Search for: All records

Quasi-periodic eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks^1–5. The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs) undergoing instabilities^6–8or interacting with a stellar object in a close orbit^9–11. It has been suggested that this disk could be created when the SMBH disrupts a passing star^8,11, implying that many QPEs should be preceded by observable tidal disruption events (TDEs). Two known QPE sources show long-term decays in quiescent luminosity consistent with TDEs^4,12and two observed TDEs have exhibited X-ray flares consistent with individual eruptions^13,14. TDEs and QPEs also occur preferentially in similar galaxies¹⁵. However, no confirmed repeating QPEs have been associated with a spectroscopically confirmed TDE or an optical TDE observed at peak brightness. Here we report the detection of nine X-ray QPEs with a mean recurrence time of approximately 48 h from AT2019qiz, a nearby and extensively studied optically selected TDE¹⁶. We detect and model the X-ray, ultraviolet (UV) and optical emission from the accretion disk and show that an orbiting body colliding with this disk provides a plausible explanation for the QPEs.

 

The Miocene (23.03–5.33 Ma) is recognized as a period with close to modern‐day paleogeography, yet a much warmer climate. With large uncertainties in future hydroclimate projections, Miocene conditions illustrate a potential future analog for the Earth system. A recent opportunistic Miocene Model Intercomparison Project 1 (MioMIP1) focused on synthesizing published Miocene climate simulations and comparing them with available temperature reconstructions. Here, we build on this effort by analyzing the hydrological cycle response to Miocene forcings across early‐to‐middle (E2MMIO; 20.03–11.6 Ma) and middle‐to‐late Miocene (M2LMIO; 11.5–5.33 Ma) simulations with CO₂concentrations ranging from 200 to 850 ppm and providing a model‐data comparison against available precipitation reconstructions. We find global precipitation increases by ∼2.1 and 2.3% per degree of warming for E2MMIO and M2LMIO simulations, respectively. Models generally agree on a wetter than modern‐day tropics; mid and high‐latitude, however, do not agree on the sign of subtropical precipitation changes with warming. Global monsoon analysis suggests most monsoon regions, except the North American Monsoon, experience higher precipitation rates under warmer conditions. Model‐data comparison shows that mean annual precipitation is underestimated by the models regardless of CO₂concentration, particularly in the mid‐ to high‐latitudes. This suggests that the models may not be (a) resolving key processes driving the hydrological cycle response to Miocene boundary conditions and/or (b) other boundary conditions or processes not considered here are critical to reproducing Miocene hydroclimate. This study highlights the challenges in modeling and reconstructing the Miocene hydrological cycle and serves as a baseline for future coordinated MioMIP efforts.

 

Extreme weather events are major causes of loss of life and damage infrastructure worldwide. High temperatures cause heat stress on humans, livestock, crops and infrastructure. Heat stress exposure is projected to increase with ongoing climate change. Extremes of temperature are common in Africa and infrastructure is often incapable of providing adequate cooling. We show how easily accessible cooling technology, such as evaporative coolers, prevent heat stress in historic timescales but are unsuitable as a solution under climate change. As temperatures increase, powered cooling, such as air conditioning, is necessary to prevent overheating. This will, in turn, increase demand on already stretched infrastructure. We use high temporal resolution climate model data to estimate the demand for cooling according to two metrics, firstly the apparent temperature and secondly the discomfort index. For each grid cell we calculate the heat stress value and the amount of cooling required to turn a heat stress event into a non heat stress event. We show the increase in demand for cooling in Africa is non uniform and that equatorial countries are exposed to higher heat stress than higher latitude countries. We further show that evaporative coolers are less effective in tropical regions than in the extra tropics. Finally, we show that neither low nor high efficiency coolers are sufficient to return Africa to current levels of heat stress under climate change.

 

We report the All-Sky Automated Survey for SuperNovae discovery of the tidal disruption event (TDE) ASASSN-23bd (AT 2023clx) in NGC 3799, a LINER galaxy with no evidence of strong active galactic nucleus (AGN) activity over the past decade. With a redshift of z = 0.01107 and a peak ultraviolet (UV)/optical luminosity of (5.4 ± 0.4) × 1042 erg s−1, ASASSN-23bd is the lowest-redshift and least-luminous TDE discovered to date. Spectroscopically, ASASSN-23bd shows H α and He i emission throughout its spectral time series, there are no coronal lines in its near-infrared spectrum, and the UV spectrum shows nitrogen lines without the strong carbon and magnesium lines typically seen for AGN. Fits to the rising ASAS-SN light curve show that ASASSN-23bd started to brighten on MJD 59988$^{+1}_{-1}$, ∼9 d before discovery, with a nearly linear rise in flux, peaking in the g band on MJD $60 \, 000^{+3}_{-3}$. Scaling relations and TDE light curve modelling find a black hole mass of ∼106 M⊙, which is on the lower end of supermassive black hole masses. ASASSN-23bd is a dim X-ray source, with an upper limit of $L_{0.3-10\, \mathrm{keV}} \lt 1.0\times 10^{40}$ erg s−1 from stacking all Swift observations prior to MJD 60061, but with soft (∼0.1 keV) thermal emission with a luminosity of $L_{0.3-2 \, \mathrm{keV}}\sim 4\times 10^{39}$ erg s−1 in XMM-Newton observations on MJD 60095. The rapid (t < 15 d) light curve rise, low UV/optical luminosity, and a luminosity decline over 40 d of ΔL40 ≈ −0.7 dex make ASASSN-23bd one of the dimmest TDEs to date and a member of the growing ‘Low Luminosity and Fast’ class of TDEs.

 

NGC 5273 is a known optical and X-ray variable AGN. We analyse new and archival IR, optical, UV, and X-ray data in order to characterize its long-term variability from 2000–2022. At least one optical changing-look event occurred between 2011 and 2014 when the AGN changed from a Type 1.8/1.9 Seyfert to a Type 1. It then faded considerably at all wavelengths, followed by a dramatic but slow increase in UV/optical brightness between 2021 and 2022. Near-IR (NIR) spectra in 2022 show prominent broad Paschen lines that are absent in an archival spectrum from 2010, making NGC 5273 one of the few AGNs to be observed changing-look in the NIR. We propose that NGC 5273 underwent multiple changing-look events between 2000 and 2022 – starting as a Type 1.8/1.9, NGC 5273 changes-look to a Type 1 temporarily in 2002 and again in 2014, reverting back to a Type 1.8/1.9 by 2005 and 2017, respectively. In 2022, it is again a Type 1 Seyfert. We characterize the changing-look events and their connection to the dynamic accretion and radiative processes in NGC 5273 and propose that the variable luminosity (and thus, Eddington ratio) of the source is changing how the broad-line region (BLR) reprocesses the continuum emission.

 

We present UV and/or optical observations and models of SN 2023ixf, a type II supernova (SN) located in Messier 101 at 6.9 Mpc. Early time (flash) spectroscopy of SN 2023ixf, obtained primarily at Lick Observatory, reveals emission lines of Hi, Hei/ii, Civ, and Niii/iv/vwith a narrow core and broad, symmetric wings arising from the photoionization of dense, close-in circumstellar material (CSM) located around the progenitor star prior to shock breakout. These electron-scattering broadened line profiles persist for ∼8 days with respect to first light, at which time Doppler broadened the features from the fastest SN ejecta form, suggesting a reduction in CSM density atr≳ 10¹⁵cm. The early time light curve of SN 2023ixf shows peak absolute magnitudes (e.g.,M_u= −18.6 mag,M_g= −18.4 mag) that are ≳2 mag brighter than typical type II SNe, this photometric boost also being consistent with the shock power supplied from CSM interaction. Comparison of SN 2023ixf to a grid of light-curve and multiepoch spectral models from the non-LTE radiative transfer codeCMFGENand the radiation-hydrodynamics codeHERACLESsuggests dense, solar-metallicity CSM confined tor= (0.5–1) × 10¹⁵cm, and a progenitor mass-loss rate of$\dot{M}={10}^{-2}{M}_{\odot }$yr⁻¹. For the assumed progenitor wind velocity ofv_w= 50 km s⁻¹, this corresponds to enhanced mass loss (i.e.,superwindphase) during the last ∼3–6 yr before explosion.

 

Abstract We present extensive optical photometry of the afterglow of GRB 221009A. Our data cover 0.9–59.9 days from the time of Swift and Fermi gamma-ray burst (GRB) detections. Photometry in rizy -band filters was collected primarily with Pan-STARRS and supplemented by multiple 1–4 m imaging facilities. We analyzed the Swift X-ray data of the afterglow and found a single decline rate power law f ( t ) ∝ t −1.556±0.002 best describes the light curve. In addition to the high foreground Milky Way dust extinction along this line of sight, the data favor additional extinction to consistently model the optical to X-ray flux with optically thin synchrotron emission. We fit the X-ray-derived power law to the optical light curve and find good agreement with the measured data up to 5−6 days. Thereafter we find a flux excess in the riy bands that peaks in the observer frame at ∼20 days. This excess shares similar light-curve profiles to the Type Ic broad-lined supernovae SN 2016jca and SN 2017iuk once corrected for the GRB redshift of z = 0.151 and arbitrarily scaled. This may be representative of an SN emerging from the declining afterglow. We measure rest-frame absolute peak AB magnitudes of M g = −19.8 ± 0.6 and M r = − 19.4 ± 0.3 and M z = −20.1 ± 0.3. If this is an SN component, then Bayesian modeling of the excess flux would imply explosion parameters of M ej = 7.1 − 1.7 + 2.4 M ⊙ , M Ni = 1.0 − 0.4 + 0.6 M ⊙ , and v ej = 33,900 − 5700 + 5900 km s −1 , for the ejecta mass, nickel mass, and ejecta velocity respectively, inferring an explosion energy of E kin ≃ 2.6–9.0 × 10 52 erg. 

We present multiwavelength photometry and spectroscopy of SN 2022jli, an unprecedented Type Ic supernova discovered in the galaxy NGC 157 at a distance of ≈ 23 Mpc. The multiband light curves reveal many remarkable characteristics. Peaking at a magnitude ofg= 15.11 ± 0.02, the high-cadence photometry reveals periodic undulations of 12.5 ± 0.2 days superimposed on the 200-day supernova decline. This periodicity is observed in the light curves from nine separate filter and instrument configurations with peak-to-peak amplitudes of ≃ 0.1 mag. This is the first time that repeated periodic oscillations, over many cycles, have been detected in a supernova light curve. SN 2022jli also displays an extreme early excess that fades over ≈25 days, followed by a rise to a peak luminosity ofL_opt= 10^42.1erg s⁻¹. Although the exact explosion epoch is not constrained by data, the time from explosion to maximum light is ≳ 59 days. The luminosity can be explained by a large ejecta mass (M_ej≈ 12 ± 6M_⊙) powered by⁵⁶Ni, but we find it difficult to quantitatively model the early excess with circumstellar interaction and cooling. Collision between the supernova ejecta and a binary companion is a possible source of this emission. We discuss the origin of the periodic variability in the light curve, including interaction of the SN ejecta with nested shells of circumstellar matter and neutron stars colliding with binary companions.