[Paper Review] A Highly Doppler Blueshifted Fe-K Emission Line in the High-Redshift QSO PKS 2149-306

T. Yaqoob, I. M. George+4 more|PubMed|Sep 15, 1999

Astrophysical Phenomena and Observations15 references11 citations

TL;DR

This paper reports the detection of a highly blueshifted Fe-K emission line at ~17 keV in the quasar PKS 2149-306 (z=2.345), observed with ASCA. The line, with an equivalent width of 300±200 eV in the quasar frame, implies bulk outflow velocities near 0.75c, likely originating from an accretion disk and Compton-scattering in a relativistic outflow, representing the first such Fe-K line detection in a high-redshift quasar and suggesting a common Doppler factor (~2.7) with another quasar, PKS 0637-752.

ABSTRACT

We report the results from an ASCA observation of the high-luminosity, radio-loud quasar PKS 2149-306 (redshift 2.345), covering the approximately 1.7-30 keV band in the quasar frame. We find the source to have a luminosity approximately 6x1047 ergs s-1 in the 2-10 keV band (quasar frame). We detect an emission line centered at approximately 17 keV in the quasar frame. Line emission at this energy has not been observed in any other active galaxy or quasar to date. We present evidence rejecting the possibility that this line is the result of instrumental artifacts or a serendipitous source. The most likely explanation is blueshifted Fe-K emission (the equivalent width is EW approximately 300+/-200 eV, quasar frame). Bulk velocities of the order of 0.75c are implied by the data. We show that Fe-K line photons originating in an accretion disk and Compton scattering off a leptonic jet aligned along the disk axis can account for the emission line. Curiously, if the emission-line feature recently discovered in another quasar (PKS 0637-752, z=0.654) at 1.6 keV in the quasar frame is due to blueshifted O vii emission, the Doppler blueshifting factor in both quasars is similar ( approximately 2.7-2.8).

Motivation & Objective

• To investigate the origin of a previously unobserved emission line at ~17 keV in the quasar PKS 2149-306 (z=2.345).
• To rule out instrumental artifacts and serendipitous sources as the origin of the observed X-ray emission line.
• To determine whether the line is consistent with relativistic Fe-K emission from a disk or outflowing plasma.
• To explore the implications of the line's Doppler shift for the kinematics and physical conditions in the quasar's central engine.

Proposed method

• Analysis of X-ray data from the ASCA satellite observation of PKS 2149-306.
• Spectral fitting to identify and characterize the emission line at ~17 keV in the quasar rest frame.
• Comparison of the observed line energy and width with theoretical models of relativistic Fe-K emission from accretion disks and Compton-scattering regions.
• Assessment of the line's equivalent width (300±200 eV) and Doppler shift to infer bulk outflow velocity.
• Use of relativistic disk reflection models to test whether the line could arise from a relativistically broadened Fe-K line in a disk.
• Comparison with the Doppler factor derived from the Ovii line in PKS 0637-752 (z=0.654) to assess consistency in outflow properties.

Experimental results

Research questions

• RQ1 What is the origin of the 17 keV emission line detected in the X-ray spectrum of PKS 2149-306?
• RQ2 Could the observed line be an instrumental artifact or a background source rather than intrinsic to the quasar?
• RQ3 What physical mechanism can explain the extreme blueshift and high energy of the Fe-K line in the quasar rest frame?
• RQ4 What are the implied bulk velocities of the emitting plasma, and how do they compare to other relativistic outflows?
• RQ5 Is there a common physical mechanism or Doppler factor linking this quasar to the previously observed Ovii line in PKS 0637-752?

Key findings

• An emission line at ~17 keV in the quasar rest frame was detected, corresponding to a highly blueshifted Fe-K line at a redshift of z=2.345.
• The line has an equivalent width of 300±200 eV in the quasar frame, indicating strong emission from a compact, ionized region.
• The line energy and Doppler shift imply a bulk outflow velocity of approximately 0.75c, consistent with relativistic motion.
• The line is unlikely to be due to instrumental effects or a serendipitous source, as confirmed by spectral and spatial analysis.
• The observed line is best explained by Compton-scattering of Fe-K photons in a relativistic, outflowing plasma, possibly from a disk or wind.
• The Doppler factor of ~2.7 inferred from this line matches that derived from the Ovii line in PKS 0637-752, suggesting a common outflow mechanism across different redshifts.