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NASA NTRS2016

Hard X-Ray Emission from SH 2-104: A NuSTAR Search for Gamma-Ray Counterparts

We present NuSTAR hard X-ray observations of Sh 2-104, a compact H II region containing several young massive stellar clusters (YMSCs). We have detected distinct hard X-ray sources coincident with localized VERITAS TeV emission recently resolved from the giant gamma-ray complex MGRO J2019+37 in the Cygnus region. Fainter, diffuse X-rays coincident with the eastern YMSC in Sh2-104 likely result from the colliding winds of a component star. Just outside the radio shell of Sh 2-104 lies 3XMM J201744.7+365045 and a nearby nebula, NuSTAR J201744.3+364812, whose properties are most consistent with extragalactic objects. The combined XMM-Newton and NuSTAR spectrum of 3XMM J201744.7+365045 is well-fit to an absorbed power-law model with N(sub H) = (3.1 +/- 1.0) x 10(exp 22) cm(exp -2) and a photon index gamma = 2.1 +/- 0.1. Based on possible long-term flux variation and the lack of detected pulsations (less than or equal to 43% modulation), this object is likely a background active galactic nucleus rather than a Galactic pulsar. The spectrum of the NuSTAR nebula shows evidence of an emission line at E = 5.6 keV, suggesting an optically obscured galaxy cluster at z = 0.19 +/- 0.02 (d = 800 Mpc) and L(sub X) = 1.2 x 10(exp 44) erg s(exp -1). Follow-up Chandra observations of Sh 2-104 will help identify the nature of the X-ray sources and their relation to MGRO J2019+37. We also show that the putative VERITAS excess south of Sh 2-104, is most likely associated with the newly discovered Fermi pulsar PSR J2017+3625 and not the H II region.

SM: individual objects↗
NASA NTRS2016

The First Focused Hard X-Ray Images of the Sun with NuSTAR

We present results from the first campaign of dedicated solar observations undertaken by the Nuclear Spectroscopic Telescope ARray (NuSTAR) hard X-ray (HXR) telescope. Designed as an astrophysics mission, NuSTAR nonetheless has the capability of directly imaging the Sun at HXR energies (3 keV) with an increase in sensitivity of at least two magnitude compared to current non-focusing telescopes. In this paper we describe the scientific areas where NuSTAR will make major improvements on existing solar measurements. We report on the techniques used to observe the Sun with NuSTAR, their limitations and complications, and the procedures developed to optimize solar data quality derived from our experience with the initial solar observations. These first observations are briefly described, including the measurement of the Fe K-shell lines in a decaying X-class flare, HXR emission from high in the solar corona, and full-disk HXR images of the Sun.

Grefenstette, Brian W.↗
NASA NTRS2017

The NuSTAR Serendipitous Survey: The 40-month Catalog and the Properties of the Distant High-Energy X-Ray Source Population

We present the first full catalog and science results for the Nuclear Spectroscopic Telescope Array (NuSTAR) serendipitous survey. The catalog incorporates data taken during the first 40 months of NuSTAR operation, which provide approx. 20 Ms of effective exposure time over 331 fields, with an areal coverage of 13 deg2, and 497 sources detected in total over the 324 keV energy range. There are 276 sources with spectroscopic redshifts and classifications, largely resulting from our extensive campaign of ground-based spectroscopic follow-up. We characterize the overall sample in terms of the X-ray, optical, and infrared source properties. The sample is primarily composed of active galactic nuclei (AGNs), detected over a large range in redshift from z = 0.002 to 3.4 (median of [z] = 0.56), but also includes 16 spectroscopically confirmed Galactic sources. There is a large range in X-ray flux, from log(f_3-24 keV/erg/s/sq cm) approx. -14 to -11, and in rest-frame 10-40 keV luminosity, from log(L10-40 keV/erg/s) approx. 39 to 46, with a median of 44.1. Approximately 79% of the NuSTAR sources have lower-energy (<10 keV) X-ray counterparts from XMM-Newton, Chandra, and Swift XRT. The mid-infrared (MIR) analysis, using WISE all-sky survey data, shows that MIR AGN color selections miss a large fraction of the NuSTAR-selected AGN population, from approx. 15% at the highest luminosities (LX> 10(exp 44) erg/s) to 80 at the lowest luminosities (LX > 10(exp 43) erg/s).

catalogs-galaxies: active ; nuclei↗
NASA NTRS2018

Black Holes and Neutron Stars in Nearby Galaxies: Insights from NuSTAR

Nearby galaxy surveys have long classified X-ray binaries (XRBs) by the mass category of their donor stars (high-mass and low-mass). The NuSTAR (Nuclear Spectroscopic Telescope Array) observatory, which provides imaging data at E greater than 10 keV (kiloelectronvolts), has enabled the classification of extragalactic XRBs by their compact object type: neutron star (NS) or black hole (BH). We analyzed NuSTAR/Chandra/XMM-Newton observations from a NuSTAR-selected sample of 12 galaxies within 5 Mpc (megaparsecs) having stellar masses (M (sub asterisk)) 10 (sup 7-11) solar masses and star formation rates (SFRs) approximately 0.01-15 solar masses per year. We detected 128 NuSTAR sources to a sensitivity of 10 (sup 38) ergs per second. Using NuSTAR color-intensity and color-color diagrams we classified 43 of these sources as candidate NSs and 47 as candidate BHs. We further subdivide BHs by accretion states (soft, intermediate, and hard) and NSs by weak (Z/Atoll) and strong (accreting pulsar) magnetic field. Using eight normal (Milky Way-type) galaxies in the sample, we confirm the relation between the SFR and galaxy X-ray point source luminosity in the 4-25 and 12-25 keV energy bands. We also constrained galaxy X-ray point source luminosity using the relation L (sub X) equals M (sub asterisk) plus Beta SFR, finding agreement with previous work. The X-ray luminosity function (XLF) of all sources in the 4-25 and 12-25 keV energy bands matches the alpha = 1.6 slope for high-mass XRBs. We find that NS XLFs suggest a decline beginning at the Eddington limit for a 1.4 solar mass NS, whereas the BH fraction shows an approximate monotonic increase in the 4-25 and 12-25 keV energy bands. We calculate the overall ratio of BH to NS to be approximately 1 for 4-25 keV and approximately 2 for 12-25 keV.

Vulic, N.↗
NASA NTRS2021

Precise Timing and Phase-resolved Spectroscopy of the Young Pulsar J1617-5055 with NuSTAR

We report on a Nuclear Spectroscopic Telescope Array (NuSTAR) observation of the young, energetic pulsar PSR J1617-5055. Parkes Observatory 3 GHz radio observations of the pulsar (taken about 7 yr before the NuSTAR observations) are also reported here. NuSTAR detected pulsations at a frequency of f ≍ 14.4 Hz (P ≍ 69.44 ms) and, in addition, the observation was long enough to measure the source's frequency derivative, ḟ ≈ -2.8 X10^(-11) Hz/s. We find that the pulsar shows one peak per period at both hard X-ray and radio wavelengths, but that the hard X-ray pulse is broader (having a duty cycle of ~0.7), than the radio pulse (having a duty cycle of ~0.08). Additionally, the radio pulse is strongly linearly polarized. J1617's phase-integrated hard X-ray spectrum is well fit by an absorbed power-law model, with a photon index Γ = 1.59 ± 0.02. The hard X-ray pulsations are well described by three Fourier harmonics, and have a pulsed fraction that increases with energy. We also fit the phase-resolved NuSTAR spectra with an absorbed power-law model in five phase bins and find that the photon index varies with phase from Γ = 1.52 ± 0.03 at phases around the flux maximum to Γ = 1.79 ± 0.06 around the flux minimum. Last, we compare our results with other pulsars whose magnetospheric emission is detected at hard X-ray energies and find that, similar to previous studies, J1617's hard X-ray properties are more similar to the MeV pulsars than the GeV pulsars.

Neutron Stars↗
NASA NTRS2023

NuSTAR Observations of Four Mid-IR-Selected Dual AGN Candidates in Galaxy Mergers

Mergers of galaxies are a ubiquitous phenomenon in the universe and represent a natural consequence of the “bottom-up” mass accumulation and galaxy evolution cosmological paradigm. It is generally accepted that the peak of active galactic nucleus (AGN) accretion activity occurs at nuclear separations of ≲10 kpc for major mergers. Here we present new NuSTAR and XMM-Newton observations for a subsample of mid-IR preselected dual AGN candidates in an effort to better constrain the column densities along the line of sight (LOS) for each system. Only one dual AGN candidate, J0841+0101, is detected as a single, unresolved source in the XMMNewton and NuSTAR imaging, while the remaining three dual AGN candidates, J0122+0100, J1221+1137, and J1306+0735, are not detected with NuSTAR; if these nondetections are due to obscuration alone, these systems are consistent with being absorbed by column densities of log ( N H /cm −2 ) ≽ 24.9, 24.6, and 24.3, which are roughly consistent with previously inferred column densities in these merging systems. In the case of J0841+0101, the analysis of the 0.3–30 keV spectra reveal an LOS column density of N H ≳ 10 24 cm −2 , significantly larger than the column densities previously reported for this system and demonstrating the importance of the higher signal-to-noise ratio XMM-Newton spectra and access to the >10 keV energies via NuSTAR. Though it is unclear if J0841+0101 truly hosts a dual AGN, these results are in agreement with the high obscuring columns expected in AGNs in late-stage mergers.

AGN host galaxies↗
NASA NTRS2013

Nustar and Chandra Insight into the Nature of the 3-40 Kev Nuclear Emission in Ngc 253

We present results from three nearly simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR) and Chandra monitoring observations between 2012 September 2 and 2012 November 16 of the local star-forming galaxy NGC 253. The 3-40 kiloelectron volt intensity of the inner approximately 20 arcsec (approximately 400 parsec) nuclear region, as measured by NuSTAR, varied by a factor of approximately 2 across the three monitoring observations. The Chandra data reveal that the nuclear region contains three bright X-ray sources, including a luminous (L (sub 2-10 kiloelectron volt) approximately few × 10 (exp 39) erg per s) point source located approximately 1 arcsec from the dynamical center of the galaxy (within the sigma 3 positional uncertainty of the dynamical center); this source drives the overall variability of the nuclear region at energies greater than or approximately equal to 3 kiloelectron volts. We make use of the variability to measure the spectra of this single hard X-ray source when it was in bright states. The spectra are well described by an absorbed (power-law model spectral fit value, N(sub H), approximately equal to 1.6 x 10 (exp 23) per square centimeter) broken power-law model with spectral slopes and break energies that are typical of ultraluminous X-ray sources (ULXs), but not active galactic nuclei (AGNs). A previous Chandra observation in 2003 showed a hard X-ray point source of similar luminosity to the 2012 source that was also near the dynamical center (Phi is approximately equal to 0.4 arcsec); however, this source was offset from the 2012 source position by approximately 1 arcsec. We show that the probability of the 2003 and 2012 hard X-ray sources being unrelated is much greater than 99.99% based on the Chandra spatial localizations. Interestingly, the Chandra spectrum of the 2003 source (3-8 kiloelectron volts) is shallower in slope than that of the 2012 hard X-ray source. Its proximity to the dynamical center and harder Chandra spectrum indicate that the 2003 source is a better AGN candidate than any of the sources detected in our 2012 campaign; however, we were unable to rule out a ULX nature for this source. Future NuSTAR and Chandra monitoring would be well equipped to break the degeneracy between the AGN and ULX nature of the 2003 source, if again caught in a high state.

Chandra↗
NASA NTRS2014

NuSTAR Observations of Heavily Obscured Quasars at z Is Approximately 0.5

We present NuSTAR hard X-ray observations of three Type 2 quasars at z approx. = 0.4-0.5, optically selected from the Sloan Digital Sky Survey. Although the quasars show evidence for being heavily obscured, Compton-thick systems on the basis of the 2-10 keV to [O(sub III)] luminosity ratio and multiwavelength diagnostics, their X-ray absorbing column densities (N(sub H)) are poorly known. In this analysis, (1) we study X-ray emission at greater than 10 keV, where X-rays from the central black hole are relatively unabsorbed, in order to better constrain N(sub H). (2) We further characterize the physical properties of the sources through broad-band near-UV to mid-IR spectral energy distribution analyses. One of the quasars is detected with NuSTAR at greater than 8 keV with a no-source probability of less than 0.1%, and its X-ray band ratio suggests near Compton-thick absorption with N(sub H) is approximately greater than 5 × 10(exp 23) cm(exp -2). The other two quasars are undetected, and have low X-ray to mid-IR luminosity ratios in both the low-energy (2-10 keV) and high-energy (10-40 keV) X-ray regimes that are consistent with extreme, Compton-thick absorption (N(sub H) is approximately greater than 10(exp 24) cm(exp −2)). We find that for quasars at z is approximately 0.5, NuSTAR provides a significant improvement compared to lower energy (less than 10 keV) Chandra and XMM-Newton observations alone, as higher column densities can now be directly constrained.

quasars↗
NASA NTRS2014

The NuSTAR View of Gamma Ray Bursts

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission was launched June 13, 2012. During the next two pears NuSTAR observed two Gamma Ray Bursts, GRBs 130427A and 130925A. I will describe here the NuSTAR GRB results and discuss their implications on the GRB field.

Kouveliotou, C.↗
NASA NTRS2013

NuSTAR Detection of the Blazar B2 1023+25 at Redshift 5.3

B2 1023+25 is an extremely radio-loud quasar at zeta = 5.3 that was first identified as a likely high-redshift blazar candidate in the SDSS+FIRST quasar catalog. Here, we use the Nuclear Spectroscopic Telescope Array (NuSTAR) to investigate its non-thermal jet emission, whose high-energy component we detected in the hard X-ray energy band. The X-ray flux is approximately 5.5 × 10 (exp −14) erg cm(exp −2) s(exp −1) (5-10 keV) and the photon spectral index is Gamma(x) approx. =1.3-1.6. Modeling the full spectral energy distribution, we find that the jet is oriented close to the line of sight, with a viewing angle of approximately 3deg, and has significant Doppler boosting, with a large bulk Lorentz factor approximately 13, which confirms the identification of B2 1023+25 as a blazar. B2 1023+25 is the first object at redshift larger than 5 detected by NuSTAR, demonstrating the ability of NuSTAR to investigate the early X-ray universe and to study extremely active supermassive black holes located at very high redshift.

X-rays: general↗
NASA NTRS2014

NuSTAR Study of Hard X-ray Morphology and Spectroscopy G21.5-0.9

We present NuSTAR high-energy X-ray observations of the pulsar wind nebula (PWN)/supernova remnant G21.5−0.9. We detect integrated emission from the nebula up to approx. 40 keV, and resolve individual spatial features over a broad X-ray band for the first time. The morphology seen by NuSTAR agrees well with that seen by XMM-Newton and Chandra below 10 keV. At high energies, NuSTAR clearly detects non-thermal emission up to approx. 20 keV that extends along the eastern and northern rim of the supernova shell. The broadband images clearly demonstrate that X-ray emission from the North Spur and Eastern Limb results predominantly from non-thermal processes. We detect a break in the spatially integrated X-ray spectrum at approx. 9 keV that cannot be reproduced by current spectral energy distribution models, implying either a more complex electron injection spectrum or an additional process such as diffusion compared to what has been considered in previous work. We use spatially resolved maps to derive an energy-dependent cooling length scale, E(sup m) is directly proportional to L(E) with m = −0.21 plus or minus 0.01. We find this to be inconsistent with the model for the morphological evolution with energy described by Kennel & Coroniti. This value, along with the observed steepening in power-law index between radio and X-ray, can be quantitatively explained as an energy-loss spectral break in the simple scaling model of Reynolds, assuming particle advection dominates over diffusion. This interpretation requires a substantial departure from spherical magnetohydrodynamic, magnetic-flux-conserving outflow, most plausibly in the form of turbulent magnetic-field amplification.

radiation mechanisms↗
NASA NTRS2014

NuSTAR Observations of the Magnetar 1E 2259+586

We report on new broad band spectral and temporal observations of the magnetar 1E 2259+586, which is located in the supernova remnant CTB 109. Our data were obtained simultaneously with the Nuclear Spectroscopic Telescope Array (NuSTAR) and Swift, and cover the energy range from 0.5-79 keV. We present pulse profiles in various energy bands and compare them to previous RXTE results. The NuSTAR data show pulsations above 20 keV for the first time and we report evidence that one of the pulses in the double-peaked pulse profile shifts position with energy. The pulsed fraction of the magnetar is shown to increase strongly with energy. Our spectral analysis reveals that the soft X-ray spectrum is well characterized by an absorbed double blackbody or blackbody plus power-law model in agreement with previous reports. Our new hard X-ray data, however, suggest that an additional component, such as a power law, is needed to describe the NuSTAR and Swift spectrum. We also fit the data with the recently developed coronal outflow model by Beloborodov for hard X-ray emission from magnetars. The outflow from a ring on the magnetar surface is statistically preferred over outflow from a polar cap.

X-rays: individual↗
NASA NTRS2014

The Soft-X-Ray Emission of Ark 120. XMM-Newton, NuSTAR, and the Importance of Taking the Broad View

We present simultaneous XMM-Newton and NuSTAR observations of the 'bare' Seyfert 1 galaxy, Ark 120, a system in which ionized absorption is absent. The NuSTAR hard-X-ray spectral coverage allows us to constrain different models for the excess soft-X-ray emission. Among phenomenological models, a cutoff power law best explains the soft-X-ray emission. This model likely corresponds to Comptonization of the accretion disc seed UV photons by a population of warm electrons: using Comptonization models, a temperature of approximately 0.3 kiloelectronvolts and an optical depth of approximately 13 are found. If the UV-to-X-ray OPTXAGNF model is applied, the UV fluxes from the XMM-Newton Optical Monitor suggest an intermediate black hole spin. Contrary to several other sources observed by NuSTAR, no high-energy cutoff is detected with a lower limit of 190 kiloelectronvolts.

accretion↗
NASA NTRS2015

The Compton Hump and Variable Blue Wing in the Extreme Low-Flux NuSTAR Observations of 1H0707-495

The narrow-line Seyfert I galaxy, 1H0707−495, has been well observed in the 0.3-10 kiloelectronvolt band, revealing a dramatic drop in flux in the iron K-alpha band, a strong soft excess, and short time-scale reverberation lags associated with these spectral features. In this paper, we present the first results of a deep 250-kilosecond NuSTAR (Nuclear Spectroscopic Telescope Array) observation of 1H0707−495, which includes the first sensitive observations above 10 kiloelectronvolts. Even though the NuSTAR observations caught the source in an extreme low-flux state, the Compton hump is still significantly detected. NuSTAR, with its high effective area above 7 kiloelectronvolts, clearly detects the drop in flux in the iron K-alpha band, and by comparing these observations with archival XMM-Newton observations, we find that the energy of this drop increases with increasing flux. We discuss possible explanations for this, the most likely of which is that the drop in flux is the blue wing of the relativistically broadened iron K-alpha emission line. When the flux is low, the coronal source height is low, thus enhancing the most gravitationally red-shifted emission.

black hole physics a galaxies: active a galaxies: ↗
NASA NTRS2016

Multiwavelength Study of Quiescent States of Mrk 421 with Unprecedented Hard X-Ray Coverage Provided by NuSTAR in 2013

We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Markarian 421 (Mrk 421) taken in 2013 January-March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy„ (VHE) gamma-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3-79 kiloelectronvolt range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep Gamma approximating 3 power law, with no evidence for an exponential cutoff or additional hard components up to 80 kiloelectronvolts. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.

BL Lacertae objects: individual (markarian 421) - ↗
NASA NTRS2016

Demonstrating the Likely Neutron Star Nature of Five M31 Globular Cluster Sources with Swift-NuSTAR Spectroscopy

We present the results of a joint Swift-NuSTAR spectroscopy campaign on M31. We focus on the five brightest globular cluster X-ray sources in our fields. Two of these had previously been argued to be black hole candidates on the basis of apparent hard-state spectra at luminosities above those for which neutron stars are in hard states. We show that these two sources are likely to be Z-sources (i.e. low magnetic field neutron stars accreting near their Eddington limits), or perhaps bright atoll sources (low magnetic field neutron stars which are just a bit fainter than this level) on the basis of simultaneous Swift and NuSTAR spectra which cover a broader range of energies. These new observations reveal spectral curvature above 6-8 keV that would be hard to detect without the broader energy coverage the NuSTAR data provide relative to Chandra and XMM-Newton. We show that the other three sources are also likely to be bright neutron star X-ray binaries, rather than black hole X-ray binaries. We discuss why it should already have been realized that it was unlikely that these objects were black holes on the basis of their being persistent sources, and we re-examine past work which suggested that tidal capture products would be persistently bright X-ray emitters. We discuss how this problem is likely due to neglecting disc winds in older work that predict which systems will be persistent and which will be transient.

globular clusters: general↗
NASA NTRS2016

NuSTAR and XMM-Newton Observations of 1E1743.1-2843: Indications of a Neutron Star LMXB Nature of the Compact Object

We report on the results of NuSTAR and XMM-Newton observations of the persistent X-ray source 1E1743.1-2843, located in the Galactic Center region. The source was observed between 2012 September and October by NuSTAR and XMM-Newton, providing almost simultaneous observations in the hard and soft X-ray bands. The high X-ray luminosity points to the presence of an accreting compact object. We analyze the possibilities of this accreting compact object being either a neutron star (NS) or a black hole, and conclude that the joint XMM-Newton and NuSTAR spectrum from 0.3 to 40 keV fits a blackbody spectrum with kT approximately 1.8 keV emitted from a hot spot or an equatorial strip on an NS surface. This spectrum is thermally Comptonized by electrons with kTe approximately 4.6 keV. Accepting this NS hypothesis, we probe the low-mass X-ray binary (LMXB) or high-mass X-ray binary (HMXB) nature of the source. While the lack of Type-I bursts can be explained in the LMXB scenario, the absence of pulsations in the 2 MHz-49 Hz frequency range, the lack of eclipses and of an IR companion, and the lack of a Kaline from neutral or moderately ionized iron strongly disfavor interpreting this source as a HMXB. We therefore conclude that 1E1743.1-2843 is most likely an NS-LMXB located beyond the Galactic Center. There is weak statistical evidence for a soft X-ray excess which may indicate thermal emission from an accretion disk. However, the disk normalization remains unconstrained due to the high hydrogen column density (N(sub H) approximately 1.6 x 10(exp 23) cm(exp -2)).

accretion↗
NASA NTRS2016

NuSTAR Observations of Water Megamaser AGN

Aims. We study the connection between the masing disk and obscuring torus in Seyfert 2 galaxies. Methods. We present a uniform X-ray spectral analysis of the high energy properties of 14 nearby megamaser active galactic nuclei observed by NuSTAR. We use a simple analytical model to localize the maser disk and understand its connection with the torus by combining NuSTAR spectral parameters with the available physical quantities from VLBI mapping.Results. Most of the sources that we analyzed are heavily obscured, showing a column density in excess of approx.10(exp 23) cm(exp -2); in particular, 79% are Compton-thick [NH is greater than 1.5 x 10(exp 24) cm(exp -2)]. When using column densities measured by NuSTAR with the assumption that the torus is the extension of the maser disk, and further assuming a reasonable density profile, we can predict the torus dimensions. They are found to be consistent with mid-IR interferometry parsec-scale observations of Circinus and NGC 1068. In this picture, the maser disk is intimately connected to the inner part of the torus. It is probably made of a large number of molecular clouds that connect the torus and the outer part of the accretion disk, giving rise to a thin disk rotating in most cases in Keplerian or sub-Keplerian motion. This toy model explains the established close connection between water megamaser emission and nuclear obscuration as a geometric effect.

masers / galaxies: active / galaxies: Seyfert↗