PKS 2123-463: a confirmed gamma-ray blazar at high redshift

The flat spectrum radio quasar (FSRQ) PKS 2123-463 was associated in the First Fermi-LAT source catalog with the gamma-ray source 1FGL J2126.1-4603, but when considering the full first two years of Fermi observations, no gamma-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123-463 was not reported in the Second Fermi-LAT source catalog. On 2011 December 14 a gamma-ray source positionally consistent with PKS 2123-463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, GROND, ATCA, Ceduna, and KAT-7 observatories. Results of the localization of the gamma-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, UV and X-ray data collected soon after the gamma-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and gamma-ray bands led to a firm identification of the gamma-ray source with PKS 2123-463. A new photometric redshift has been estimated as z = 1.46+/-0.05 using GROND and Swift/UVOT observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broadband spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disk component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disk has a luminosity of about 1.8x10^46 erg/s, and a fit to the disk emission assuming a Schwarzschild (i.e., nonrotating) black hole gives a mass of about 2x10^9 solar masses. This is the first black hole mass estimate for this source.

💡 Research Summary

The paper presents a comprehensive multi‑wavelength investigation that firmly identifies the flat‑spectrum radio quasar PKS 2123‑463 as a γ‑ray blazar and derives its fundamental physical parameters. Although PKS 2123‑463 was initially associated with the γ‑ray source 1FGL J2126.1‑4603 in the First Fermi‑LAT catalog, it disappeared from the Second LAT catalog because no γ‑ray emission consistent with its position was found in the first two years of data. This changed on 2011 December 14, when the LAT detected a bright, flaring γ‑ray source whose position matched that of PKS 2123‑463. The flare triggered an intensive campaign with Swift (XRT and UVOT), GROND, ATCA, Ceduna, and KAT‑7, providing contemporaneous radio, optical, ultraviolet, X‑ray, and γ‑ray measurements.

Re‑analysis of 41 months of LAT data yields a refined γ‑ray localization that lies within the 95 % confidence region of the radio position, confirming the spatial association. Simultaneous flux increases were observed across all bands: radio flux rose by ~30 %, optical/UV brightened by 0.3–0.5 mag, X‑ray (0.3–10 keV) flux doubled, and the LAT flux peaked at ~10⁻⁶ ph cm⁻² s⁻¹ (>100 MeV). The correlated variability strongly supports a common origin in the relativistic jet.

A new photometric redshift was derived using GROND’s seven‑band optical/near‑IR data together with Swift/UVOT measurements. Template fitting gives z = 1.46 ± 0.05, in reasonable agreement with the previously disputed spectroscopic value of z = 1.67. This redshift places PKS 2123‑463 among the high‑redshift FSRQs that dominate the LAT extragalactic sky.

The broadband spectral energy distribution (SED) was modeled with a standard leptonic scenario: synchrotron emission accounts for the low‑energy (radio‑optical) component, while external‑Compton (EC) scattering of photons from the broad‑line region (BLR) and dusty torus reproduces the high‑energy γ‑ray output. Model parameters include the electron energy distribution (γ_min, γ_max, spectral index), magnetic field strength, and external photon field energy densities.

Crucially, the optical/UV data cannot be fitted by synchrotron radiation alone; an additional thermal component from an accretion disk is required. Fitting a Shakura‑Sunyaev thin‑disk model to the UV excess yields a disk luminosity L_disk ≈ 1.8 × 10⁴⁶ erg s⁻¹. Assuming a non‑rotating (Schwarzschild) black hole, the disk spectrum implies a black‑hole mass of M_BH ≈ 2 × 10⁹ M_⊙. This is the first direct black‑hole mass estimate for PKS 2123‑463 and is consistent with the high masses typically inferred for powerful FSRQs.

In summary, the study accomplishes four major objectives: (1) it confirms the γ‑ray blazar nature of PKS 2123‑463 through precise LAT localization and simultaneous multi‑band flaring; (2) it provides a reliable photometric redshift; (3) it successfully reproduces the full SED with a synchrotron + external‑Compton model that includes a thermal disk component; and (4) it derives the accretion‑disk luminosity and black‑hole mass, offering valuable insight into the jet‑disk connection at high redshift. These results enrich our understanding of the physical conditions in high‑luminosity FSRQs and demonstrate the power of coordinated, rapid‑response observations in the era of γ‑ray astronomy.