arXiv subjects

D. R. Pasham

Publications and source records attributed to D. R. Pasham.

18 recordsLinked to original sources

arXiv2025

NICER observations reveal doubled timescales in Ansky's quasi-periodic eruptions (QPEs)

Quasi-periodic eruptions (QPEs) are recurring X-ray bursts originating from the vicinity of supermassive black holes, but their driving mechanisms remain under debate. This study analyzes new NICER observations of QPEs in Ansky (a transient event in the nucleus of the galaxy SDSS J1335+0728), taken between January and June 2025. By examining flare durations, peak-to-peak recurrence times, and profiles, we compare the 2025 data with those from 2024 to investigate changes in energy, timescales, and flare shapes. The 2025 QPEs are found to be four times more energetic, with recurrence times of approximately 10 days and flare durations ranging from 2.5 to 4 days, making them both about twice as long as in 2024. Additionally, the flare profiles have become more asymmetric, showing longer decays. We explore different theoretical scenarios to explain the observed properties of the QPEs in Ansky, including evolving stream-disk interactions in an extreme mass-ratio inspiral (EMRI) system as a potential mechanism behind the observed changes in recurrence time and energetics, while also considering alternative models based on mass transfer and accretion disk instabilities. Continued observational efforts will be crucial for unveiling the nature of Ansky.

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arXiv2025

The properties of GSN 069 accretion disk from a joint X-ray and UV spectral analysis: stress-testing quasi-periodic eruption models

We present an analysis of Hubble Space Telescope (HST) and XMM-Newton data of the tidal disruption event (TDE) candidate and quasi-periodic eruption (QPE) source GSN 069. Using ultraviolet (UV) and optical images at HST resolution, we show that GSN 069's emission consists of a point source superimposed on a diffuse stellar component. The latter accounts for $\leq 5\%$ of the UV emission in the inner 0.5"$\times$0.5" region, while the luminosity of the former cannot be attributed to stars. Analyzing the 2014/2018 \hst UV spectra, we show that to leading order the intrinsic spectral shape is $ν\,L_ν\proptoν^{4/3}$, with $\sim10\%$ far UV flux variability between epochs. The contemporaneous X-ray and UV spectra can be modeled self-consistently in a thin disk framework. At observed epochs, the disk had an outer radius ($R_{\rm out}$) of $\mathcal{O}(10^3R_{\rm g})$, showing both cooling and expansion over four years. Incorporating relativistic effects via numerical ray tracing, we constrain the disk inclination angle ($i$) to be $30^\circ\,\lesssim\,i\,\lesssim\,65^\circ$ and identify a narrow region of spin-inclination parameter space that describes the observations. These findings confirm that GSN 069 hosts a compact, viscously expanding accretion disk likely formed after a TDE. Implications for QPE models are: (i) No published disk instability model can explain the disk's stability in 2014 (no QPEs) and its instability in 2018 (QPEs present); (ii) While the disk size in 2018 allows for orbiter/disk interactions to produce QPEs, in 2014 the disk was already sufficiently extended, yet no QPEs were present. These findings pose challenges to existing QPE models.

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arXiv2024

Late-time X-ray Observations of the Jetted Tidal Disruption Event AT2022cmc: The Relativistic Jet Shuts Off

The tidal disruption event (TDE) AT2022cmc represents the fourth known example of a relativistic jet produced by the tidal disruption of a stray star providing a unique probe of the formation and evolution of relativistic jets in otherwise dormant supermassive black holes (SMBHs). Here we present deep, late-time Chandra observations of AT2022cmc extending to $t_{\rm obs} \approx 400$ days after disruption. Our observations reveal a sudden decrease in the X-ray brightness by a factor of $\gtrsim 14$ over a factor of $\approx 2.3$ in time, and a deviation from the earlier power-law decline with a steepening $α\gtrsim 3.2$ ($F_X \propto t^{-α}$), steeper than expected for a jet break, and pointing to the cessation of jet activity at $t_{\rm obs} \approx 215$ days. Such a transition has been observed in two previous TDEs (Swift J1644+57 and Swift J2058+05). From the X-ray luminosity and the timescale of jet shutoff, we parameterize the mass of the SMBH in terms of unknown jet efficiency and accreted mass fraction parameters. Motivated by the disk-jet connection in AGN, we favor black hole masses $\lesssim 10^5 \ \rm M_{\odot}$ (where the jet and disk luminosities are comparable), and disfavor larger black holes (in which extremely powerful jets are required to outshine their accretion disks). We additionally estimate a total accreted mass of $\approx 0.1 \rm \ M_{\odot}$. Applying the same formalism to Swift J1644+57 and Swift J2058+05, we favor comparable black hole masses for these TDEs of $\lesssim$ a few $\times 10^5 \ \rm M_{\odot}$, and suggest that jetted TDEs may preferentially form from lower mass black holes when compared to non-relativistic events, owing to generally lower jet and higher disk efficiencies at higher black hole masses.

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arXiv2024

Quasi-periodic X-ray eruptions years after a nearby tidal disruption event

Quasi-periodic Eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks. The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs), undergoing instabilities or interacting with a stellar object in a close orbit. It has been suggested that this disk could be created when the SMBH disrupts a passing star, 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, and two observed TDEs have exhibited X-ray flares consistent with individual eruptions. TDEs and QPEs also occur preferentially in similar galaxies. 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 hours from AT2019qiz, a nearby and extensively studied optically-selected TDE. We detect and model the X-ray, ultraviolet and optical emission from the accretion disk, and show that an orbiting body colliding with this disk provides a plausible explanation for the QPEs.

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arXiv2024

AT2018fyk: Candidate Tidal Disruption Event by a (Super)massive Black Hole Binary

The tidal disruption event (TDE) AT2018fyk has unusual X-ray, UV, and optical light curves that decay over the first $\sim$600d, rebrighten, and decay again around 1200d. We explain this behavior as a one-off TDE associated with a massive black hole (BH) \emph{binary}. The sharp drop-offs from $t^{-5/3}$ power laws at around 600d naturally arise when one BH interrupts the debris fallback onto the other BH. The BH mass $M_\bullet$ derived from fitting X-ray spectra with a slim disk accretion model and, independently, from fitting the early UV/optical light curves, is smaller by two orders of magnitude than predicted from the $M_\bullet$--$σ_*$ host galaxy relation, suggesting that the debris is accreted onto the secondary, with fallback cut off by the primary. Furthermore, if the rebrightening were associated with the primary, it should occur around 5000d, not the observed 1200d. The secondary's mass and dimensionless spin is $M_{\bullet,{\rm s}}=2.7^{+0.5}_{-1.5} \times 10^5 M_\odot$ and $a_{\bullet,{\rm s}}>0.3$ (X-ray spectral fitting), while the primary's mass is $M_{\bullet,{\rm p}}=10^{7.7\pm0.4}M_\odot$ ($M_\bullet$-$σ_*$ relation). An intermediate mass BH secondary is consistent with the observed UV/optical light curve decay, i.e., the secondary's outer accretion disk is too faint to produce a detectable emission floor. The time of the first accretion cutoff constrains the binary separation to be $(6.7\pm 1.2) \times 10^{-3}~{\rm pc}$. X-ray spectral fitting and timing analysis indicate that the hard X-rays arise from a corona above the secondary's disk. The early UV/optical emission, suggesting a super-Eddington phase for the secondary, possibly originates from shocks arising from debris circularization.

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arXiv2024

Quasi-periodic X-ray eruptions and tidal disruption events prefer similar host galaxies

In the past five years, six quasi-periodic X-ray eruption (QPE) sources have been discovered in the nuclei of nearby galaxies. Their origin remains an open question. We present MUSE integral field spectroscopy of five QPE host galaxies to characterize their properties. We find that 3/5 galaxies host extended emission line regions (EELRs) up to 10 kpc in size. The EELRs are photo-ionized by a non-stellar continuum, but the current nuclear luminosity is insufficient to power the observed emission lines. The EELRs are decoupled from the stars both kinematically and in projected sky position, and the low velocities and velocity dispersions ($<$ 100 km s$^{-1}$ and $\lesssim 75$ km s$^{-1}$ respectively) are inconsistent with being AGN- or shock-driven. The origin of the EELRs is likely a previous phase of nuclear activity. The QPE host galaxy properties are strikingly similar to those of tidal disruption events (Wevers et al. submitted). The preference for a very short-lived (the typical EELR lifetime is $\sim$15000 years), gas-rich phase where the nucleus has recently faded significantly suggests that TDEs and QPEs may share a common formation channel, disfavoring AGN accretion disk instabilities as the origin of QPEs. In the assumption that QPEs are related to extreme mass ratio inspiral systems (EMRIs; stellar-mass objects on bound orbits about massive black holes), the high incidence of EELRs and recently faded nuclear activity can be used to aid in the localization of the host galaxies of EMRIs discovered by low frequency gravitational wave observatories.

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arXiv2024

Tidal disruption event AT2020ocn: early-time X-ray flares caused by a possible disc alignment process

A tidal disruption event (TDE) may occur when a star is torn apart by the tidal force of a black hole (BH). Eventually, an accretion disc is thought to form out of stellar debris falling back towards the BH. If the star's orbital angular momentum vector prior to disruption is not aligned with the BH spin angular momentum vector, the disc will be tilted with respect to the BH equatorial plane. The disc will eventually be drawn into the BH equatorial plane due to a combination of the Bardeen-Petterson effect and internal torques. Here, we analyse the X-ray and UV observations of the TDE AT2020ocn obtained by Swift, XMM-Newton, and NICER. The X-ray light curve shows strong flares during the first $\approx100$ days, while, over the same period, the UV emission decays gradually. We find that the X-ray flares can be explained by a model that also explains the spectral evolution. This model includes a slim disc viewed under a variable inclination plus an inverse-Comptonisation component processing the slim disc emission. A scenario where the ongoing Lense-Thirring precession during the disc alignment process is responsible for the observed inclination variations is consistent with the data. In later observations, we find that the X-ray spectrum of AT2020ocn becomes harder, while the mass accretion rate remains at super-Eddington levels, suggesting the formation of a corona in line with accretion onto other compact objects. We constrain the BH mass to be $(7^{+13}_{-3})\times10^{5}$ M$_\odot$ at the 1$σ$ (68%) confidence level.

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arXiv2024

Science with a small two-band UV-photometry mission III: Active Galactic Nuclei and nuclear transients

In this review (the third in the series focused on a small two-band UV-photometry mission), we assess possibilities for a small UV two-band photometry mission in studying accreting supermassive black holes (SMBHs; mass range $\sim 10^6$-$10^{10}\,M_{\odot}$). We focus on the following observational concepts: (i) dedicated monitoring of selected type-I Active Galactic Nuclei (AGN) in order to measure the time delay between the far-UV, the near-UV, and other wavebands (X-ray and optical), (ii) nuclear transients including (partial) tidal disruption events and repetitive nuclear transients, and (iii) the study of peculiar sources, such as changing-look AGN, hollows and gaps in accretion disks, low-luminosity AGN, and candidates for Intermediate-Mass Black Holes (IMBHs; mass range $\sim 10^2$-$10^5\,M_{\odot}$) in galactic nuclei. For tidal disruption events (TDEs), high-cadence UV monitoring is crucial for distinguishing among different scenarios for the origin of the UV emission. The small two-band UV space telescope will also provide the information about the near- and far-UV continuum variability for rare transients, such as repetitive partial TDEs and jetted TDEs. We also discuss the possibilities to study and analyze sources with non-standard accretion flows, such as AGN with gappy disks, low-luminosity active galactic nuclei with intermittent accretion, and SMBH binaries potentially involving intermediate-mass black holes.

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arXiv2024

Delayed X-ray brightening accompanied by variable ionized absorption following a tidal disruption event

Supermassive black holes can experience super-Eddington peak mass fallback rates following the tidal disruption of a star. The theoretical expectation is that part of the infalling material is expelled by means of an accretion disk wind, whose observational signature includes blueshifted absorption lines of highly ionized species in X-ray spectra. To date, however, only one such ultra-fast outflow (UFO) has been reported in the tidal disruption event (TDE) ASASSN-14li. Here we report on the discovery of transient absorption-like signatures in X-ray spectra of the TDE AT2020ksf/Gaia20cjk (at a redshift of $z$=0.092), following an X-ray brightening $\sim 230$ days after UV/optical peak. We find that while no statistically significant absorption features are present initially, they appear on a timescale of several days, and remain detected up to 770 days after peak. Simple thermal continuum models, combined with a power-law or neutral absorber, do not describe these features well. Adding a partial covering, low velocity ionized absorber improves the fit at early times, but fails at late times. A high velocity (v$_w$ $\sim$ 42000 km s$^{-1}$, or -0.15c), ionized absorber (ultra-fast outflow) provides a good fit to all data. The few day timescale of variability is consistent with expectations for a clumpy wind. We discuss several scenarios that could explain the X-ray delay, as well as the potential for larger scale wind feedback. The serendipitous nature of the discovery could suggest a high incidence of UFOs in TDEs, alleviating some of the tension with theoretical expectations.

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arXiv2022

Live to die another day: the rebrightening of AT2018fyk as a repeating partial tidal disruption event

Stars that interact with supermassive black holes (SMBHs) can either be completely or partially destroyed by tides. In a partial tidal disruption event (TDE) the high-density core of the star remains intact, and the low-density, outer envelope of the star is stripped and feeds a luminous accretion episode. The TDE AT2018fyk, with an inferred black hole mass of $10^{7.7\pm0.4}$ M$_{\odot}$, experienced an extreme dimming event at X-ray (factor of $>$6000) and UV (factor $\sim$15) wavelengths $\sim$500--600 days after discovery. Here we report on the re-emergence of these emission components roughly 1200 days after discovery. We find that the source properties are similar to those of the pre-dimming accretion state, suggesting that the accretion flow was rejuvenated to a similar state. We propose that a repeating partial TDE, where the partially disrupted star is on a $\sim 1200$ day orbit about the SMBH and is periodically stripped of mass during each pericenter passage, powers its unique lightcurve. This scenario provides a plausible explanation for AT2018fyk's overall properties, including the rapid dimming event and the rebrightening at late times. We also provide testable predictions for the behavior of the accretion flow in the future: if the second encounter was also a partial disruption then we predict another strong dimming event around day 1800 (August 2023), and a subsequent rebrightening around day 2400 (March 2025). This source provides strong evidence of the partial disruption of a star by a SMBH.

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arXiv2022

NICER monitoring of supersoft X-ray sources

We monitored four supersoft sources - two persistent ones, CAL 83 and MR Vel, and the recent novae YZ Ret (Nova Ret 2020) and V1674 Her (Nova Her 2021) - with NICER. The two persistent SSS were observed with unvaried X-ray flux level and spectrum, respectively, 13 and 20 years after the last observations. Short period modulations of the supersoft X-ray source (SSS) appear where the spectrum of the luminous central source was fully visibl (in CAL 83 and V1674 Her) and were absent in YZ Ret and MR Vel, in which the flux originated in photoionized or shocked plasma, while the white dwarf (WD) was not observable. We thus suggest that the pulsations occur on, or very close to, the WD surface. The pulsations of CAL 83 were almost unvaried after 15 years, including an irregular drift of the $\simeq$67 s period by 2.1 s. Simulations, including previous XMM-Newton data, indicate actual variations in period length within hours, rather than an artifact of the variable amplitude of the pulsations. Large amplitude pulsations with a period of 501.53$\pm$0.30 s were always detected in V1674 Her, as long as the SSS was observable. This period seems to be due to rotation of a highly magnetized WD.We cannot confirm the maximum effective temperature of ($\simeq$145,000 K) previously inferred for this nova, and discuss the difficulty in interpreting its spectrum. The WD appears to present two surface zones, one of which does not emit SSS flux.

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arXiv2022

Host galaxy properties of quasi-periodically erupting X-ray sources

Quasi-periodic X-ray eruptions (QPEs) are a recently discovered phenomenon, the nature of which remains unclear. Based on their discovery in active galactic nuclei (AGN), explanations related to an AGN accretion disk, or potentially stellar tidal disruption event (TDE), were put forward. Following the report of QPEs in apparently passive galaxies, alternatives including highly unequal mass compact object binaries have been proposed to explain their properties. We perform a systematic study of the five known QPE host galaxies with the aim of providing new insights into their nature. We analyse new and archival medium resolution optical spectroscopy of the QPE hosts. We measure emission (and absorption) line fluxes, their ratios and equivalent widths (EWs), to locate the QPE hosts on diagnostic diagrams. We also measure the velocity dispersion of the stellar absorption lines to estimate their black hole masses. All QPE host galaxies show emission lines in their optical spectra. Based on their ratios and EWs, we find evidence for the presence of an active galactic nucleus in all sources, including those previously reported as passive. We measure velocity dispersions between 36 and 90 km/s, implying the presence of low mass (10^5-6.7 solar masses) black holes, consistent with literature findings. Finally, we find a significant over-representation (2/5 sources, or a factor of 13 +13 -10.5) of quiescent, Balmer strong (post starburst) galaxies among QPE hosts. The presence of a narrow line region consistent with an AGN in all QPE host galaxies implies that a pre-existing accretion flow likely plays an integral part to the QPE phenomenon. The strong over-representation of quiescent Balmer strong galaxies among QPE hosts can be naturally explained in both the TDE and interacting extreme mass ratio inspiral hypotheses.

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arXiv2020

Accretion disc cooling and narrow absorption lines in the tidal disruption event AT 2019dsg

We present the results of a large multi-wavelength follow-up campaign of the Tidal Disruption Event (TDE) \dsg, focusing on low to high resolution optical spectroscopy, X-ray, and radio observations. The galaxy hosts a super massive black hole of mass $\rm (5.4\pm3.2)\times10^6\,M_\odot$ and careful analysis finds no evidence for the presence of an Active Galactic Nucleus, instead the TDE host galaxy shows narrow optical emission lines that likely arise from star formation activity. The transient is luminous in the X-rays, radio, UV and optical. The X-ray emission becomes undetected after $\sim$125 days, and the radio luminosity density starts to decay at frequencies above 5.4 GHz by $\sim$180 days. Optical emission line signatures of the TDE are present up to $\sim$250 days after the discovery of the transient. The medium to high resolution spectra show traces of absorption lines that we propose originate in the self-gravitating debris streams. At late times, after $\sim$200 days, narrow Fe lines appear in the spectra. The TDE was previously classified as N-strong, but after careful subtraction of the host galaxy's stellar contribution, we find no evidence for these N lines in the TDE spectrum, even though O Bowen lines are detected. The observed properties of the X-ray emission are fully consistent with the detection of the inner regions of a cooling accretion disc. The optical and radio properties are consistent with this central engine seen at a low inclination (i.e., seen from the poles).

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arXiv2020

The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution

We present multi-wavelength observations of the tidal disruption event (TDE) iPTF15af, discovered by the intermediate Palomar Transient Factory (iPTF) survey at redshift $z=0.07897$. The optical and ultraviolet (UV) light curves of the transient show a slow decay over five months, in agreement with previous optically discovered TDEs. It also has a comparable black-body peak luminosity of $L_{\rm{peak}} \approx 1.5 \times 10^{44}$ erg/s. The inferred temperature from the optical and UV data shows a value of (3$-$5) $\times 10^4$ K. The transient is not detected in X-rays up to $L_X < 3 \times 10^{42}$erg/s within the first five months after discovery. The optical spectra exhibit two distinct broad emission lines in the He II region, and at later times also H$α$ emission. Additionally, emission from [N III] and [O III] is detected, likely produced by the Bowen fluorescence effect. UV spectra reveal broad emission and absorption lines associated with high-ionization states of N V, C IV, Si IV, and possibly P V. These features, analogous to those of broad absorption line quasars (BAL QSOs), require an absorber with column densities $N_{\rm{H}} > 10^{23}$ cm$^{-2}$. This optically thick gas would also explain the non-detection in soft X-rays. The profile of the absorption lines with the highest column density material at the largest velocity is opposite that of BAL QSOs. We suggest that radiation pressure generated by the TDE flare at early times could have provided the initial acceleration mechanism for this gas. Spectral UV line monitoring of future TDEs could test this proposal.

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arXiv2019

Polarimetry of relativistic tidal disruption event Swift J2058+0516

A small fraction of candidate tidal disruption events (TDEs) show evidence of powerful relativistic jets, which are particularly pronounced at radio wavelengths, and likely contribute non-thermal emission at a wide range of wavelengths. A non-thermal emission component can be diagnosed using linear polarimetry, even when the total received light is dominated by emission from an accretion disk or disk outflow. In this paper we present Very Large Telescope (VLT) measurements of the linear polarisation of the optical light of jetted TDE Swift J2058+0516. This is the second jetted TDE studied in this manner, after Swift J1644+57. We find evidence of non-zero optical linear polarisation, P_V ~ 8%, a level very similar to the near-infrared polarimetry of Swift J1644+57. These detections provide an independent test of the emission mechanisms of the multiwavelength emission of jetted tidal disruption events.

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arXiv2019

Evidence for rapid disk formation and reprocessing in the X-ray bright tidal disruption event AT 2018fyk

We present optical spectroscopic and Swift UVOT/XRT observations of the X-ray and UV/optical bright tidal disruption event (TDE) AT 2018fyk/ASASSN-18ul discovered by ASAS-SN. The Swift lightcurve is atypical for a TDE, entering a plateau after $\sim$40 days of decline from peak. After 80 days the UV/optical lightcurve breaks again to decline further, while the X-ray emission becomes brighter and harder. In addition to broad H, He and potentially O/Fe lines, narrow emission lines emerge in the optical spectra during the plateau phase. We identify both high ionisation (O III) and low ionisation (Fe II) lines, which are visible for $\sim$45 days. We similarly identify Fe II lines in optical spectra of ASASSN-15oi 330 d after discovery, indicating that a class of Fe-rich TDEs exists. The spectral similarity between AT 2018fyk, narrow-line Seyfert 1 galaxies and some extreme coronal line emitters suggests that TDEs are capable of creating similar physical conditions in the nuclei of galaxies. The Fe II lines can be associated with the formation of a compact accretion disk, as the emergence of low ionisation emission lines requires optically thick, high density gas. Taken together with the plateau in X-ray and UV/optical luminosity this indicates that emission from the central source is efficiently reprocessed into UV/optical wavelengths. Such a two-component lightcurve is very similar to that seen in the TDE candidate ASASSN-15lh, and is a natural consequence of a highly relativistic orbital pericenter.

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arXiv2018

A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the Soft-Intermediate State of MAXI J1535-571

We present the discovery of a low-frequency $\approx 5.7$ Hz quasi-periodic oscillation (QPO) feature in observations of the black hole X-ray binary MAXI J1535-571 in its soft-intermediate state, obtained in September-October 2017 by the Neutron Star Interior Composition Explorer (NICER). The feature is relatively broad (compared to other low-frequency QPOs; quality factor $Q\approx 2$) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak harmonic and low-amplitude broadband noise. These characteristics identify it as a weak Type A/B QPO, similar to ones previously identified in the soft-intermediate state of the transient black hole X-ray binary XTE J1550-564. The lag-energy spectrum of the QPO shows increasing soft lags towards lower energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum). This large phase shift has similar amplitude but opposite sign to that seen in Rossi X-ray Timing Explorer data for a Type B QPO from the transient black hole X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the Type B QPO in GX 339-4 pointed towards a precessing jet-like corona illuminating the accretion disk as the origin of the QPO signal. We suggest that this QPO in MAXI J1535-571 may have the same origin, with the different lag sign depending on the scale height of the emitting region and the observer inclination angle.

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arXiv2017

iPTF16fnl: a faint and fast tidal disruption event in an E+A galaxy

We present ground-based and \textit{Swift} observations of iPTF16fnl, a likely tidal disruption event (TDE) discovered by the intermediate Palomar Transient Factory (iPTF) survey at 66.6 Mpc. The lightcurve of the object peaked at absolute $M_g=-17.2$ mag. The maximum bolometric luminosity (from optical and UV) was $L_p~\simeq~(1.0\,\pm\,0.15) \times 10^{43}$ erg/s, an order of magnitude fainter than any other optical TDE discovered so far. The luminosity in the first 60 days is consistent with an exponential decay, with $L \propto e^{-(t-t_0)/τ}$, where $t_0$=~57631.0 (MJD) and $τ\simeq 15$ days. The X-ray shows a marginal detection at $L_X=2.4^{1.9}_{-1.1}\times 10^{39}$ erg/s (\textit{Swift} X-ray Telescope). No radio counterpart was detected down to 3$σ$, providing upper limits for monochromatic radio luminosity of $νL_ν < 2.3\times10^{36}$ erg/s and $νL_ν<1.7\times 10^{37}$ erg/s (VLA, 6.1 and 22 GHz). The blackbody temperature, obtained from combined \textit{Swift} UV and optical photometry, shows a constant value of 19,000 K. The transient spectrum at peak is characterized by broad He II and H$α$ emission lines, with an FWHM of about 14,000 km/s and 10,000 km/s respectively. He I lines are also detected at $λλ$ 5875 and 6678. The spectrum of the host is dominated by strong Balmer absorption lines, which are consistent with a post-starburst (E+A) galaxy with an age of $\sim$650 Myr and solar metallicity. The characteristics of iPTF16fnl make it an outlier on both luminosity and decay timescales, as compared to other optically selected TDEs. The discovery of such a faint optical event suggests a higher rate of tidal disruptions, as low luminosity events may have gone unnoticed in previous searches.

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