wavelengths as possible to either prove or disprove the current understanding of these amazing yet enigmatic objects.

In the paper I’m reviewing, the only fairly concrete conclusion that they could make after this six year project was that the object they were observing appeared bluer as it got brighter. This effect is seen in other BL Lacertae objects. They also seemed to suffer from some pretty bad luck when it came to catching this object being active. Despite that, they were able to collect 1399 data points covering five different spectral bands, or colors, which they claim “is the largest optical multi-color database for the variability of the object to date.” Sadly, the authors of this paper don’t indicate whether they’re going to continue this work. I hope they do.

Until next apogee — I bid you Peace.
End of podcast:

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