Wednesday, October 27, 2010

Cometary BAL Clouds?

If BAL clouds have optically thick cores surrounded by lower column density envelopes, then the envelopes will be more affected by radiation pressure than the cores and will be accelerated outwards from the quasar more than the core of the cloud will. This might form a tail analogous to a comet's dust tail, trailing the cloud opposite to its direction of motion. Such an effect might be detectable by looking to see if variable BAL troughs get stronger more quickly than they get weaker. That is, as a cometary BAL cloud moves across our line of sight, it should first produce a sharp jump in absorption from the core, followed by a slower decline from the larger envelope. That kind of variability was seen in X-rays by Maiolino et al. [that's where I got the idea] and in the UV at one point in FBQS 1408+3054 (see the 3rd-5th data points in Figure 3 of my paper); the SDSS DR5 and DR6 BAL catalogs should have many more examples to use to look for this effect (should also check the Gibson et al. 2008 and 2010 papers to see if they've already looked for this in their datasets).

Crap, is it Wednesday Already?

Recent activities include:

* conference call with Niel Brandt and his grad student Nur Filiz Ak about targeted BAL variability studies in SDSS-III
* useful exercise of writing a units conversion program (e.g., how many grams/second is a mass loss rate of 1 solar mass/year?)
* writing a macro version of my SM code for generating animated gif movies of BAL variability (now including the uncertainties in interpolated spectra, calculated using the observed sqrt[1-exp(-t/tau)] increase to a maximum sigma of 0.23 magnitudes over a timescale tau of 101 rest-frame days)
* experimenting with the Shareflow website as a Google Wave replacement for collaborative commenting on astronomy papers

Right now I am reading & commenting a draft of my grad student Jesse Rogerson's Master's Thesis. (Finished that; had a Skype meeting w/Jesse about it, and a Skype meeting w/my other grad student Laura; read & commented on a draft paper from my former grad student Alireza.)

Monday, October 25, 2010

Working for the Weekend

Friday's work included the previous blog post and, after
an epic battle with SM, the creation of a generic macro for plotting a
quasar spectrum with logarithmic wavelength axes in both quasar and rest frames.

Sunday included tinkering with with an MDM telescope proposal,
and getting lots of minor items crossed off my to-do list. Good times.

Friday, October 22, 2010

Color (or Colour) Variability of BAL Quasars

Here's a summary of ideas for studying BAL quasar variability.

Most BAL trough variability comes from transverse motion of absorbing clouds. The timescale of variation constrains the clouds' transverse velocity. That provides a constraint on the cloud's distance from the black hole (via angular momentum conservation plus the fact that a disk wind's terminal velocity is a few times the circular velocity at its launch radius). Uncertainties on the transverse velocity translate directly to uncertainties on the distance; thus, we want to accurately measure transverse velocities of BAL clouds, which requires spectroscopy during a trough variability event.

It is fairly common to see that BAL troughs _have_ varied, but much less common to see them actually vary. That is, either trough changes are very quick so that we almost never see them happening or they are very gradual so that we only notice them after sufficient time has passed. The two possibilities can be discriminated using time series photometry of BAL and non-BAL quasars (e.g., SDSS Stripe 82).

First, a prediction: trough variability will cause BAL quasars to show more color variability than non-BAL quasars, as measured by some variant of the structure function S: if you have N individual measurements i of an object's color c, then S = [\sum_i \sqrt{c_i^2 - sig_i^2}]/N, where sig is the uncertainty on the color.
Problem: colors aren't like magnitudes; a zero or negative color is perfectly reasonable.  What needs to be measured is whether or not the distribution of color measurements differs from that expected from random variations due to noise.  So probably best to just use reduced chi^2:
\chi_\nu^2=[\sum_i (c_i-c_0)^2 / sig_i^2]/(N-1), where c_0 is the mean color of the object.  Now if the c_i values are drawn from Gaussian distributions with variance sig_i^2, then on avg. \chi_\nu^2 = 1, and if there is colour variability, it will be larger.  But what matters is the distribution of \chi_\nu^2 for BALs and nonBALs - we expect the distributions to be different.  However, a complication is that the \chi_\nu^2 distribution changes as N increases, so you can't intercompare \chi_\nu^2 values from objects with different N.  So I'm thinking we should just use the distribution of individual \chi_i^2 = (c_i-c_0)^2/sig_i^2.  By plotting the distribution of all those individual values for nonBAL quasars (each with a different c_0 of course), and then the equivalent distribution for BAL quasars, we should see a difference.
[In fact, may want to just plot distribution of \chi=(c_i-c_0)/sig_i, to retain maximum information.  An object with a few outlying large negative \chi values may not be the same as an object with a few outlying large positive \chi values, but could have an indistinguishable \chi^2 distribution.  So that's a reason for using just \chi.]
(Note that a quick look at the Stripe 82 dataset shows that some method for excluding outliers (X sigma from the median color for each quasar?) needs to be implemented before calculating \chi_i^2.)

A refined prediction is that some subset of BAL quasars will show more color variability than non-BALs; any BALs which only show absorption from distant (kpc-scale) gas will show variability consistent with non-BALs, but BALs with pc-scale gas will show more color variability. (One would also have to keep an eye out for objects which were non-BALs during a past spectroscopic epoch but which later developed a BAL trough.)

The \chi or \chi_i^2 distribution won't answer the question of BAL trough variability timescales; to do so, I think you need to look at the distribution of the rate of colour changes r_ij=(c_j-c_i)/(t_j-t_i) where the t values are the rest-frame times of observation. If trough variations are slow, you'll have lots of small values of r_ij. If trough variations are fast, you'll have lots of even smaller values of r_ij, plus a few large values. (One complication is that the uncertainty on r_jk will be correlated with that on both r_ij and r_kl. A crude workaround is to separate each quasar into two datasets: ij in 1, jk in 2, kl in 1, lm in 2, etc., so errors are uncorrelated within each dataset.) One would have to compare the BAL colour change rate distribution to that of non-BALs, as colour changes caused by trough variability will be in addition to any intrinsic colour variability (e.g., quasars usually get slightly bluer when they get brighter). And comparisons should be done in bins of similar rest-frame wavelength [<- most important], luminosity and black hole mass, [and BAL subtype (Hi/Lo/FeLo) and maybe Eddington ratio] to remove any effects of those parameters on intrinsic variability and to search for any dependency of trough variability on those parameters.

A plot of c_j-c_i vs. t_j-t_i for all i,j pairs would also be useful to see just how fast color changes of a given magnitude can happen.

If that timescale is not too short, then it might be worth monitoring BAL quasars photometrically to catch new cases of trough variability as they start to happen and then trigger spectroscopic followup. It might be possible to demonstrate the feasibility of such an approach using SDSS: establish a threshold of |delta_c/sig_c| > 3 (say), look for quasars whose delta_c/sig_c exceeds that threshold _after_ a spectroscopic epoch in which the quasar appears as a non-BAL, and then look for the subset of objects with a 2nd epoch spectrum after that color change. The 2nd epoch spectrum should show a new BAL, or other unusual effect like a transient increase in reddening or strong emission line variation.

I'll link back to this post y/o post a comment if/when I pick up this idea again. It's worth doing, but I need to finish other projects first.

Thursday, October 21, 2010

BAL quasars

Bit more research chatting with OSU grad students.

Skype conversation with my grad student Laura Chajet, about fitting absorption troughs.

Worked on two new cases of BAL variability from SDSS-III, including adapting existing software to more easily deal with SDSS-III spectra. Time spent writing code now will pay off in not having to repeat the same processing steps in the future (obvious, yes; but when I'm teaching I find I opt for the what's-quickest-now approach instead of what's-best-in-the-long-run).

Investigated whether a list of BALs bright enough for MDM spectroscopy have pre-existing X-ray observations. Some do, making them good targets for spectroscopic monitoring (and a 2nd epoch of X-ray data if their troughs are seen to vary).

Wednesday, October 20, 2010

Tue-Wed Oct 19-20

Met with Matthias Dietrich & Smita Mathur about observing proposal to monitor bright BAL quasars.

Discussed paper on variable BAL quasar at Astro Coffee; general interest among AGN folk in the paper and in searching for similar objects in light-curve datasets.

Skype meeting with my grad student Jesse Rogerson.

Isolated, downloaded and examined 135 SDSS-III repeat spectra of BAL quasars. Found one new case of dramatic variability!

Monday, October 18, 2010

Monday Oct. 18th

Read the latest draft of a paper on PHL 1811 analogs (intrinsically X-ray-weak quasars), and did some measurements on the spectra of possibly similar moderately X-ray-weak quasars, which turn out to appear intermediate in other properties as well.

Posted the recently accepted paper on; it should appear in Tuesday evening's mailing.

Switched desks in my office at Ohio State.

Friday, October 15, 2010

Historical Photometry of BAL quasars

The paper on dramatic variability in a BAL quasar that I submitted last week to MNRAS (see Wed. Oct. 6 post) is now accepted; look for it on arXiv next week.

York undergrad Ted Rudyk visited Wednesday and Thursday. We were investigating whether old photographic magnitudes could be used to look for other cases of 'overlapping trough' BAL quasars transitioning between heavily absorbed (red) and unabsorbed (blue) states. We found a few cases of potential long-term variations in BAL trough strength as traced by g-r color (see graph for SDSS J090212.4+592415 above), but we're limited by the photometric accuracy of the old POSS-I and DPOSS magnitudes. My colleague Zeljko Ivezic and his former grad student Branimir Sesar have produced recalibrated photographic magnitudes for quasars from SDSS DR2, but there are no plans to extend the recalibration to DR7 quasars. If such a recalibration did get done, we could look at long-term quasar color variability in BAL vs. non-BAL quasars, including searching for quasars which aren't BALs now but were in the past. Could also take a stab at this using SDSS Stripe 82 data from Zeljko's current grad student Chelsea MacLeod.

Ted and I talked to Smita Mathur and Matthias Dietrich about possible BAL variability projects with the MDM 2.4-meter telescope to which OSU has access. Biweekly to monthly monitoring of quasars with high-velocity outflows and known X-ray properties seems to be the best bet.

Talked to Phil Hopkins about searching for radial inflow (with ALMA and in quasar spectra) as a signature of the single-armed spiral waves that feed AGNs in his simulations.

On Friday gave this journal club on distance measurements to BAL outflows.

Tuesday, October 12, 2010

Monday & Tuesday

Played around with SDSS-III spectra to familiarize myself with data access. Already have one cool result I can't post because the data isn't public yet.

Wrote referee reports for two Gemini proposals.

Spent a bit of time thinking about PHL 1811 analogues.

Spent time discussing research with grad students and visiting postdoc Phil Hopkins.

Friday, October 8, 2010

Rest of Friday

Refereed a Canada-France-Hawaii Telescope proposal.

Discussed some plots over at my grad student Jesse Rogerson's research blog.

Looked at some talks from this summer's Accretion Processes in X-Rays Workshop.

Retrieved SDSS magnitudes & epochs of overlapping trough BALs for Ted Rudyk.

Past 48 hours

Not quite yet in the habit of daily posts.

Spent part of Wed. evening conferring with York undergrad Ted Rudyk about a nascent project to look for historical variability of "overlapping-trough" FeLoBALs like FBQS J1408+3054 seen in the previous post's animated gifs.

Thursday and Friday up till now spent in extremely productive conversations with colloquium visitor Daniel Proga (UNLV), who does numerical simulations of disk winds. Started my review of the literature on disk winds and quasars in general with his papers. The goal is to refresh my picture of how quasars work; to aid in this I am investigating the Papers software as well as other ways of disseminating research thoughts (like this blog).

Wednesday, October 6, 2010

Revised FBQS 1408 paper submitted

Today I submitted this revised paper to MNRAS. The gist of the paper is shown in the animated gif below. The animation shows the spectrum of the BAL quasar FBQS 1408+3054 from 1995 to 2009 (black curves are data, grey curves are interpolations). Note that starting sometime after 2000, the Fe II absorption starts to go away. (more below animation)

The disappearing Fe II absorber is even more remarkable when historical photographic-plate photometry is taken into account. That photometry shows that this quasar is consistent with having been an FeLoBAL quasar for 50 years before its Fe II absorber vanished over a period of 5 years. That history is shown schematically in the animated gif below (which takes a while to run).

(Other activities today so far included submission of merit exercise paperwork to York and going around and introducing myself to some of the grad students and postdocs here at OSU and asking about their research.)

Tuesday, October 5, 2010


This blog is intended to keep my students, collaborators and other interested parties up to date with my astronomy research activities.

For example, today I put the finishing touches on a paper for MNRAS (Monthly Notices of the Royal Astronomical Society) about a broad absorption line (BAL) quasar whose singly-ionized Fe II absorption region disappeared. This is the first time an FeLoBAL (Iron Low-Ionization BAL) quasar has been seen to change into a LoBAL.

You can read quick summaries of most of my recent research interests