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William Forman

Publications and source records attributed to William Forman.

At least 37 records · Page 2Linked to original sources
arXiv2024

Weak-Lensing Analysis of the Complex Cluster Merger Abell 746 with Subaru/Hyper Suprime-Cam

The galaxy cluster Abell 746 (A746; $z$=0.214), featuring a double radio relic system, two isolated radio relics, a possible radio halo, disturbed V-shaped X-ray emission, and intricate galaxy distributions, is a unique and complex merging system. We present a weak-lensing analysis of A746 based on wide-field imaging data from Subaru/Hyper Suprime-Cam observations. The mass distribution is characterized by a main peak which coincides with the center of the X-ray emission. At this main peak, we detect two extensions toward the north and west, tracing the cluster galaxy and X-ray distributions. Despite the ongoing merger, our estimate of the A746 global mass $M_{500}=4.4\pm1.0\times10^{14}~M_{\odot}$ is consistent with the previous results from SZ and X-ray observations. We conclude that reconciling the distributions of mass, galaxies, and intracluster medium with the double radio relic system and other radio features remains challenging.

astro-ph.GA↗
arXiv2024

Merger-driven multi-scale ICM density perturbations: testing cosmological simulations and constraining plasma physics

The hot intracluster medium (ICM) provides a unique laboratory to test multi-scale physics in numerical simulations and probe plasma physics. Utilizing archival Chandra observations, we measure density fluctuations in the ICM in a sample of 80 nearby (z<1) galaxy clusters and infer scale-dependent velocities within regions affected by mergers (r<R2500c), excluding cool-cores. Systematic uncertainties (e.g., substructures, cluster asymmetries) are carefully explored to ensure robust measurements within the bulk ICM. We find typical velocities ~220 (300) km/s in relaxed (unrelaxed) clusters, which translate to non-thermal pressure fractions ~4 (8) per cent, and clumping factors ~1.03 (1.06). We show that density fluctuation amplitudes could distinguish relaxed from unrelaxed clusters in these regions. Comparison with density fluctuations in cosmological simulations shows good agreement in merging clusters. Simulations underpredict the amplitude of fluctuations in relaxed clusters on length scales <0.75 R2500c, suggesting these systems are most sensitive to missing physics in the simulations. In clusters hosting radio halos, we examine correlations between gas velocities, turbulent dissipation rate, and radio emission strength/efficiency to test turbulent re-acceleration of cosmic ray electrons. We measure a weak correlation, driven by a few outlier clusters, in contrast to some previous studies. Finally, we present upper limits on effective viscosity in the bulk ICM of 16 clusters, showing it is systematically suppressed by at least a factor of 8, and the suppression is a general property of the ICM. Confirmation of our results with direct velocity measurements will be possible soon with XRISM.

astro-ph.HE↗
arXiv2024

On the Particle Acceleration Mechanisms in a Double Radio Relic Galaxy Cluster, Abell 1240

We present a 368 ks deep Chandra observation of Abell~1240, a binary merging galaxy cluster at a redshift of 0.195 with two Brightest Cluster Galaxies (BCGs) may have passed each other 0.3 Gyr ago. Building upon previous investigations involving GMRT, VLA, and LOFAR data, our study focuses on two prominent extended radio relics at the north-west (NW) and south-east (SE) of the cluster core. By leveraging the high-resolution Chandra imaging, we have identified two distinct surface brightness edges at $\sim$ 1 Mpc and 1.2 Mpc NW and SE of the cluster center, respectively, coinciding with the outer edges of both relics. Our temperature measurements hint the edges to be shock front edges. The Mach numbers, derived from the gas density jumps, yield $\cal{M}_{\rm SE}$ = 1.49$^{+0.22}_{-0.24}$ for the South Eastern shock and $\cal{M}_{\rm NW}$ = 1.41$^{+0.17}_{-0.19}$ for the North Western shock. Our estimated Mach numbers are remarkably smaller compared to those derived from radio observations ($\cal{M}_{\rm SE}$ = 2.3 and $\cal{M}_{\rm NW}$ = 2.4), highlighting the prevalence of a re-acceleration scenario over direct acceleration of electrons from the thermal pool. Furthermore, we compare the observed temperature profiles across both shocks with that of predictions from collisional vs. collisionless models. Both shocks favor the Coulomb collisional model, but we could not rule out a purely collisionless model due to pre-shock temperature uncertainties.

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arXiv2023

X-Ray Constraints on the Hot Gaseous Corona of Edge-on Late-type Galaxies in Virgo

We present a systematic study of the putative hot gas corona around late-type galaxies (LTGs) residing in the Virgo cluster, based on archival Chandra observations. Our sample consists of 21 nearly edge-on galaxies representing a star formation rate (SFR) range of ($0.2-3\rm~M_\odot~yr^{-1}$) a stellar mass ($M_*$) range of $(0.2-10) \times 10^{10}\rm~M_{\odot}$, the majority of which have not been explored with high-sensitivity X-ray observations so far. Significant extraplanar diffuse X-ray (0.5-2 keV) emission is detected in only three LTGs, which are also the three galaxies with the highest SFR. A stacking analysis is performed for the remaining galaxies without individual detection, dividing the whole sample into two subsets based on SFR, stellar mass, or specific SFR. Only the high-SFR bin yields a significant detection, which has a value of $L\rm_X \sim3\times10^{38}\rm~erg~s^{-1}$ per galaxy. The stacked extraplanar X-ray signals of the Virgo LTGs are consistent with the empirical $L\rm_X - SFR$ and $L\rm_X - M_*$ relations found among highly inclined disk galaxies in the field, but appear to be systematically lower than that of a comparison sample of simulated cluster star-formation galaxies identified from the Illustris-TNG100 simulation. The apparent paucity of hot gas coronae in the sampled Virgo LTGs might be understood as the net outcome of the long-lasting effect of ram pressure stripping exerted by the hot intra-cluster medium and in-disk star-forming activity acting on shorter timescales. A better understanding of the roles of environmental effects in regulating the hot gas content of cluster galaxies invites sensitive X-ray observations for a large galaxy sample.

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arXiv2023

Jet reorientation in central galaxies of clusters and groups: insights from VLBA and Chandra data

Recent observations of galaxy clusters and groups with misalignments between their central AGN jets and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet - bubble connection in cooling cores, and the processes responsible for jet realignment. To investigate the frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample and selected subsets, we consistently find that there is a 30% - 38% chance to find a misalignment larger than $ΔΨ= 45^{\circ}$ when observing a cluster/group with a detected jet and at least one cavity. We determine that projection may account for an apparently large $ΔΨ$ only in a fraction of objects ($\sim$35%), and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned systems, we exclude environmental perturbation as the main driver of cavity - jet misalignment. Moreover, we find that large misalignments (up to $\sim90^{\circ}$) are favored over smaller ones ($45^{\circ}\leqΔΨ\leq70^{\circ}$), and that the change in jet direction can occur on timescales between one and a few tens of Myr. We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we discuss several engine-based mechanisms that may cause these dramatic changes.

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arXiv2023

LEM All-Sky Survey: Soft X-ray Sky at Microcalorimeter Resolution

The Line Emission Mapper (LEM) is an X-ray Probe with with spectral resolution ~2 eV FWHM from 0.2 to 2.5 keV and effective area >2,500 cm$^2$ at 1 keV, covering a 33 arcmin diameter Field of View with 15 arcsec angular resolution, capable of performing efficient scanning observations of very large sky areas and enabling the first high spectral resolution survey of the full sky. The LEM-All-Sky Survey (LASS) is expected to follow the success of previous all sky surveys such as ROSAT and eROSITA, adding a third dimension provided by the high resolution microcalorimeter spectrometer, with each 15 arcsec pixel of the survey including a full 1-2 eV resolution energy spectrum that can be integrated over any area of the sky to provide statistical accuracy. Like its predecessors, LASS will provide both a long-lasting legacy and open the door to the unknown, enabling new discoveries and delivering the baseline for unique GO studies. No other current or planned mission has the combination of microcalorimeter energy resolution and large grasp to cover the whole sky while maintaining good angular resolution and imaging capabilities. LASS will be able to probe the physical conditions of the hot phases of the Milky Way at multiple scales, from emission in the Solar system due to Solar Wind Charge eXchange, to the interstellar and circumgalactic media, including the North Polar Spur and the Fermi/eROSITA bubbles. It will measure velocities of gas in the inner part of the Galaxy and extract the emissivity of the Local Hot Bubble. By maintaining the original angular resolution, LASS will also be able to study classes of point sources through stacking. For classes with ~$10^4$ objects, it will provide the equivalent of 1 Ms of high spectral resolution data. We describe the technical specifications of LASS and highlight the main scientific objectives that will be addressed. (Abridged)

astro-ph.IM↗
arXiv2023

Evidence for heavy seed origin of early supermassive black holes from a z~10 X-ray quasar

Observations of quasars reveal that many supermassive black holes (BHs) were in place less than 700 million years after the Big Bang. However, the origin of the first BHs remains a mystery. Seeds of the first BHs are postulated to be either light (i.e., $10-100~\rm{M_{\odot}})$, remnants of the first stars or heavy (i.e., $10^4-10^5~\rm{M_{\odot}})$, originating from the direct collapse of gas clouds. Harnessing recent data from the Chandra X-ray Observatory, we report the detection of an X-ray-luminous massive BH in a gravitationally-lensed galaxy identified by JWST at $z\approx10.3$ behind the cluster lens Abell 2744. This heavily-obscured quasar with a bolometric luminosity of $L_{\rm bol}\sim5\times10^{45}~\rm{erg\ s^{-1}}$ harbors a $M_{\rm BH}\sim10^7-10^8~\rm{M_{\odot}}$ BH assuming accretion at the Eddington limit. This mass is comparable to the inferred stellar mass of its host galaxy, in contrast to what is found in the local Universe wherein the BH mass is $\sim0.1\%$ of the host galaxy's stellar mass. The combination of such a high BH mass and large BH-to-galaxy stellar mass ratio just $\sim$500 Myrs after the Big Bang was theoretically predicted and is consistent with a picture wherein BHs originated from heavy seeds.

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arXiv2023

Circumgalactic Medium on the Largest Scales: Detecting X-ray Absorption Lines with Large-Area Microcalorimeters

The circumgalactic medium (CGM) plays a crucial role in galaxy evolution as it fuels star formation, retains metals ejected from the galaxies, and hosts gas flows in and out of galaxies. For Milky Way-type and more massive galaxies, the bulk of the CGM is in hot phases best accessible at X-ray wavelengths. However, our understanding of the CGM remains largely unconstrained due to its tenuous nature. A promising way to probe the CGM is via X-ray absorption studies. Traditional absorption studies utilize bright background quasars, but this method probes the CGM in a pencil beam, and, due to the rarity of bright quasars, the galaxy population available for study is limited. Large-area, high spectral resolution X-ray microcalorimeters offer a new approach to exploring the CGM in emission and absorption. Here, we demonstrate that the cumulative X-ray emission from cosmic X-ray background sources can probe the CGM in absorption. We construct column density maps of major X-ray ions from the Magneticum simulation and build realistic mock images of nine galaxies to explore the detectability of X-ray absorption lines arising from the large-scale CGM. We conclude that the OVII absorption line is detectable around individual massive galaxies at the $3σ-6σ$ confidence level. For Milky Way-type galaxies, the OVII and OVIII absorption lines are detectable at the $\sim\,6σ$ and $\sim\,3σ$ levels even beyond the virial radius when co-adding data from multiple galaxies. This approach complements emission studies, does not require additional exposures, and will allow probing of the baryon budget and the CGM at the largest scales.

astro-ph.GA↗
arXiv2023

Inferences from surface brightness fluctuations of Zwicky 3146 via the Sunyaev-Zeldovich effect and X-ray observations

The galaxy cluster Zwicky 3146 is a sloshing cool core cluster at $z{=}0.291$ that in SZ imaging does not appear to exhibit significant pressure substructure in the intracluster medium (ICM). We perform a surface brightness fluctuation analysis via Fourier amplitude spectra on SZ (MUSTANG-2) and X-ray (XMM-Newton) images of this cluster. These surface brightness fluctuations can be deprojected to infer pressure and density fluctuations from the SZ and X-ray data, respectively. In the central region (Ring 1, $r < 100^{\prime\prime} = 440$ kpc, in our analysis) we find fluctuation spectra that suggest injection scales around 200 kpc ($\sim 140$ kpc from pressure fluctuations and $\sim 250$ kpc from density fluctuations). When comparing the pressure and density fluctuations in the central region, we observe a change in the effective thermodynamic state from large to small scales, from isobaric (likely due to the slow sloshing) to adiabatic (due to more vigorous motions). By leveraging scalings from hydrodynamical simulations, we find an average 3D Mach number $\approx0.5$. We further compare our results to other studies of Zwicky 3146 and, more broadly, to other studies of fluctuations in other clusters.

astro-ph.CO↗
arXiv2023

X-ray polarization evidence for a 200 years-old flare of Sgr A$^*$

The center of the Milky Way Galaxy hosts a $\sim$4 million solar mass black hole (Sgr A$^*$) that is currently very quiescent with a luminosity many orders of magnitude below those of active galactic nuclei. Reflection of X-rays from Sgr A$^*$ by dense gas in the Galactic Center region offers a means to study its past flaring activity on times scales of hundreds and thousands of years. The shape of the X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A$^*$ are consistent with the reflection scenario. If this interpretation is correct, the reflected continuum emission should be polarized. Here we report observations of polarized X-ray emission in the direction of the Galactic center molecular clouds using the Imaging X-ray Polarimetry Explorer (IXPE). We measure a polarization degree of 31\% $\pm$ 11\%, and a polarization angle of $-$48$^\circ$ $\pm$ 11$^\circ$. The polarization angle is consistent with Sgr A$^*$ being the primary source of the emission, while the polarization degree implies that some 200 years ago the X-ray luminosity of Sgr A$^*$ was briefly comparable to a Seyfert galaxy.

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arXiv2023

Line Emission Mapper (LEM): Probing the physics of cosmic ecosystems

The Line Emission Mapper (LEM) is an X-ray Probe for the 2030s that will answer the outstanding questions of the Universe's structure formation. It will also provide transformative new observing capabilities for every area of astrophysics, and to heliophysics and planetary physics as well. LEM's main goal is a comprehensive look at the physics of galaxy formation, including stellar and black-hole feedback and flows of baryonic matter into and out of galaxies. These processes are best studied in X-rays, and emission-line mapping is the pressing need in this area. LEM will use a large microcalorimeter array/IFU, covering a 30x30' field with 10" angular resolution, to map the soft X-ray line emission from objects that constitute galactic ecosystems. These include supernova remnants, star-forming regions, superbubbles, galactic outflows (such as the Fermi/eROSITA bubbles in the Milky Way and their analogs in other galaxies), the Circumgalactic Medium in the Milky Way and other galaxies, and the Intergalactic Medium at the outskirts and beyond the confines of galaxies and clusters. LEM's 1-2 eV spectral resolution in the 0.2-2 keV band will make it possible to disentangle the faintest emission lines in those objects from the bright Milky Way foreground, providing groundbreaking measurements of the physics of these plasmas, from temperatures, densities, chemical composition to gas dynamics. While LEM's main focus is on galaxy formation, it will provide transformative capability for all classes of astrophysical objects, from the Earth's magnetosphere, planets and comets to the interstellar medium and X-ray binaries in nearby galaxies, AGN, and cooling gas in galaxy clusters. In addition to pointed observations, LEM will perform a shallow all-sky survey that will dramatically expand the discovery space.

astro-ph.IM↗
arXiv2022

Testing galaxy feedback models with the first resolved profiles of the circumgalactic medium

The hot ($>10^6$ K) phase of the circumgalactic medium (CGM) contains a large fraction of baryons in galaxies. It also retains signatures of the processes that shaped the galaxies, such as feedback from active galactic nuclei (AGNs) and supernovae, and offers a uniquely powerful way to constrain theoretical models of feedback. It is, however, notoriously difficult to detect. By stacking 2643 optically selected galaxies in the eROSITA Final Equatorial Depth Survey (eFEDS), we present spatially resolved properties of the extended CGM in both star-forming and quiescent galaxies spanning an order of magnitude in stellar mass. We mask out resolved point sources and galaxy groups/clusters and model the contribution from X-ray binaries and the hot ISM, producing accurate radial profiles. We compare the profiles to mock X-ray observations of galaxy stacks in the IllustrisTNG100 (TNG) and EAGLE cosmological simulations. We detect extended emission from both the high-mass ($10.7<\log(M_*/M_\odot)<11.2$) and low-mass ($10.2<\log(M_*/M_\odot)<10.7$) galaxy stacks. Galaxies have somewhat more luminous CGM between $10-100$~kpc if they are more massive or star-forming. However, the luminosity increases slower with stellar mass than predicted in simulations. Simulated quenched galaxies are far dimmer than observed, suggesting that they rely too heavily on CGM ejection for quenching. Star-forming galaxies are observed to have flatter and more extended profiles than in simulations, suggesting under-efficient stellar feedback models. Our results highlight the need to modify future prescriptions of galaxy feedback models.

astro-ph.GA↗
arXiv2022

Discovery of a pre-merger shock in an intercluster filament in Abell 98

We report the first unambiguous detection of an axial merger shock in the early-stage merging cluster Abell 98 using deep (227 ks) Chandra observations. The shock is about 420 kpc south from the northern subcluster of Abell 98, in between the northern and central subclusters, with a Mach number of M $\approx$ 2.3 $\pm$ 0.3. Our discovery of the axial merger shock front unveils a critical epoch in the formation of a massive galaxy cluster, when two subclusters are caught in the early phase of the merging process. We find that the electron temperature in the post-shock region favors the instant collisionless model, where electrons are strongly heated at the shock front, by interactions with the magnetic field. We also report on the detection of an intercluster gas filament, with a temperature of kT = 1.07 $\pm$ 0.29 keV, along the merger axis of Abell 98. The measured properties of the gas in the filament are consistent with previous observations and numerical simulations of the hottest, densest parts of the warm-hot intergalactic medium (WHIM), where WHIM filaments interface with the virialization regions of galaxy clusters.

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arXiv2022

Intracluster magnetic filaments and an encounter with a radio jet

Thin synchrotron-emitting filaments are increasingly seen in the intracluster medium (ICM). We present the first example of a direct interaction between a magnetic filament, a radio jet, and a dense ICM clump in the poor cluster Abell 194. This enables the first exploration of the dynamics and possible histories of magnetic fields and cosmic rays in such filaments. Our observations are from the MeerKAT Galaxy Cluster Legacy Survey and the LOFAR Two Metre Sky Survey. Prominent 220 kpc long filaments extend east of radio galaxy 3C40B, with very faint extensions to 300 kpc, and show signs of interaction with its northern jet. They curve around a bend in the jet and intersect the jet in Faraday depth space. The X-ray surface brightness drops across the filaments; this suggests that the relativistic particles and fields contribute significantly to the pressure balance and evacuate the thermal plasma in a $\sim$35 kpc cylinder. We explore whether the relativistic electrons could have streamed along the filaments from 3C40B, and present a plausible alternative whereby magnetized filaments are a) generated by shear motions in the large-scale, post-merger ICM flow, b) stretched by interactions with the jet and flows in the ICM, amplifying the embedded magnetic fields, and c) perfused by re-energized relativistic electrons through betatron-type acceleration or diffusion of turbulently accelerated ICM cosmic ray electrons. We use the Faraday depth measurements to reconstruct some of the 3D structures of the filaments and of 3C40A and B.

astro-ph.HE↗
arXiv2022

Resilience of Sloshing Cold Fronts against Subsequent Minor Mergers

Minor mergers are common in galaxy clusters. They have the potential to create sloshing cold fronts (SCFs) in the intracluster medium (ICM) of the cluster. However, the resilience of SCFs to subsequent minor mergers is unknown. Here we investigate the extent to which SCFs established by an off-axis minor merger are disrupted by a subsequent minor merger. We perform a suite of 13 hydrodynamic + N-body simulations of idealised triple cluster mergers in which we vary the approach direction and impact parameter of the tertiary cluster. Except for ~1 Gyr after the first core passage of the tertiary cluster, clear SCFs are present in all merger configurations. Subsequent head-on minor mergers reduce the number of SCFs significantly, while subsequent off-axis minor mergers only moderately reduce the number of SCFs. In particular, outer (>~500 kpc) SCFs are resilient. The results of this work indicate that SCFs are easily formed in the course of a minor merger and are long-lived even if a further minor merger takes place. SCFs thus should be ubiquitous, but deriving the merger history of a given cluster based on its observed SCFs might be more complex than previously thought.

astro-ph.GA↗
arXiv2022

Chandra view of Abell 407: the central compact group of galaxies and the interaction between the radio AGN and the ICM

Abell 407 (A407) is a unique galaxy cluster hosting a central compact group of nine galaxies (named as 'Zwicky's Nonet'; G1 - G9 in this work) within a 30 kpc radius region. The cluster core also hosts a luminous radio active galactic nucleus (AGN), 4C 35.06 with helically twisted jets extending over 200 kpc. With a 44 ks Chandra observation of A407, we characterize the X-ray properties of its intracluster medium (ICM) and central galaxies. The mean X-ray temperature of A407 is 2.7 keV and the $M_{200}$ is $1.9 \times 10^{14} {M_{\odot}}$. We suggest that A407 has a weak cool core at $r < 60$ kpc scales and at its very center, $< 1$-2 kpc radius, a small galaxy corona associated with the strong radio AGN. We also conclude that the AGN 4C 35.06 host galaxy is most likely G3. We suggest that the central group of galaxies is undergoing a `slow merge' procedure. The range of the merging time-scale is $0.3\sim2.3$ Gyr and the stellar mass of the future brightest cluster galaxy (BCG) will be $7.4\times10^{11} M_{\odot}$. We find that the regions which overlap with the radio jets have higher temperature and metallicity. This is consistent with AGN feedback activity. The central entropy is higher than that for other clusters, which may be due to the AGN feedback and/or merging activity. With all these facts, we suggest that A407 is a unique and rare system in the local universe that could help us to understand the formation of a massive BCG.

astro-ph.GA↗
arXiv2022

The velocity structure of the intracluster medium during a major merger: simulated microcalorimeter observations

Major mergers between galaxy clusters can produce large turbulent and bulk flow velocities in the intra-cluster medium and thus imprint diagnostic features in X-ray spectral emission lines from heavy ions. As demonstrated by Hitomi in observations of the Perseus cluster, measurements of gas velocities in clusters from high-resolution X-ray spectra will be achievable with upcoming X-ray calorimeters like those on board XRISM, Athena, or a Lynx like mission. We investigate this possibility for interesting locations across a major cluster merger from a hydrodynamical simulation, via X-ray synthetic spectra with a few eV energy resolution. We observe the system from directions perpendicular to the plane of the merger and along the merger axis. In these extreme geometrical configurations, we find clear non-Gaussian shapes of the iron He-like K_alpha line at 6.7keV. The velocity dispersion predicted from the simulations can be retrieved for the brightest 100ks pointings with XRISM Resolve, despite some discrepancy related to the complex non-Gaussian line shapes. Measurements in faint regions require however high S/N and the larger collecting area of the Athena X-IFU calorimeter is thus needed. With the latter, we also investigate the gas temperature and velocity gradient across the merger bow shock edge, from 20"-wide annuli extracted from a single 1Ms X-IFU pointing. We find best-fit temperature and velocity dispersion values that are consistent with predictions from the simulations within 1-sigma, but the uncertainties on the inferred velocity dispersion are too large to place any stringent constraints on the shallow gradient downstream of the shock. We also present simulated images of the thermal and kinetic Sunyaev-Zeldovich effects, using the above viewing configurations, and compare the results at angular resolutions appropriate for future observatories such as CMB-S4 and AtLAST.

astro-ph.CO↗
arXiv2021

The deepest $Chandra$ view of RBS 797: evidence for two pairs of equidistant X-ray cavities

We present the first results of a deep $Chandra$ observation of the galaxy cluster RBS 797, whose previous X-ray studies revealed two pronounced X-ray cavities in the east-west (E-W) direction. Follow-up VLA radio observations of the central active galactic nucleus (AGN) uncovered different jet and lobe orientations, with radio lobes filling the E-W cavities and perpendicular jets showing emission in the north-south (N-S) direction over the same scale ($\approx$30 kpc). With the new $\sim$427 ks total exposure, we report the detection of two additional, symmetric X-ray cavities in the N-S direction at nearly the same radial distance as the E-W ones. The newly discovered N-S cavities are associated with the radio emission detected at 1.4 GHz and 4.8 GHz in archival VLA data, making RBS 797 the first galaxy cluster found to have four equidistant, centrally-symmetric, radio-filled cavities. We derive the dynamical and radiative ages of the four cavities from X-ray and radio data, respectively, finding that the two outbursts are approximately coeval, with an age difference of $\lessapprox$10 Myr between the E-W and N-S cavities. We discuss two scenarios for the origin of the two perpendicular, equidistant cavity systems: either the presence of a binary AGN which is excavating coeval pairs of cavities in perpendicular directions, or a fast ($<$10 Myr) jet reorientation event which produced subsequent, misaligned outbursts.

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