High Energy Blazars and Optical Counterparts of Gamma-ray Sources

Summary

A search for the optical counterparts of Fermi gamma-ray sources in a the SDSS Data Release 7 survey area (7,650 square degrees)

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.

Description

We targeted candidate optical counterparts of sources detected (or likely to be detected) by NASA’s Fermi Gamma-ray Space Telescope (Atwood et al. 2009), with the goal of spectroscopically confirming, and providing redshifts for, candidate gamma-ray blazars.

Target Selection

We require:

• Model magnitude < 21 in any of the three bandpasses g, r, or i
• 3″ fiber magnitudes > 16.5 (to minimize impact of fiber cross-talk)

Ranked in approximate order of priority, fibers are assigned to targets from the following subprograms:

• BLAZGXR: blazar candidates are assigned at highest priority to DR7 optical sources within Fermi gamma-ray error ellipses. Targets must also lie within the < 1′ radius error circle for X-ray sources in the ROSAT All-Sky Survey (RASS; Voges et al. 1999; Voges et al. 2000) and within 2″ of a FIRST (Becker et al. 1995) radio source.
• BLAZGRFLAT: blazar candidates detected with Fermi and the Combined Radio All-Sky Targeted Eight GHz Survey (CRATES; Healey et al. 2007). Objects from the DR7 catalog within 2″ of a CRATES radio source and within a Fermi error ellipse were targeted.
• BLAZGXQSO: candidate X-ray and gamma-ray emitting quasars/blazars, including photometric quasar/blazar candidates (Richards et al. 2009), as well as confirmed DR7 quasars/blazars (Schneider et al. 2010) revisited to assess optical spectral variability. Targets are selected that lie within < 1′ of a RASS X-ray source and within Fermi error ellipses.
• BLAZGRQSO: candidate radio and gamma-ray emitting quasars/blazars, including both photometric candidates (Richards et al. 2009), and DR7 confirmations (Schneider et al. 2010) revisited to assess optical spectral variability. Targets are selected that lie within 2″ of a FIRST radio source and within Fermi error ellipses.
• BLAZGX: targets that are candidate high-energy counterparts but which lack typical (e.g., radio emission, unusual optical color, etc.) blazar properties were targeted to probe unknown classes of gamma ray sources. The optically brightest objects from DR7 within the Fermi error ellipses and within 1′ of a RASS X-ray source were preferentially targeted.
• BLAZXR: targets are selected that may plausibly emerge as Fermi sources, but are still below the detection limits in the early Fermi source catalogs. The approach is similar to the “ROSAT_A” target selection scheme described in Anderson et al. (2003) and the “pre-selection” approach of Healey et al. (2008) that provided many of the gamma-ray counterpart associations reported in the first Fermi catalogs (Abdo et al. 2010b, Abdo et al. 2010a). Targets are chosen from the DR7 photometry catalog with radio coincidence (within 2″ of a FIRST source) and X-ray coincidence (< 1′ of a RASS source). This sample overlaps heavily with the BONUS quasar sample, but includes quasars at lower redshift.

In addition, there were ten miscellaneous candidate blazar spectra taken in an early trial of this program. These targets were assigned subcategory names using the following flags: BLAZGVAR, BLAZR, and BLAZXRSAM.

REFERENCES

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