Broad Absorption Line (BAL) Quasar Variability Survey


Photo of Niel Brandt
Niel Brandt
Pennsylvania State University


Spectra for broad absorption lines (BAL) quasars in a 5740 deg2 survey area

Finding Targets

An object whose ANCILLARY_TARGET1 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers Number of Unique Primary Objects
FBQSBAL 15 Broad absorption line (BAL) quasar with spectrum from the FIRST Bright Quasar Survey (FBQS) 13 10
LBQSBAL 16 Broad absorption line (BAL) quasar with spectrum from the Large Bright Quasar Survey (LBQS) 6 6
ODDBAL 17 Broad absorption line (BAL) quasar with various unusual properties 37 27
OTBAL 18 Photometrically-selected overlapping-trough (OT) broad absorption line (BAL) quasar 16 9
PREVBAL 19 Broad absorption line (BAL) quasar with prior spectrum from SDSS-I/-II 34 14
VARBAL 20 Photometrically-selected candidate broad absorption line (BAL) quasar 1,699 1,327


Thousands of broad absorption line (BAL) quasars were discovered in the SDSS-I and SDSS-II (e.g. Gibson et al. 2009). In some cases, repeat spectroscopy showed variable absorption, providing clues to the nature of the BAL phenomenon (e.g., Lundgren et al. 2007, Gibson et al. 2008, Gibson et al. 2010). Returning with BOSS to obtain repeat spectra for a much larger sample of these quasars allows for large-scale study of BAL variability on multi-year timescales in the rest frame. The resulting data provide insight into the dynamics, structure, and energetics of quasar winds.

The first science results from this ancillary project have appeared in ApJ (Filiz Ak et al. 2012, Filiz Ak et al. 2013), and were the subject of an SDSS-III Press Release of October 1, 2012: Gone, with the Wind.

Target Selection

Targets for this ancillary project were selected before the decision was made to re-target known quasars at z > 2.15 in the BOSS Quasar target selection algorithm; thus there is some overlap between the two samples. However, this ancillary project does provide many unique targets at z < 2.15.

The main sample of BAL quasars chosen for study contains objects assigned the ancillary target flag VARBAL; this sample is about two orders of magnitude larger than those previously used to investigate BAL variability on multi-year timescales. These objects were selected to be optically bright (iPSF < 19.28 with no correction for extinction), and to have at least moderately strong absorption in one of their BAL troughs (with a “balnicity index” of BI0 > 100 km/s as measured by Gibson et al. (2009).

In addition, only quasars in redshift ranges such that strong BAL transitions are fully covered by SDSS-I/-II/BOSS spectra (from outflow velocities of 0-25000 km/s) were included; see Section 4 of Gibson et al. (2009) for further explanation. The corresponding redshift ranges are 1.96 < z < 5.55 for Si IV BALs, 1.68 < z < 4.93 for C IV BALs, 1.23 < z < 3.93 for Al III BALs, and 0.48 < z < 2.28 for Mg II BALs.

Finally, for those objects in the Gibson et al. (2009) catalog that have measurements of the signal-to-noise ratio at rest-frame 1700 Å (SN1700), we require SN1700 > 6; this criterion ensures that high-quality SDSS-I/-II spectra are available for these targets.

In addition to the primary sample objects described above as VARBAL, the BAL quasar variability survey also targets additional BAL quasars selected through other target selection approaches. These targets may violate one or more of the selection criteria used for the VARBAL targets, but they have been identified as worthy of new observations nonetheless.

The relevant source types for these additional BAL quasars are the following:

  • LBQSBAL and FBQSBAL are BAL quasars identified in the Large Bright Quasar Survey (LBQS; e.g., Hewett et al. 1995) and the FIRST Bright Quasar Survey (FBQS; e.g., White et al. 2000), respectively. These targets thus have LBQS or FBQS spectra predating the SDSS-I/-II spectra by up to a decade or more.
  • OTBAL (Overlapping-Trough BAL quasars) are BAL quasars with nearly complete absorption at wavelengths shortward of Mg II in one epoch, and which in one case have already shown extreme variability (e.g., Hall et al. 2002). These objects were selected by hand as likely to be highly unusual.
  • PREVBAL are BAL quasars observed more than once by SDSS-I/-II. They thus already possess more than one observation epoch for comparison to BOSS spectra.
  • ODDBAL are BAL quasars selected to have various unusual properties (e.g., Hall et al. 2002). For these objects, variability (or lack thereof) between SDSS-I/-II and BOSS may help to unravel the processes responsible for their unusual spectra.


Filiz Ak, N., Brandt, W. N., Hall, P. B., Schneider, D. P., Anderson, S. F., Gibson, R. R., Lundgren, B. F., Myers, A. D., Petitjean, P., Ross, N. P.; Shen, Y., York, D. G., Bizyaev, D., Brinkmann, J., Malanushenko, E., Oravetz, D. J., Pan, K., Simmons, A. E., & Weaver, B. A. 2012 (Abstract from ADS), ApJ, 757, 114, doi:10.1088/0004-637X/757/2/114

Filiz Ak, N., Brandt, W. N., Hall, P. B., Schneider, D. P., Anderson, S. F., Hamann, F., Lundgren, B. F., Myers, A. D., Pâris, I., Petitjean, P., Ross, N. P., Shen, Y., York, D. G. 2013 (Abstract from ADS), ApJ, 777, 168, doi:10.1088/0004-637X/777/2/168

Gibson, R.R., Brandt, W.N., Schneider, D.P., & Gallagher, S.C., 2008, AJ, 675, 985, doi:10.1086/527462

Gibson, R.R., Jiang, L., Brandt, W.N., Hall, P.B., Shen, Y., Wu, J., Anderson, S.F., Schneider, D.P., Vanden Berk, D., Gallagher, S.C., Fan, X., & York, D.G., 2009, AJ, 692, 758, doi:10.1088/0004-637X/692/1/758

Gibson, R.R., Brandt, W.N., Gallagher, S.C., Hewett, P.C., & Schneider, D.P., 2010, ApJ, 713, 200, doi:10.1088/0004-637X/713/1/220

Hall, P.B., et al., 2002, ApJS, 141, 267, doi:10.1086/340546

Hewett, P.C., Foltz, C.B. & Chaffee, F.H., 1995, AJ, 109, 1498, doi:10.1086/117380

Lundgren, B.F., Wilhite, B.C., Brunner, R.J., Hall, P.B., Schneider, D.P., York, D.G., Vanden Berk, D.E., & Brinkmann, J., 2007, AJ, 656, 73, doi:10.1086/510202

White, R.L., Becker, R.H., Gregg, M.D., Laurent-Muehleisen, S.A., Brotherton, M.S., Impey, C.D., Petry, C.E., Foltz, C.B., Chaffee, F.H., Richards, G.T., Oegerle, W.R., Helfand, D.J., McMahon, R.G., & Cabanela, J.E., 2000, ApJS, 126, 133, doi:10.1086/313300