5BZQ J1243+4043: Dual Role as Quasar & Blazar

• 5BZQ J1243+4043 is a radio-loud quasar/blazar that serves as both a background beacon for detailed 21-cm absorption studies through UGC 07904 and a changing-look blazar with variable optical spectra.
• Its multi-scale H I 21-cm spectroscopy reveals DLA-level neutral hydrogen columns, a harmonic-mean spin temperature near 400 K, and a coherent cold medium spanning parsec to tens of parsec scales.
• Despite its optical state changes, the source maintains HERG-like accretion and powerful jet activity, making it a promising candidate for multi-messenger studies including high-energy neutrino emission.

Searching arXiv for the specified papers and closely related records. 5BZQ J1243+4043, also referred to as SDSS J124357.5+404346.5 or simply J1243+4043, is a radio-loud quasar/blazar of particular interest in two distinct but connected observational contexts. In one, it serves as the background source in a low-redshift quasar–galaxy pair with the foreground galaxy UGC 07904, enabling a detailed study of neutral hydrogen in both 21-cm absorption and 21-cm emission from arcsecond to milliarcsecond scales (Gupta et al., 2017). In the other, it is studied as a changing-look blazar candidate and as a candidate high-energy neutrino emitter, with optical spectra alternating between broad-line FSRQ-like and featureless BL Lac-like states while retaining HERG-like, radiatively efficient accretion properties (Azzollini et al., 21 Sep 2025). Reported source redshifts differ between these observational settings: the quasar–galaxy-pair study quotes $z_q = 1.5266 \pm 0.0032$ from IGO optical spectroscopy, whereas the later blazar study confirms a literature value $z = 1.5181 \pm 0.0002$ from broad emission lines.

1. Identification and astrophysical role

In the absorber study, J1243+4043 is the background quasar behind UGC 07904, and the sight line is treated as a physically direct probe through the galaxy’s optical extent rather than as a generic intervening absorber (Gupta et al., 2017). The quasar is compact in FIRST at arcsecond resolution, polarized, and flat-spectrum overall, with integrated radio spectral index $\alpha \approx -0.3$. On VLBI scales it exhibits a core–jet morphology.

In the later blazar study, the same source is designated 5BZQ J1243+4043 and is analyzed as a changing-look blazar candidate that has shown broad optical emission lines in some epochs and a featureless continuum in others (Azzollini et al., 21 Sep 2025). The paper explicitly frames it as both a candidate high-energy neutrino emitter and an object relevant to the BL Lac/FSRQ connection. This combination of roles makes the source unusual: it is simultaneously a background beacon for foreground interstellar-medium tomography and a variable relativistic jet source whose own optical classification changes with epoch.

2. Sight-line geometry through UGC 07904

The foreground galaxy UGC 07904 lies at $z_g = 0.0169$, with the absorption analysis using $z_g = 0.01693 \pm 0.00001$ for the galaxy center (Gupta et al., 2017). The quasar sight line passes through the outer disk / spiral arm of the galaxy at an impact parameter of $20.3$ arcsec, corresponding to $6.9$ kpc. The study emphasizes that this geometry samples a region within the galaxy’s optical extent.

The local gas-phase environment near the quasar position is moderately metal-rich, with metallicity estimated at $\sim 0.5\,Z_\odot$, and the line of sight is unusually dust free. The quasar’s relative color is reported as $\Delta(g-i) = -0.05 \pm 0.02$, indicating that it is slightly bluer than typical quasars at comparable redshift. The paper also notes non-detections of Na I and Ca II absorption at the foreground-galaxy redshift, while adding that the data quality and spectral resolution are not ideal for such weak lines.

This geometry is central to the scientific value of the system. It allows direct comparison between optical diagnostics of metallicity, reddening, and kinematics and radio diagnostics of neutral gas on multiple physical scales.

3. Multi-scale H I 21-cm spectroscopy

A defining result is the detection of H I 21-cm absorption toward J1243+4043 in GMRT, WSRT, and global-VLBI observations (Gupta et al., 2017). On arcsecond scales, the GMRT spectrum yields a total integrated optical depth

$\int \tau(v)\,dv = 2.24 \pm 0.10,$

with about 90% of the optical depth confined within $z = 1.5181 \pm 0.0002$0. The absorption peak is redshifted by roughly $z = 1.5181 \pm 0.0002$1 relative to the galaxy systemic redshift. The absorption kinematics are consistent with co-rotation with the stellar disk and match the H$z = 1.5181 \pm 0.0002$2 velocity field derived from optical slit data.

Using the standard 21-cm absorption formalism with $z = 1.5181 \pm 0.0002$3, the authors note that for a canonical CNM spin temperature of $z = 1.5181 \pm 0.0002$4 K, the inferred column density is

$z = 1.5181 \pm 0.0002$5

so the absorber qualifies as a damped Ly$z = 1.5181 \pm 0.0002$6 absorber for typical cold-gas temperatures. This establishes the system as DLA-level in neutral hydrogen column density under standard CNM assumptions.

The WSRT data are particularly important because they detect both 21-cm emission from UGC 07904 at the quasar position and 21-cm absorption superposed on that emission. The emission is modeled with a single Gaussian of FWHM $z = 1.5181 \pm 0.0002$7. From optically thin emission, the derived column density is

$z = 1.5181 \pm 0.0002$8

Combining this value with the GMRT integrated optical depth gives a harmonic mean spin temperature

$z = 1.5181 \pm 0.0002$9

An independent upper limit derived from the local star-formation rate and the Kennicutt–Schmidt relation gives

$\alpha \approx -0.3$0

implying

$\alpha \approx -0.3$1

The preferred characterization is therefore $\alpha \approx -0.3$2 K.

These measurements place the absorber in an intermediate thermal regime: cold enough to produce strong 21-cm absorption and DLA-level columns, but with a harmonic-mean spin temperature substantially above the canonical $\alpha \approx -0.3$3 K CNM benchmark.

4. Parsec-scale radio structure and cold neutral medium constraints

Global-VLBI resolves J1243+4043 on milliarcsecond scales and detects 21-cm absorption at full spectral resolution of $\alpha \approx -0.3$4 (Gupta et al., 2017). The VLBI continuum image has a beam of $\alpha \approx -0.3$5 and rms $\alpha \approx -0.3$6. The quasar shows a core–jet morphology extending southward over $\alpha \approx -0.3$7 mas, corresponding to $\alpha \approx -0.3$8 pc at the foreground-galaxy redshift. A lower-resolution VLBI image recovers diffuse emission extending to $\alpha \approx -0.3$9 mas, or $z_g = 0.0169$0 pc, south of the core. About 85% of the arcsecond-scale flux is recovered on VLBI scales.

The 21-cm absorption is detected against the VLBI core, and the line profiles across six $z_g = 0.0169$1 subregions are very similar. The study concludes that absorption variations across the inner $z_g = 0.0169$2 pc are not significant, that the absorbing clouds must be at least $z_g = 0.0169$3 pc in size, and that the absorbing structures likely cover the full source extent and may extend over $z_g = 0.0169$4 pc. The principal parsec-scale result is therefore that the CNM is not confined to a tiny clump but instead traces a coherent structure spanning many parsecs.

The VLBI absorption profile is modeled with three Gaussian components, labeled A, B, and C. Their fitted values are:

• Component A: $z_g = 0.0169$5, FWHM $z_g = 0.0169$6, $z_g = 0.0169$7
• Component B: $z_g = 0.0169$8, FWHM $z_g = 0.0169$9, $z_g = 0.01693 \pm 0.00001$0
• Component C: $z_g = 0.01693 \pm 0.00001$1, FWHM $z_g = 0.01693 \pm 0.00001$2, $z_g = 0.01693 \pm 0.00001$3

The narrowest component, B, is interpreted as cold gas; assuming pure thermal broadening, its width implies a kinetic temperature

$z_g = 0.01693 \pm 0.00001$4

The study adopts this as representative of the CNM spin temperature in the absorbing phase. Using a simple two-phase medium relation, the inferred CNM fraction is

$z_g = 0.01693 \pm 0.00001$5

The paper emphasizes that this value is very similar to that observed in the Milky Way and less than that of high-redshift DLAs.

5. Polarization, rotation measure, and the galaxy-group environment

J1243+4043 has a published $z_g = 0.01693 \pm 0.00001$6 GHz rotation measure

$z_g = 0.01693 \pm 0.00001$7

and polarization fraction

$z_g = 0.01693 \pm 0.00001$8

The Galactic foreground contribution in this direction is small,

$z_g = 0.01693 \pm 0.00001$9

The absorber study reports a Bayesian probability of $20.3$0 that the absolute RM of J1243+4043 exceeds that of the high-$20.3$1 DLA quasar sample from Farnes et al. (Gupta et al., 2017). At the same time, it does not find statistically significant differences in RM or polarization fraction between sight lines with and without high-$20.3$2 ($20.3$3) DLAs or low-$20.3$4 ($20.3$5) 21-cm absorbers.

This distinction is important. The source has an unusually high absolute RM relative to the comparison high-$20.3$6 DLA sample, but the paper does not claim a secure intrinsic magneto-ionic enhancement caused by the absorber itself. Instead, it argues that the apparent difference may reflect selection effects or the quasar environment.

The foreground galaxy UGC 07904 is also a member of a small group. WSRT observations serendipitously detect 21-cm emission from all four group members—UGC 07904, UGC 07921 / IC 3726, IC 3723, and SDSS J124423.25+404148.5—and reveal an $20.3$7 kpc H I bridge connecting two of them (Gupta et al., 2017). For UGC 07904, the integrated H I flux is

$20.3$8

corresponding to an H I mass of about

$20.3$9

The group members are described as mostly low-mass, blue, star-forming galaxies. Disturbed morphologies and gas redistribution, especially in IC 3723, lead the paper to characterize the group as a highly interactive environment.

6. Changing-look blazar behavior and accretion diagnostics

A later study analyzes 5BZQ J1243+4043 as a changing-look blazar using three optical spectra spanning about 12 years (Azzollini et al., 21 Sep 2025).

Epoch	Observation	Reported behavior
#1	NOT, 2012 April 18, MJD 56035.11	Prominent broad emission lines
#2	LAMOST, 2013 March 9, MJD 56360.75	Featureless
#3	GTC, 2024 January 9, MJD 60318.21	Broad emission lines return

The GTC spectrum was obtained with OSIRIS/R1000B on the Gran Telescopio Canarias and reduced with PypeIt. Spectrum #1 shows broad lines at observed wavelengths $6.9$0 Å and $6.9$1 Å, consistent with the source redshift. Spectrum #2 appears featureless, yielding a BL Lac-like optical appearance. Spectrum #3 again shows broad emission lines at $6.9$2 Å, $6.9$3 Å, $6.9$4 Å, $6.9$5 Å, and $6.9$6 Å, corresponding to C III], C IV, $6.9$7, $6.9$8, and Mg II-related rest-frame features at the known redshift. The paper also notes double absorption components on the blue wing of the C III] profile at $6.9$9–$\sim 0.5\,Z_\odot$0 Å and $\sim 0.5\,Z_\odot$1–$\sim 0.5\,Z_\odot$2 Å, possibly associated with intervening or host-galaxy ISM or with outflowing material.

The confirmed redshift in this later study is

$\sim 0.5\,Z_\odot$3

The continuum is modeled as a power law,

$\sim 0.5\,Z_\odot$4

The reported continua are:

• Epoch #1: $\sim 0.5\,Z_\odot$5, $\sim 0.5\,Z_\odot$6
• Epoch #2: $\sim 0.5\,Z_\odot$7, $\sim 0.5\,Z_\odot$8
• Epoch #3: $\sim 0.5\,Z_\odot$9, $\Delta(g-i) = -0.05 \pm 0.02$0

The highest continuum state is therefore the featureless LAMOST epoch. For that epoch, upper limits on undetected broad-line fluxes were obtained by simulating lines with fixed FWHM of $\Delta(g-i) = -0.05 \pm 0.02$1, giving limits of $\Delta(g-i) = -0.05 \pm 0.02$2, $\Delta(g-i) = -0.05 \pm 0.02$3, and $\Delta(g-i) = -0.05 \pm 0.02$4 in units of $\Delta(g-i) = -0.05 \pm 0.02$5. The interpretation advanced in the paper is that the jet continuum in epoch #2 outshines the line emission rather than that the broad-line region disappears.

The study explicitly connects the source to the empirical optical classification boundary

$\Delta(g-i) = -0.05 \pm 0.02$6

while preferring the more physical HERG/LERG framework. Using $\Delta(g-i) = -0.05 \pm 0.02$7 as the accretion-state diagnostic, with a dividing line of approximately $\Delta(g-i) = -0.05 \pm 0.02$8, it reports $\Delta(g-i) = -0.05 \pm 0.02$9 in epoch #1, $\int \tau(v)\,dv = 2.24 \pm 0.10,$0 in epoch #3, and $\int \tau(v)\,dv = 2.24 \pm 0.10,$1 in epoch #2. These values are all above the dividing line, so the source is classified as HERG-like throughout. The reported radio power at $\int \tau(v)\,dv = 2.24 \pm 0.10,$2 is $\int \tau(v)\,dv = 2.24 \pm 0.10,$3, above the radio-power boundary of roughly $\int \tau(v)\,dv = 2.24 \pm 0.10,$4 used in the paper, again consistent with HERG-like behavior.

Within that framework, the optical changing-look behavior does not imply a change in the underlying engine. The paper concludes that the source remains characterized by radiatively efficient accretion, intense external radiation fields, and powerful jets across all observed epochs. In the multi-messenger context, it therefore remains a plausible environment for high-energy neutrino production, because leptohadronic scenarios favor strong external radiation fields, radiatively efficient accretion, and powerful jets (Azzollini et al., 21 Sep 2025).

7. Integrated physical picture

Taken together, the available studies portray 5BZQ J1243+4043 as a source of dual astrophysical utility. As a background quasar behind UGC 07904, it probes a metal-rich, weakly dusty outer-disk region in which 21-cm absorption and emission can be combined to infer DLA-level H I columns, a harmonic-mean spin temperature of $\int \tau(v)\,dv = 2.24 \pm 0.10,$5 K, and a CNM fraction of $\int \tau(v)\,dv = 2.24 \pm 0.10,$6 (Gupta et al., 2017). The parsec-scale VLBI data further indicate that the absorbing cold gas is organized in coherent structures extending at least several parsecs and plausibly tens of parsecs.

As a blazar, it alternates observationally between broad-line and featureless optical states without evidence for a transition out of the radiatively efficient, HERG-like regime (Azzollini et al., 21 Sep 2025). This suggests that the BL Lac-like epochs are dominated by continuum dilution from the jet rather than by a disappearance of the line-emitting or accretion structures. A plausible implication is that the source’s changing optical appearance and its utility as a stable background radio probe are not contradictory but complementary: the same compact, polarized, flat-spectrum source that resolves foreground neutral gas on parsec scales also hosts the powerful jet and external radiation fields relevant to changing-look blazar phenomenology and neutrino-emission models.

The result is a system that links several domains usually treated separately: low-redshift DLA analogs, parsec-scale CNM structure, galaxy-group gas dynamics, blazar optical state changes, and multi-messenger source characterization.