Steinn Sigurdsson

Spectroscopic observations of the quasar J0950+5128 spanning 22 yr reveal monotonic radial velocity variations in its broad H β emission line. Moreover, the line profile becomes broader over time, necessitating careful measurements. We present robust H β velocity shift measurements obtained via cross correlation, applied to both the full spectra and to isolated broad H β components derived from spectral decomposition. We also examine the light curves for variability consistent with the spectroscopic trends. Using Lomb–Scargle periodogram analysis, we find no significant periodic signal. We consider several interpretations for the observed changes, including a binary supermassive black hole, dust-cloud obscuration, outflows, a recoiling black hole, and a single perturbed, disk-like broad-line region (BLR). We deem the binary and perturbed BLR scenarios to be physically plausible. The binary interpretation is the only one for which we can immediately compare a physical model to the available data. Thus, we incorporate radial velocity “jitter” to emulate typical quasar variability and fit the radial velocity curve with a Keplerian model to examine whether it can reproduce the observations. In this context, the available observations trace only a segment of the putative orbit. The fit yields a period of 33 yr (observed frame) and an eccentricity of 0.65, with lower limits on the semimajor axis and black hole mass of 10 ⁻² pc and 10 ⁷ M _⊙ , respectively. Thus, J0950+5128 is a binary candidate deserving further study. The single, perturbed BLR interpretation remains viable but requires additional observations and modeling for further evaluation. Continued monitoring is, therefore, essential.

We present Hubble Space Telescope ultraviolet (UV) spectroscopy of 13 quasars at z < 0.7, along with contemporaneous optical spectra from ground-based telescopes. The targets were selected to have broad Hβ emission lines with substantial velocity offsets relative to the rest frame of their host galaxy. By analogy to single-line spectroscopic binary stars, these objects have been regarded as supermassive black hole binary (SBHB) candidates where the offset emission lines may be caused by bulk orbital motion. The best alternative explanation is that the Hβ line profile is the result of nonaxisymmetric emission from a disk-like broad-line region associated with a single supermassive black hole. We use the broad UV line profiles to discriminate between these two scenarios. We describe our methodology for isolating the broad optical and UV line profiles and the criteria we apply for comparing them. Of the 13 SBHB candidates, three have strong evidence in support of the SBHB hypothesis, five have tentative support, one is disfavored, and four have such severely absorbed UV line profiles that the results are inconclusive.

The Laser Interferometer Space Antenna (LISA) will soon detect gravitational waves (GWs) emitted by massive black hole (MBH) mergers. Some theoretical models have predicted transient electromagnetic (EM) emission from these mergers, enabling the association of LISA GW sources with their EM counterparts via telescope follow-up. However, the number of unrelated EM transients that might contaminate telescope searches for the true transient counterparts of LISA MBH mergers is unknown. We investigate the expected numbers of unrelated EM transients that will coincide with simulated LISA localization volumes of MBH mergers as a function of the merger total mass and redshift. We find that the number of potential contaminants in LISA localization volumes drops to unity for mergers at z ≲ 0.8 and at 1 hr before coalescence. After coalescence, the parameter space corresponding to a maximum of one potential contaminant expands to z ≲ 1.5. In contrast, if the redshifts for all transients detected in LISA sky localization regions are not available, the number of potential contaminants increases by an average factor of ∼100 and never drops below unity. Overall, we expect the average number of contaminating transients in telescope follow-up of LISA MBH mergers to be nonnegligible, especially without redshift information for the detected transients. We recommend that endeavors designing follow-up strategies of LISA events should focus on (1) building large redshift catalogs for host galaxies, (2) developing robust real-time transient classification algorithms, and (3) coordinating telescope resources to obtain redshifts for candidate transient EM counterparts in a timely manner.

Whitepaper #038 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: outer heliosphere; space-based missions/projects; emerging opportunities.

We present QUOTAS, a novel research platform for the data-driven investigation of supermassive black hole (SMBH) populations. While SMBH data—observations and simulations—have grown in complexity and abundance, our computational environments and tools have not matured commensurately to exhaust opportunities for discovery. To explore the BH, host galaxy, and parent dark matter halo connection—in this pilot version—we assemble and colocate the high-redshift, z > 3 quasar population alongside simulated data at the same cosmic epochs. As a first demonstration of the utility of QUOTAS, we investigate correlations between observed Sloan Digital Sky Survey (SDSS) quasars and their hosts with those derived from simulations. Leveraging machine-learning algorithms (ML), to expand simulation volumes, we show that halo properties extracted from smaller dark-matter-only simulation boxes successfully replicate halo populations in larger boxes. Next, using the Illustris-TNG300 simulation that includes baryonic physics as the training set, we populate the larger LEGACY Expanse dark-matter-only box with quasars, and show that observed SDSS quasar occupation statistics are accurately replicated. First science results from QUOTAS comparing colocated observational and ML-trained simulated data at z3 are presented. QUOTAS demonstrates the power of ML, in analyzing and exploring large data sets, while also offering a unique opportunity to interrogate theoretical assumptions that underpin accretion and feedback models. QUOTAS and all related materials are publicly available at the Google Kaggle platform. (The full data set—observational data and simulation data—are available at: https://www.kaggle.com/ and the codes are available at:https://www.kaggle.com/datasets/quotasplatform/quotas)

In several previous studies, quasars exhibiting broad emission lines with ≳1000 km s⁻¹ velocity offsets with respect to the host galaxy rest frame have been discovered. One leading hypothesis for the origin of these velocity-offset broad lines is the dynamics of a binary supermassive black hole (SMBH). We present high-resolution radio imaging of 34 quasars showing these velocity-offset broad lines with the Very Long Baseline Array (VLBA), aiming to find evidence for the putative binary SMBHs (such as dual radio cores), and testing the competing physical models. We detect exactly half of the target sample from our VLBA imaging, after implementing a 5σ detection limit. While we do not resolve double radio sources in any of the targets, we obtain limits on the instantaneous projected separations of a radio-emitting binary for all of the detected sources under the assumption that a binary still exists within our VLBA angular resolution limits. We also assess the likelihood that a radio-emitting companion SMBH exists outside of our angular resolution limits, but its radio luminosity is too weak to produce a detectable signal in the VLBA data. Additionally, we compare the precise sky positions afforded by these data to optical positions from both the SDSS and Gaia DR2 source catalogs. We find projected radio/optical separations on the order of ∼10 pc for three quasars. Finally, we explore how future multi-wavelength campaigns with optical, radio, and X-ray observatories can help discriminate further between the competing physical models.

We estimate the merger timescale of spectroscopically selected, subparsec supermassive black hole binary (SMBHB) candidates by comparing their expected contribution to the gravitational-wave background (GWB) with the sensitivity of current pulsar timing array (PTA) experiments and in particular, with the latest upper limit placed by the North American Nanohertz Observatory for Gravitational Waves. We find that the average timescale to coalescence of such SMBHBs is yr, assuming that their orbital evolution in the PTA frequency band is driven by emission of gravitational waves. If some fraction of SMBHBs do not reside in spectroscopically detected active galaxies, and their incidence in active and inactive galaxies is similar, then the merger timescale could be ∼10 times longer, yr. These limits are consistent with the range of timescales predicted by theoretical models and imply that all the SMBHB candidates in our spectroscopic sample could be binaries without violating the observational constraints on the GWB. This result illustrates the power of the multimessenger approach, facilitated by the PTAs, in providing an independent statistical test of the nature of SMBHB candidates discovered in electromagnetic searches.

We present a method for comparing the Hβ emission-line profiles of observed supermassive black hole (SBHB) candidates and models of subparsec SBHBs in circumbinary disks. Using the approach based on principal component analysis, we infer the values of the binary parameters for the spectroscopic SBHB candidates and evaluate the parameter degeneracies, representative of the uncertainties intrinsic to such measurements. We find that as a population, the SBHB candidates favor the average value of the semimajor axis corresponding to log(a/M) ≈ 4.20 ± 0.42 and comparable mass ratios, q > 0.5. If the SBHB candidates considered are true binaries, this result would suggest that there is a physical process that allows initially unequal-mass systems to evolve toward comparable mass ratios (e.g., accretion that occurs preferentially onto the smaller of the black holes) or point to some, yet unspecified, selection bias. Our method also indicates that the SBHB candidates equally favor configurations in which the mini-disks are coplanar or misaligned with the binary orbital plane. If confirmed for true SBHBs, this finding would indicate the presence of a physical mechanism that maintains misalignment of the mini-disks down to subparsec binary separations (e.g., precession driven by gravitational torques). The probability distributions of the SBHB parameters inferred for the observed SBHB candidates and our control group of AGNs are statistically indistinguishable, implying that this method can in principle be used to interpret the observed emission-line profiles once a sample of confirmed SBHBs is available but cannot be used as a conclusive test of binarity.