Hunting for Gravitationally Lensed AGN

The Abell 2218 Galaxy Cluster (HST).

The Abell 2218 Galaxy Cluster (HST).

Gravitational lensing is a powerful tool for probing high-redshift galaxies. Massive clusters of galaxies magnify (increase the size) and amplify (increase the number of photon counts) background sources, allowing for studies of these sources at higher spatial resolution as well as the detection and characterization of sources that would have been too distant and therefore faint for current instrumentation to detect (see Kneib & Natarajan 2011 for a recent review of the field). Typically, surveys for lensing candidates are carried out at optical and near- infrared wavelengths. More recently, one massive cluster (MACS J0717.5+3745) was studied at radio and X-ray wavelengths (van Weeren et al. 2016), resulting in part in the discovery of two lensed X-ray sources.

Massive clusters contain an X-ray continuum from the intracluster medium (ICM), which can be a concern for X-ray source searches as it produces significant back- ground signal even in the hard (~3.0-7.0 keV) bands (massive clusters tend to be X-ray hot (~6-12 keV)), which may obscure or wash our the regions of highest amplification near the cluster center. However, the payoff to such a search is that detected sources are al- most always active galactic nuclei (AGN), which can be verified by determinations of their luminosities as AGN have typical X-ray luminosities of 10e43-10e46 erg/s, while the most luminous star-forming galaxies (excluding rare, extreme starbursts in the early universe) have X-ray luminosities of 3e42 erg/s (Brandt & Alexander 2015). Deep imaging of each cluster is required, but generally most science goals oriented around clusters also require deep imaging, setting up lensing candidate searches as promising avenues for archival work.

To date, only four X-ray sources have been found be- hind clusters, two in MACSJ0717.5+3745 (van Weeren et al. 2016) and two in Abell 370 (Bautz et al. 2000). Based on these two results alone, it was unclear how many detectable lensed X-ray sources could be expected behind massive clusters—whether such sources would be highly rare or fairly common.

Because the chances of finding well-aligned AGN via gravitational lensing is lower than that for normal galax- ies (due to their rarity), a large sample of clusters is required to find a significant sample of these objects. CLASH (Cluster Lensing And Supernova survey with Hubble) is one such sample (see Postman et al. 2012 for details). The CLASH survey observed twenty-five massive clusters between z = 0.187 and z = 0.89 over the course of 524 HST orbits with 16 broadband HST filters, allowing for fairly precise (~ ±0.1) photometric redshifts to be obtained. CLASH is well-suited to carrying out an X-ray source search because (1) photometric redshifts are provided by the CLASH collaboration for all objects in the field, (2) magnification maps of each cluster region are provided based on the models of Zitrin et al. 2011, and (3) CLASH contains several very high- mass clusters, which are the best candidates for finding highly amplified objects (MACS J0717, for example, is included in the sample).

The Chandra X-ray Observatory has observed all of the CLASH clusters at various depths (see Table 1 for exposure times). Used concurrently with the HST imaging available for the CLASH sample, we extend the analysis employed on MACSJ0717.5+3745 (van Weeren et al. 2016) to all twenty-five clusters in the survey, result- ing in a new sample of lensed X-ray sources. In addition to increasing the number of known gravitationally lensed X-ray sources behind clusters significantly, the quantity of total sources found also points to the usefulness of carrying out such searches on new cluster surveys as they are completed.