arXiv subjects

Jay Strader

Publications and source records attributed to Jay Strader.

At least 73 records · Page 4Linked to original sources

arXiv2022

1RXH J082623.6-505741: a new long-period cataclysmic variable with an evolved donor and a low mass transfer rate

We report the discovery of 1RXH J082623.6-505741, a 10.4 hr orbital period compact binary. Modeling extensive optical photometry and spectroscopy reveals a $\sim 0.4 M_{\odot}$ K-type secondary transferring mass through a low-state accretion disk to a non-magnetic $\sim 0.8 M_{\odot}$ white dwarf. The secondary is overluminous for its mass and dominates the optical spectra at all epochs, and must be evolved to fill its Roche Lobe at this orbital period. The X-ray luminosity $L_X \sim 1$-$2 \times 10^{32}$ erg s$^{-1}$ derived from both new XMM-Newton and archival observations, although high compared to most CVs, still only requires a modest accretion rate onto the white dwarf of $\dot{M} \sim 3 \times 10^{-11}$ to $3 \times 10^{-10} M_{\odot}$ yr$^{-1}$, lower than expected for a cataclysmic variable with an evolved secondary. No dwarf nova outbursts have yet been observed from the system, consistent with the low derived mass transfer rate. Several other cataclysmic variables with similar orbital periods also show unexpectedly low mass transfer rates, even though selection effects disfavor the discovery of binaries with these properties. This suggests the abundance and evolutionary state of long-period, low mass transfer rate cataclysmic variables is worthy of additional attention.

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arXiv2022

SOAR/Goodman Spectroscopic Assessment of Candidate Counterparts of the LIGO-Virgo Event GW190814

On 2019 August 14 at 21:10:39 UTC, the LIGO/Virgo Collaboration (LVC) detected a possible neutron star-black hole merger (NSBH), the first ever identified. An extensive search for an optical counterpart of this event, designated GW190814, was undertaken using the Dark Energy Camera (DECam) on the 4m Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory. Target of Opportunity interrupts were issued on 8 separate nights to observe 11 candidates using the 4.1m Southern Astrophysical Research (SOAR) telescope's Goodman High Throughput Spectrograph in order to assess whether any of these transients was likely to be an optical counterpart of the possible NSBH merger. Here, we describe the process of observing with SOAR, the analysis of our spectra, our spectroscopic typing methodology, and our resultant conclusion that none of the candidates corresponded to the gravitational wave merger event but were all instead other transients. Finally, we describe the lessons learned from this effort. Application of these lessons will be critical for a successful community spectroscopic follow-up program for LVC observing run 4 (O4) and beyond.

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arXiv2022

The first nova eruption in a novalike variable: YZ Ret as seen in X-rays and gamma-rays

Peaking at 3.7 mag on 2020 July 11, YZ Ret was the second-brightest nova of the decade. The nova's moderate proximity (2.7 kpc from Gaia) provided an opportunity to explore its multi-wavelength properties in great detail. Here we report on YZ Ret as part of a long-term project to identify the physical mechanisms responsible for high-energy emission in classical novae. We use simultaneous Fermi/LAT and NuSTAR observations complemented by XMM-Newton X-ray grating spectroscopy to probe the physical parameters of the shocked ejecta and the nova-hosting white dwarf. The XMM-Newton observations revealed a super-soft X-ray emission which is dominated by emission lines of CV, CVI, NVI, NVII, and OVIII rather than a blackbody-like continuum, suggesting CO-composition of the white dwarf in a high-inclination binary system. Fermi/LAT detected YZ Ret for 15 days with the gamma-ray spectrum best described by a power law with an exponential cut-off at 1.9 +/-0.6 GeV. In stark contrast with theoretical predictions and in keeping with previous NuSTAR observations of Fermi-detected classical novae (V5855 Sgr and V906 Car), the 3.5-78 keV X-ray emission is found to be two orders of magnitude fainter than the GeV emission. The X-ray emission observed by NuSTAR is consistent with a single-temperature thermal plasma. We detect no non-thermal tail of the GeV emission expected to extend down to the NuSTAR band. NuSTAR observations continue to challenge theories of high-energy emission from shocks in novae.

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arXiv2022

An Exceptional Dimming Event for a Massive, Cool Supergiant in M51

We present the discovery of an exceptional dimming event in a cool supergiant star in the Local Volume spiral M51. The star, dubbed M51-DS1, was found as part of a Hubble Space Telescope (HST) search for failed supernovae (SNe). The supergiant, which is plausibly associated with a very young ($\lesssim6$ Myr) stellar population, showed clear variability (amplitude $ΔF814W\approx0.7$ mag) in numerous HST images obtained between 1995 and 2016, before suddenly dimming by $>$2 mag in $F814W$ sometime between late 2017 and mid-2019. In follow-up data from 2021, the star rebrightened, ruling out a failed supernova. Prior to its near-disappearance, the star was luminous and red ($M_{F814W}\lesssim-7.6$ mag, $F606W-F814W=1.9$ - $2.2$ mag). Modeling of the pre-dimming spectral energy distribution of the star favors a highly reddened, very luminous ($\log[L/L_{\odot}] = 5.4$ - $5.7$) star with $T_{\mathrm{eff}}\approx3700$ - $4700$ K, indicative of a cool yellow or post-red supergiant (RSG) with an initial mass of $\approx26$ - $40$ $M_{\odot}$. However, the local interstellar extinction and circumstellar extinction are uncertain, and could be lower: the near-IR colors are consistent with an RSG, which would be cooler ($T_{\mathrm{eff}}\lesssim3700$ K) and slightly less luminous ($\log[L/L_{\odot}] = 5.2$ - $5.3$), giving an inferred initial mass of $\approx19$ - $22$ $M_{\odot}$. In either case, the dimming may be explained by a rare episode of enhanced mass loss that temporarily obscures the star, potentially a more extreme counterpart to the 2019 - 2020 "Great Dimming" of Betelgeuse. Given the emerging evidence that massive evolved stars commonly exhibit variability that can mimic a disappearing star, our work highlights a substantial challenge in identifying true failed SNe.

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arXiv2022

The MAVERIC survey: A catalogue of radio sources in southern globular clusters from the Australia Telescope Compact Array

Radio continuum observations offer a new window on compact objects in globular clusters compared to typical X-ray or optical studies. As part of the MAVERIC survey, we have used the Australia Telescope Compact Array to carry out a deep (median central noise level of approximately 4 microJy per beam) radio continuum survey of 26 southern globular clusters at central frequencies of 5.5 and 9.0 GHz. This paper presents a catalogue of 1285 radio continuum sources in the fields of these 26 clusters. Considering the surface density of background sources, we find significant evidence for a population of radio sources in seven of the 26 clusters, and also identify at least 11 previously known compact objects (6 pulsars and 5 X-ray binaries). While the overall density of radio continuum sources with 7.25-GHz flux densities greater than about 20 microJy in typical globular clusters is relatively low, the survey has already led to the discovery of several exciting compact binaries, including a candidate ultracompact black hole X-ray binary in 47 Tuc. Many of the unclassified radio sources near the centres of the clusters are likely to be true cluster sources, and multi-wavelength follow-up will be necessary to classify these objects and better understand the demographics of accreting compact binaries in globular clusters.

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arXiv2022

The MAVERIC Survey: Radio catalogs and source counts from deep Very Large Array imaging of 25 Galactic globular clusters

The MAVERIC survey is the first deep radio continuum imaging survey of Milky Way globular clusters, with a central goal of finding and classifying accreting compact binaries, including stellar-mass black holes. Here we present radio source catalogs for 25 clusters with ultra-deep Karl G. Jansky Very Large Array observations. The median observing time was 10 hr per cluster, resulting in typical rms sensitivities of 2.3 and 2.1 uJy per beam at central frequencies of 5.0 and 7.2 GHz, respectively. We detect nearly 1300 sources in our survey at 5 sigma, and while many of these are likely to be background sources, we also find strong evidence for an excess of radio sources in some clusters. The radio spectral index distribution of sources in the cluster cores differs from the background, and shows a bimodal distribution. We tentatively classify the steep-spectrum sources (those much brighter at 5.0 GHz) as millisecond pulsars and the flat-spectrum sources as compact or other kinds of binaries. These provisional classifications will be solidified with the future addition of X-ray and optical data. The outer regions of our images represent a deep, relatively wide field (~ 0.4/sq. deg) and high resolution C band background survey, and we present source counts calculated for this area. We also release radio continuum images for these 25 clusters to the community.

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arXiv2022

AGC 226178 and NGVS 3543: Two deceptive dwarfs towards Virgo

The two sources AGC 226178 and NGVS 3543, an extremely faint, clumpy, blue stellar system and a low surface brightness dwarf spheroidal, are adjacent systems in the direction of the Virgo cluster. Both have been studied in detail previously, with it being suggested that they are unrelated normal dwarf galaxies or that NGVS 3543 recently lost its gas through ram pressure stripping, and that AGC 226178 formed from this stripped gas. However, with HST ACS imaging we demonstrate that the stellar population of NGVS 3543 is inconsistent with being at the distance of the Virgo cluster, and that it is likely a foreground object at approximately 10 Mpc. Whereas the stellar population of AGC 226178 is consistent with it being a very young (10-100 Myr) object in the Virgo cluster. Through a re-analysis of the original ALFALFA HI detection we show that AGC 226178 likely formed from gas stripped from the nearby dwarf galaxy VCC 2034, a hypothesis strengthened by the high metallicity measured with MUSE VLT observations. However, it is unclear whether ram pressure or a tidal interaction is responsible for stripping the gas. AGC 226178 is one of at least five similar objects now known towards Virgo. These objects are all young and unlikely to remain visible for over ~500 Myr, suggesting that they are continually produced in the cluster.

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arXiv2022

A population of luminous globular clusters and stripped nuclei with elevated mass to light ratios around NGC 5128

The dense central regions of tidally disrupted galaxies can survive as ultra-compact dwarfs (UCDs) that hide among the luminous globular clusters (GCs) in the halo of massive galaxies. An exciting confirmation of this model is the detection of overmassive black holes in the centers of some UCDs, which also lead to elevated dynamical mass-to-light ratios ($M/L_{dyn}$). Here we present new high-resolution spectroscopic observations of 321 luminous GC candidates in the massive galaxy NGC 5128/Centaurus A. Using these data we confirm 27 new luminous GCs, and measure velocity dispersions for 57 luminous GCs (with $g$-band luminosities between $2.5 \times 10^5$ and $2.5 \times 10^7 L_{\odot}$), of which 48 are new measurements. Combining these data with size measurements from Gaia, we determine the $M/L_{dyn}$ for all 57 luminous GCs. We see a clear bimodality in the $M/L_{dyn}$ distribution, with a population of normal GCs with mean $M/L_{dyn}=1.51\pm0.31$, and a second population of $\sim$20 GCs with elevated mean $M/L_{dyn}=2.68\pm0.22$. We show that black holes with masses $\sim4$-18 % of the luminous GCs can explain the elevated mass-to-light ratios. Hence, it is plausible that the NGC 5128 sources with elevated $M/L_{dyn}$ are mostly stripped galaxy nuclei that contain massive central black holes, though future high spatial resolution observations are necessary to confirm this hypothesis for individual sources. We also present a detailed discussion of an extreme outlier, \textit{VHH81-01}, one of the largest and most massive GC in NGC 5128, making it an exceptionally strong candidate to be a tidally stripped nucleus.

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arXiv2022

4FGL J1120.0-2204: A Unique Gamma-ray Bright Neutron Star Binary with an Extremely Low Mass Proto-White Dwarf

We have discovered a new X-ray emitting compact binary that is the likely counterpart to the unassociated Fermi-LAT GeV $γ$-ray source 4FGL J1120.0-2204, the second brightest Fermi source that still remains formally unidentified. Using optical spectroscopy with the SOAR telescope, we have identified a warm ($T_{\textrm{eff}}\sim8500$ K) companion in a 15.1-hr orbit around an unseen primary, which is likely a yet-undiscovered millisecond pulsar. A precise Gaia parallax shows the binary is nearby, at a distance of only $\sim 820$ pc. Unlike the typical "spider" or white dwarf secondaries in short-period millisecond pulsar binaries, our observations suggest the $\sim 0.17\,M_{\odot}$ companion is in an intermediate stage, contracting on the way to becoming an extremely low-mass helium white dwarf (a "pre-ELM" white dwarf). Although the companion is apparently unique among confirmed or candidate millisecond pulsar binaries, we use binary evolution models to show that in $\sim 2$ Gyr, the properties of the binary will match those of several millisecond pulsar-white dwarf binaries with very short ($< 1$ d) orbital periods. This makes 4FGL J1120.0-2204 the first system discovered in the penultimate phase of the millisecond pulsar recycling process.

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arXiv2022

The MAVERIC Survey: The first radio and X-ray limits on the detached black holes in NGC 3201

The Galactic globular cluster NGC 3201 is the first Galactic globular cluster observed to host dynamically-confirmed stellar-mass black holes, containing two confirmed and one candidate black hole. This result indicates that globular clusters can retain black holes, which has important implications for globular cluster evolution. NGC 3201 has been observed as part of the MAVERIC survey of Galactic globular clusters. We use these data to confirm that there is no radio or X-ray detection of the three black holes, and present the first radio and X-ray limits on these sources. These limits indicate that any accretion present is at an extremely low rate and may be extremely inefficient. In particular, for the system ACS ID #21859, by assuming the system is tidally locked and any accretion is through the capture of the companion's winds, we constrain the radiative efficiency of any accretion to $\leq1.5\times10^{-5}$. We also combine the radio and X-ray source catalogues from the MAVERIC survey with the existing MUSE spectroscopic surveys and the HUGS catalogue of NGC 3201 to provide a catalogue of 42 multiwavelength sources in this cluster. We identify a new red straggler source with X-ray emission, and investigate the multiwavelength properties of the sub-subgiant population in the cluster.

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arXiv2021

Transients from the Cataclysmic Deaths of Cataclysmic Variables

We explore the observational appearance of the merger of a low-mass star with a white dwarf (WD) binary companion. We are motivated by Schreiber et al. (2016), who found that multiple tensions between the observed properties of cataclysmic variables (CVs) and standard evolution models are resolved if a large fraction of CV binaries merge as a result of unstable mass transfer. Tidal disruption of the secondary forms a geometrically thick disk around the WD, which subsequently accretes at highly super-Eddington rates. Analytic estimates and numerical hydrodynamical simulations reveal that outflows from the accretion flow unbind a large fraction >~ 90% of the secondary at velocities ~500-1000 km/s within days of the merger. Hydrogen recombination in the expanding ejecta powers optical transient emission lasting about a month with a luminosity > 1e38 erg/s, similar to slow classical novae and luminous red novae from ordinary stellar mergers. Over longer timescales the mass accreted by the WD undergoes hydrogen shell burning, inflating the remnant into a giant of luminosity ~300-5000 L_sun, effective temperature T_eff ~ 3000 K and lifetime ~1e4-1e5 yr. We predict that ~1e3-1e4 Milky Way giants are CV merger products, potentially distinguishable by atypical surface abundances. We explore whether any Galactic historical slow classical novae are masquerading CV mergers by identifying four such post-nova systems with potential giant counterparts for which a CV merger origin cannot be ruled out. We address whether the historical transient CK Vul and its gaseous/dusty nebula resulted from a CV merger.

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arXiv2021

Detection of a $\sim$100,000 M$_\odot$ black hole in M31's most massive globular cluster: A tidally stripped nucleus

We investigate the presence of a central black hole (BH) in B023-G078, M31's most massive globular cluster. We present high-resolution, adaptive-optics assisted, integral-field spectroscopic kinematics from Gemini/NIFS that shows a strong rotation ($\sim$20 km/s) and a velocity dispersion rise towards the center (37 km/s). We combine the kinematic data with a mass model based on a two-component fit to $HST$ ACS/HRC data of the cluster to estimate the mass of a putative BH. Our dynamical modeling suggests a $>$3$σ$ detection of a BH component of 9.1$^{+2.6}_{-2.8}\times$10$^4$ M$_\odot$ (1$σ$ uncertainties). The inferred stellar mass of the cluster is 6.22$^{+0.03}_{-0.05}\times$10$^6$ M$_\odot$, consistent with previous estimates, thus the BH makes up 1.5% of its mass. We examine whether the observed kinematics are caused by a collection of stellar mass BHs by modeling an extended dark mass as a Plummer profile. The upper limit on the size scale of the extended mass is 0.56 pc (95% confidence), which does not rule out an extended mass. There is compelling evidence that B023-G078 is the tidally stripped nucleus of a galaxy with a stellar mass $>$10$^9$ M$_{\odot}$, including its high mass, two-component luminosity profile, color, metallicity gradient, and spread in metallicity. Given the emerging evidence that the central BH occupation fraction of $>$10$^9$ M$_{\odot}$ galaxies is high, the most plausible interpretation of the kinematic data is that B023-G078 hosts a central BH. This makes it the strongest BH detection in a lower mass ($<$10$^7$ M$_{\odot}$) stripped nucleus, and one of the few dynamically detected intermediate-mass BHs.

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arXiv2021

A Deep Search for Faint Chandra X-ray Sources, Radio Sources, and Optical Counterparts in NGC 6752

We report the results of a deep search for faint Chandra X-ray sources, radio sources, and optical counterparts in the nearby, core-collapsed globular cluster, NGC 6752. We combined new and archival Chandra imaging to detect 51 X-ray sources (12 of which are new) within the 1.9 arcmin half-light radius. Three radio sources in deep ATCA 5 and 9 GHz radio images match with Chandra sources. We have searched for optical identifications for the expanded Chandra source list using deep Hubble Space Telescope photometry in B, R, H-alpha, UV, and U. Among the entire sample of 51 Chandra sources, we identify 18 cataclysmic variables (CVs), 9 chromospherically active binaries (ABs), 3 red giants (RGs), 3 galaxies (GLXs), and 6 active galactic nuclei (AGNs). Three of the sources are associated with millisecond pulsars (MSPs). As in our previous study of NGC 6752, we find that the brightest CVs appear to be more centrally concentrated than are the faintest CVs, although the effect is no longer statistically significant as a consequence of the inclusion in the faint group of two intermediate brightness CVs. This possible difference in the radial distributions of the bright and faint CV groups appears to indicate that mass segregation has separated them. We note that photometric incompleteness in the crowded central region of the cluster may also play a role. Both groups of CVs have an inferred mass above that of the main-sequence turnoff stars. We discuss the implications for the masses of the CV components.

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arXiv2021

Ultraluminous X-ray Sources in Seven Edge-On Spiral Galaxies

We investigate a sample of seven edge-on spiral galaxies using Chandra observations. Edge-on spiral galaxies allow us to clearly separate source associated with their star-forming regions versus the outer edges of the system; offering a clear advantage over other systems. We uncover a number of X-ray point sources across these galaxies, and after eliminating contaminating foreground and background sources, we identify 12 candidate ultraluminous X-ray sources. All of these sources are projected onto the central regions, implying that the majority of ULXs in this sample of spiral galaxies are disk/bulge, and thus not halo sources. This also includes two transient ULXs, which may be long-duration transients and low mass X-ray binaries. This finding illustrates the need for further studies of transient ULXs.

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arXiv2021

Optimization of the Observing Cadence for the Rubin Observatory Legacy Survey of Space and Time: a pioneering process of community-focused experimental design

Vera C. Rubin Observatory is a ground-based astronomical facility under construction, a joint project of the National Science Foundation and the U.S. Department of Energy, designed to conduct a multi-purpose 10-year optical survey of the southern hemisphere sky: the Legacy Survey of Space and Time. Significant flexibility in survey strategy remains within the constraints imposed by the core science goals of probing dark energy and dark matter, cataloging the Solar System, exploring the transient optical sky, and mapping the Milky Way. The survey's massive data throughput will be transformational for many other astrophysics domains and Rubin's data access policy sets the stage for a huge potential users' community. To ensure that the survey science potential is maximized while serving as broad a community as possible, Rubin Observatory has involved the scientific community at large in the process of setting and refining the details of the observing strategy. The motivation, history, and decision-making process of this strategy optimization are detailed in this paper, giving context to the science-driven proposals and recommendations for the survey strategy included in this Focus Issue.

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arXiv2021

The Blue Supergiant Progenitor of the Supernova Imposter AT~2019krl

Extensive archival \textit{Hubble Space Telescope}, \textit{Spitzer Space Telescope}, and Large Binocular Telescope imaging of the recent intermediate-luminosity transient, AT~2019krl in M74, reveal a bright optical and mid-infrared progenitor star. While the optical peak of the event was missed, a peak was detected in the infrared with an absolute magnitude of $M_{4.5\,μ{\rm m}} = -18.4$ mag, leading us to infer a visual-wavelength peak absolute magnitude of $-$13.5 to $-$14.5. The pre-discovery light curve indicated no outbursts over the previous 16\,yr. The colors, magnitudes, and inferred temperatures of the progenitor best match a 13--14 M$_{\sun}$ yellow or blue supergiant (BSG), if only foreground extinction is taken into account, or a hotter and more massive star, if any additional local extinction is included. A pre-eruption spectrum of the star reveals strong H$α$ and [N~{\sc ii}] emission with wings extending to $\pm 2000$\,km\,s$^{-1}$. The post-eruption spectrum is fairly flat and featureless with only H$α$, \ion{Na}{1}~D, [\ion{Ca}{2}], and the \ion{Ca}{2} triplet in emission. As in many previous intermediate-luminosity transients, AT~2019krl shows remarkable observational similarities to luminous blue variable (LBV) giant eruptions, SN~2008S-like events, and massive-star mergers. However, the information about the pre-eruption star favors either a relatively unobscured BSG or a more extinguished LBV with $M > 20$\,M$_{\sun}$ likely viewed pole-on.

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arXiv2021

Hubble Space Telescope Observations of NGC 253 Dwarf Satellites: Discovery of Three Ultra-faint Dwarf Galaxies

We present deep Hubble Space Telescope imaging of five faint dwarf galaxies associated with the nearby spiral NGC 253 (D$\approx$3.5 Mpc). Three of these are newly discovered ultra-faint dwarf galaxies, while all five were found in the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), a Magellan$+$Megacam survey to identify faint dwarfs and other substructures in resolved stellar light around massive galaxies outside of the Local Group. Our HST data reach $\gtrsim$3 magnitudes below the tip of the red giant branch for each dwarf, allowing us to derive their distances, structural parameters, and luminosities. All five systems contain predominantly old, metal-poor stellar populations (age$\sim$12 Gyr, [M/H]$\lesssim$$-$1.5) and have sizes ($r_{h}$$\sim$110-3000 pc) and luminosities ($M_V$$\sim$$-7$ to $-12$ mag) largely consistent with Local Group dwarfs. The three new NGC 253 satellites are among the faintest systems discovered beyond the Local Group. We also use archival HI data to place limits on the gas content of our discoveries. Deep imaging surveys such as our program around NGC 253 promise to elucidate the faint end of the satellite luminosity function and its scatter across a range of galaxy masses, morphologies, and environments in the decade to come.

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arXiv2021

Classical Novae at Radio Wavelengths

We present radio observations (1--40 GHz) for 36 classical novae, representing data from over five decades compiled from the literature, telescope archives, and our own programs. Our targets display a striking diversity in their optical parameters (e.g., spanning optical fading timescales, t_2 = 1--263 days), and we find a similar diversity in the radio light curves. Using a brightness temperature analysis, we find that radio emission from novae is a mixture of thermal and synchrotron emission, with non-thermal emission observed at earlier times. We identify high brightness temperature emission (T_B > 5x10^4 K) as an indication of synchrotron emission in at least 9 (25%) of the novae. We find a class of synchrotron-dominated novae with mildly evolved companions, exemplified by V5589 Sgr and V392 Per, that appear to be a bridge between classical novae with dwarf companions and symbiotic binaries with giant companions. Four of the novae in our sample have two distinct radio maxima (the first dominated by synchrotron and the later by thermal emission), and in four cases the early synchrotron peak is temporally coincident with a dramatic dip in the optical light curve, hinting at a common site for particle acceleration and dust formation. We publish the light curves as tables and encourage use of these data by the broader community in multi-wavelength studies and modeling efforts.

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