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We present the discovery with Keck of the extremely infrared (IR) luminous transient AT 2017gbl, coincident with the Northern nucleus of the luminous infrared galaxy (LIRG) IRAS 23436+5257. Our extensive multi-wavelength follow-up spans ~900 days, including photometry and spectroscopy in the optical and IR, and (very long baseline interferometry) radio and X-ray observations. Radiative transfer modelling of the host galaxy spectral energy distribution and long-term pre-outburst variability in the mid-IR indicate the presence of a hitherto undetected dust obscured active galactic nucleus (AGN). The optical and near-IR spectra show broad 2000 km/s hydrogen, He I and O I emission features that decrease in flux over time. Radio imaging shows a fast evolving compact source of synchrotron emission spatially coincident with AT 2017gbl. We infer a lower limit for the radiated energy of 7.3 x 10^50 erg from the IR photometry. An extremely energetic supernova would satisfy this budget, but is ruled out by the radio counterpart evolution. Instead, we propose AT 2017gbl is related to an accretion event by the central supermassive black hole, where the spectral signatures originate in the AGN broad line region and the IR photometry is consistent with re-radiation by polar dust. Given the fast evolution of AT 2017gbl, we deem a tidal disruption event (TDE) of a star a more plausible scenario than a dramatic change in the AGN accretion rate. This makes AT 2017gbl the third TDE candidate to be hosted by a LIRG, in contrast to the so far considered TDE population discovered at optical wavelengths and hosted preferably by post-starburst galaxies.
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We present the discovery of ASASSN-18jd (AT 2018bcb), a luminous optical/UV/X-ray transient located in the nucleus of the galaxy 2MASX J22434289$-$1659083 at $z=0.1192$. Swift UVOT photometry shows the UV SED of the transient to be well modeled by a slowly shrinking blackbody with temperature $T sim 2.5 times 10^{4} rm ~K$, a maximum observed luminosity of $L_text{max} = 4.5^{+0.6}_{-0.3} times 10^{44} rm ~erg ~s^{-1}$, and a total radiated energy of $E = 9.6^{+1.1}_{-0.6} times 10^{51} rm ~erg$. X-ray data from Swift XRT and XMM-Newton show a transient, variable X-ray flux with blackbody and power-law components. Optical spectra show strong, roughly constant broad Balmer emission as well as transient features attributable to He II, N III-V, O III, and coronal Fe. While ASASSN-18jd shares similarities with Tidal Disruption Events (TDEs), it is also similar to the rapid turn-on events seen in quiescent galaxies and in faint Active Galactic Nuclei (AGNs).
Hot, Dust-Obscured Galaxies (Hot DOGs), selected from the WISE all sky infrared survey, host some of the most powerful Active Galactic Nuclei (AGN) known, and might represent an important stage in the evolution of galaxies. Most known Hot DOGs are at $z> 1.5$, due in part to a strong bias against identifying them at lower redshift related to the selection criteria. We present a new selection method that identifies 153 Hot DOG candidates at $zsim 1$, where they are significantly brighter and easier to study. We validate this approach by measuring a redshift $z=1.009$, and an SED similar to higher redshift Hot DOGs for one of these objects, WISE J1036+0449 ($L_{rm,Bol}simeq 8times 10^{46}rm,erg,s^{-1}$), using data from Keck/LRIS and NIRSPEC, SDSS, and CSO. We find evidence of a broadened component in MgII, which, if due to the gravitational potential of the supermassive black hole, would imply a black hole mass of $M_{rm,BH}simeq 2 times 10^8 M_{odot}$, and an Eddington ratio of $lambda_{rm,Edd}simeq 2.7$. WISE J1036+0449 is the first Hot DOG detected by NuSTAR, and the observations show that the source is heavily obscured, with a column density of $N_{rm,H}simeq(2-15)times10^{23}rm,cm^{-2}$. The source has an intrinsic 2-10 keV luminosity of $sim 6times 10^{44}rm,erg,s^{-1}$, a value significantly lower than that expected from the mid-infrared/X-ray correlation. We also find that the other Hot DOGs observed by X-ray facilities show a similar deficiency of X-ray flux. We discuss the origin of the X-ray weakness and the absorption properties of Hot DOGs. Hot DOGs at $zlesssim1$ could be excellent laboratories to probe the characteristics of the accretion flow and of the X-ray emitting plasma at extreme values of the Eddington ratio.
We present observations of the dark Gamma-Ray Burst GRB 051008 provided by Swift/BAT, Swift/XRT, Konus-WIND, INTEGRAL/SPI-ACS in the high-energy domain and the Shajn, Swift/UVOT, Tautenburg, NOT, Gemini and Keck I telescopes in the optical and near-infrared bands. The burst was detected only in gamma- and X-rays and neither a prompt optical nor a radio afterglow were detected down to deep limits. We identified the host galaxy of the burst, which is a typical Lyman-break Galaxy (LBG) with R-magnitude of 24.06 +/- 0.10. A redshift of the galaxy of z = 2.77 (-0.20,+0.15) is measured photometrically due to the presence of a clear, strong Lyman-break feature. The host galaxy is a small starburst galaxy with moderate intrinsic extinction (A_V = 0.3 mag) and has a SFR of ~ 60 M_Sun / yr typical for LBGs. It is one of the few cases where a GRB host has been found to be a classical Lyman-break galaxy. Using the redshift we estimate the isotropic-equivalent radiated energy of the burst to be E_iso = (1.15 +/- 0.20) x 10^54 erg. We also provide evidence in favour of the hypothesis that the darkness of GRB 051008 is due to local absorption resulting from a dense circumburst medium.
The known host galaxies of short-hard gamma-ray bursts (GRBs) to date are characterized by low to moderate star-formation rates and a broad range of stellar masses. In this paper, we positionally associate the recent unambiguously short-hard Swift GRB 100206A with a disk galaxy at redshift z=0.4068 that is rapidly forming stars at a rate of ~30 M_sun/yr, almost an order of magnitude higher than any previously identified short GRB host. Using photometry from Gemini, Keck, PAIRITEL, and WISE, we show that the galaxy is very red (g-K = 4.3 AB mag), heavily obscured (A_V ~ 2 mag), and has the highest metallicity of any GRB host to date (12 + log[O/H]_KD02 = 9.2): it is a classical luminous infrared galaxy (LIRG), with L_IR ~ 4 x 10^11 L_sun. While these properties could be interpreted to support an association of this GRB with very recent star formation, modeling of the broadband spectral energy distribution also indicates that a substantial stellar mass of mostly older stars is present. The current specific star-formation rate is modest (specific SFR ~ 0.5 Gyr^-1), the current star-formation rate is not substantially elevated above its long-term average, and the host morphology shows no sign of recent merger activity. Our observations are therefore equally consistent with an older progenitor, similar to what is inferred for other short-hard GRBs. Given the precedent established by previous short GRB hosts and the significant fraction of the Universes stellar mass in LIRG-like systems at z >~0.3, an older progenitor represents the most likely origin of this event.
Supernova (SN) rates serve as an important probe of star-formation models and initial mass functions. Near-infrared seeing-limited ground-based surveys typically discover a factor of 3-10 fewer SNe than predicted from far-infrared (FIR) luminosities owing to sensitivity limitations arising from both a variable point-spread function (PSF) and high dust extinction in the nuclear regions of star-forming galaxies. This inconsistency has potential implications for our understanding of star-formation rates and massive-star evolution, particularly at higher redshifts, where star-forming galaxies are more common. To resolve this inconsistency, a successful SN survey in the local universe must be conducted at longer wavelengths and with a space-based telescope, which has a stable PSF to reduce the necessity for any subtraction algorithms and thus residuals. Here we report on a two-year Spitzer/IRAC 3.6 um survey for dust-extinguished SNe in the nuclear regions of forty luminous infrared galaxies (LIRGs) within 200 Mpc. The asymmetric Spitzer PSF results in worse than expected subtraction residuals when implementing standard template subtraction. Forward-modeling techniques improve our sensitivity by ~1.5 magnitudes. We report the detection of 9 SNe, five of which were not discovered by optical surveys. After adjusting our predicted rates to account for the sensitivity of our survey, we find that the number of detections is consistent with the models. While this search is nonetheless hampered by a difficult-to-model PSF and the relatively poor resolution of Spitzer, it will benefit from future missions, such as Roman Space Telescope and JWST, with higher resolution and more symmetric PSFs.
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