arXiv subjects

L. E. Rivera Sandoval

Publications and source records attributed to L. E. Rivera Sandoval.

12 recordsLinked to original sources

arXiv2024

Variable radio emission of neutron star X-ray binary Ser X-1 during its persistent soft state

Ser X-1 is a low mass neutron star X-ray binary and has been persistently accreting since its discovery in the 1960s. It has always been observed to be in a soft spectral state and has never showed substantial long-term X-ray variability. Ser X-1 has one previous radio observation in the literature in which radio emission was detected during this soft state, which is contrary to the behavior of black hole X-ray binaries. We have recently obtained 10 randomly sampled radio epochs of Ser X-1 in order to further investigate its anomalous soft state radio emission. Out of 10 epochs, we find 8 non-detections and 2 detections at 10 GHz flux densities of 19.9 +/- 4.2 uJy and 32.2 +/- 3.6 uJy. We do not detect polarization in either epoch, ruling out very high polarization levels (< 63% and 34%). We compare these Ser X-1 results to other X-ray binaries and consider explanations for its long term variable radio behavior.

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arXiv2023

The Fast Evolving, Tremendous and Blue Superoutburst in ASASSN-21au Reveals a Dichotomy in the Outbursts of Long-period AM CVns

ASASSN-21au is an ultracompact accreting white dwarf binary (AM CVn) with a period of approx. 58 min. Using multiwavelength observations of the system, we discovered a dichotomy in the behavior of outbursts in AM CVns. The binary showed an initial brightness increase which lasted for at least 82 days, followed by an additional increase which lasted 2 weeks. Afterwards ASASSN-21au went into superoutburst with a total duration of 19 days, showing an amplitude with respect to quiescence of approx. 7.5 mags in g, with a precursor and an echo outburst. A correlation between X-rays, UV and optical was identified for the first time in an AM CVn during this stage. The color evolution of ASASSN-21au indicates that during the superoutburst the dominant component was the accretion disk. The short duration, large amplitude and color evolution of the superoutburst agree with expectations from the disk instability model. These characteristics are opposite to the ones observed in SDSS J080710+485259 and SDSS J113732+405458, which have periods of approx. 53 min and 60 min, respectively. The initially slow brightness increase in the light curve of ASASSN-21au and the behavior after the superoutburst favors a scenario in which changes in the mass-transfer rate led to disk instabilities, while the outburst mechanism of SDSS J080710+485259 and SDSS J113732+405458 has been attributed to enhanced mass-transfer alone. Further observations are needed to understand the origin of this dichotomy.

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arXiv2021

The outburst of a 60 min AM CVn reveals peculiar color evolution: implications for outbursts in long period double white dwarfs

We report on multi-wavelength observations during quiescence and of the first detected outburst of the ~60 min orbital period AM CVn SDSS J113732+405458. Using X-ray and UV observations we determined an upper limit duration of the event of about one year. The amplitude of the outburst was remarkably small, of around one magnitude in r and 0.5 magnitudes in g. We have also investigated the color variations of SDSS J113732+405458 and other long period AM CVns in outbursts and identified a track on the color-magnitude diagram that is not compatible with the predictions of the disk instability model, suggesting that some outbursts in long period AM CVns are caused by enhanced mass-transfer. To our knowledge, these are the first studies of the color evolution in AM CVns. During quiescence we measured an X-ray luminosity for SDSS J113732+405458 of ~3E29 erg/s in the 0.5-10 keV band. This indicates a very low accretion rate, in agreement with the disk instability model for long period systems. However, such a model predicts stable disks at somewhat long periods. The discovery of this system outburst, along with similarities to the long period system SDSS J080710+485259 with a comparably long, weak outburst, indicates that these enhanced mass-transfer events may be more common in long period AM CVns. A larger sample would be needed to determine empirically at what period, if any, the disk instability stops functioning entirely. Finally, we identified an infrared excess in the quiescence spectrum attributable to the donor. This makes SDSS J113732+405458 the second AM CVn to have a directly detected donor.

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arXiv2021

A 2 day orbital period for a redback millisecond pulsar candidate in the globular cluster NGC 6397

We report optical modulation of the companion to the X-ray source U18 in the globular cluster NGC 6397. U18, with combined evidence from radio and X-ray measurements, is a strong candidate as the second redback in this cluster, initially missed in pulsar searches. This object is a bright variable star with an anomalous red color and optical variability (\sim 0.2 mag in amplitude) with a periodicity \sim 1.96 days that can be interpreted as the orbital period. This value corresponds to the longest orbital period for known redback candidates and confirmed systems in Galactic globular clusters and one of the few with a period longer than 1 day.

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arXiv2020

The Swift Bulge Survey: Motivation, Strategy, and First X-ray Results

Very faint X-ray transients (VFXTs) are X-ray transients with peak X-ray luminosities ($L_X$) of $L_X\lesssim10^{36}$ erg/s, which are not well-understood. We carried out a survey of 16 square degrees of the Galactic Bulge with the Swift Observatory, using short (60 s) exposures, and returning every 2 weeks for 19 epochs in 2017-18 (with a gap from November 2017 to February 2018, when the Bulge was in sun-constraint). Our main goal was to detect and study VFXT behaviour in the Galactic Bulge across various classes of X-ray sources. In this work, we explain the observing strategy of the survey, compare our results with the expected number of source detections per class, and discuss the constraints from our survey on the Galactic VFXT population. We detected 91 X-ray sources, 25 of which have clearly varied by a factor of at least 10. 45 of these X-ray sources have known counterparts: 17 chromospherically active stars, 12 X-ray binaries, 5 cataclysmic variables (and 4 candidates), 3 symbiotic systems, 2 radio pulsars, 1 AGN, and a young star cluster. The other 46 are of previously undetermined nature. We utilize X-ray hardness ratios, searches for optical/infrared counterparts in published catalogs, and flux ratios from quiescence to outburst to constrain the nature of the unknown sources. Of these 46, 7 are newly discovered hard transients, which are likely VFXT X-ray binaries. Furthermore, we find strong new evidence for a symbiotic nature of 4 sources in our full sample, and new evidence for accretion power in 6 X-ray sources with optical counterparts. Our findings indicate that a large subset of VXFTs is likely made up of symbiotic systems.

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arXiv2020

A year long superoutburst from an ultracompact white dwarf binary reveals the importance of donor star irradiation

SDSS J080710+485259 is the longest period outbursting ultracompact white dwarf binary. Its first ever detected superoutburst started in November of 2018 and lasted for a year, the longest detected so far for any short orbital period accreting white dwarf. Here we show that the superoutburst duration of SDSS J080710+485259 exceeds the 2 months viscous time of its accretion disk by a factor of about 5. Consequently it follows neither the empirical relation nor the theoretical relation between the orbital period and the superoutburst duration for AM CVn systems. Six months after the end of the superoutburst the binary remained 0.4 mag brighter than its quiescent level before the superoutburst. We detect a variable X-ray behavior during the post-outburst cooling phase, demonstrating changes in the mass accretion rate. We discuss how irradiation of the donor star, a scenario poorly explored so far and which ultimately can have important consequences for AM CVns as gravitational wave sources, might explain the unusual observed features of the superoutburst.

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arXiv2020

The Swift Bulge Survey: optical and near-IR follow-up featuring a likely symbiotic X-ray binary & a focused wind CV

The nature of very faint X-ray transients (VFXTs) - transient X-ray sources that peak at luminosities $L_X\lesssim10^{36} {\rm erg s^{-1}}$ - is poorly understood. The faint and often short-lived outbursts make characterising VFXTs and their multi-wavelength counterparts difficult. In 2017 April we initiated the Swift Bulge Survey, a shallow X-ray survey of $\sim$16 square degrees around the Galactic centre with the Neil Gehrels Swift Observatory. The survey has been designed to detect new and known VFXTs, with follow-up programmes arranged to study their multi-wavelength counterparts. Here we detail the optical and near-infrared follow-up of four sources detected in the first year of the Swift Bulge Survey. The known neutron star binary IGR J17445-2747 has a K4III donor, indicating a potential symbiotic X-ray binary nature and the first such source to show X-ray bursts. We also find one nearby M-dwarf (1SXPS J174215.0-291453) and one system without a clear near-IR counterpart (Swift J175233.9-290952). Finally, 3XMM J174417.2-293944 has a subgiant donor, an 8.7 d orbital period, and a likely white dwarf accretor; we argue that this is the first detection of a white dwarf accreting from a gravitationally focused wind. A key finding of our follow-up campaign is that binaries containing (sub)giant stars may make a substantial contribution to the VFXT population.

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arXiv2019

X-ray Swift observations of SN 2018cow

Supernova (SN) 2018cow (or AT2018cow) is an optical transient detected in the galaxy CGCG 137-068. It has been classified as a SN due to various characteristics in its optical spectra. The transient is also a bright X-ray source. We present results of the analysis of ~62ks of X-ray observations taken with the Neil Gehrels Swift Observatory over 27 days. We found a variable behavior in the 0.3-10 keV X-ray light curve of SN 2018cow, with variability timescales of days. The observed X-ray variability could be due to the interaction between the SN ejecta and a non-uniform circumstellar medium, perhaps related to previous mass ejections from a luminous-blue-variable-like progenitor.

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arXiv2018

Swift X-ray and UV Observations of SDSS J141118.31+481257.6 During its First Ever Recorded Superoutburst

SDSS J141118.31+481257.6 is an ultracompact white dwarf binary (or AM CVn system) with an orbital period of 46 min. We analyze ~23 ks of X-ray and UV data taken with the Neil Gehrels Swift Observatory during its first ever recorded outbursts. The events took place 13 years after the system was discovered. We detected 3 events in our UV data, all with amplitudes of ~7 mags with respect to quiescence, the largests detected for an AM CVn system so far. The first 2 events correspond to a superoutburst and the third one to another detected outburst. The 3 episodes that we identified occurred in a period of 24 days, each one displaying very rapid brightness changes. At ~120 days since the detection of the superoutburst, the system remains 1 magnitude brighter in UV compared to the quiescence level. The X-ray observations suggest that the X-ray emission is not correlated with the UV.

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arXiv2018

Extreme Quiescent Variability of the Transient Neutron Star Low-mass X-ray Binary EXO 1745-248 in Terzan 5

EXO 1745-248 is a transient neutron-star low-mass X-ray binary that resides in the globular cluster Terzan 5. We studied the transient during its quiescent state using 18 Chandra observations of the cluster acquired between 2003 and 2016. We found an extremely variable source, with a luminosity variation in the 0.5-10 keV energy range of $\sim3$ orders of magnitude (between $3\times10^{31}$ erg s$^{-1}$ and $2\times10^{34}$ erg s$^{-1}$) on timescales from years down to only a few days. Using an absorbed power-law model to fit its quiescent spectra, we obtained a typical photon index of $\sim1.4$, indicating that the source is even harder than during outburst and much harder than typical quiescent neutron stars if their quiescent X-ray spectra are also described by a single power-law model. This indicates that EXO 1745-248 is very hard throughout the entire observed X-ray luminosity range. At the highest luminosity, the spectrum fits better when an additional (soft) component is added to the model. All these quiescent properties are likely related to strong variability in the low-level accretion rate in the system. However, its extreme variable behavior is strikingly different from the one observed for other neutron star transients that are thought to still accrete in quiescence. We compare our results to these systems. We also discuss similarities and differences between our target and the transitional millisecond pulsar IGR J18245-2452, which also has hard spectra and strong variability during quiescence.

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arXiv2018

Mid-UV Studies of the Transitional Millisecond Pulsars XSS J12270-4859 and PSR J1023+0038 During Their Radio Pulsar States

We report mid-UV (MUV) observations taken with HST/WFC3, Swift/UVOT and GALEX/NUV of the transitional millisecond pulsars XSS J12270-4859 and PSR J1023+0038 during their radio pulsar states. Both systems were detected in our images and showed MUV variability. At similar orbital phases, the MUV luminosities of both pulsars are comparable. This suggests that the emission processes involved in both objects are similar. We estimated limits on the mass ratio, companion's temperature, inclination, and distance to XSS J12270-4859 by using a Markov Chain Monte Carlo algorithm to fit published folded optical light curves. Using the resulting parameters, we modeled MUV light curves in our HST filters. The resulting models failed to fit our MUV observations. Fixing the mass ratio of XSS J12270-4859 to the value reported in other studies, we obtained a distance of ~3.2 kpc. This is larger than the one derived from radio measurements (~1.4 kpc). Assuming a uniform prior for the mass ratio, the distance is similar to that from radio measurements. However, it requires an undermassive companion (~0.01 M_sun). We conclude that a direct heating model alone can not fully explain the observations in optical and MUV. Therefore, an additional radiation source is needed. The source could be an intrabinary shock which contributes to the MUV flux and likely to the optical one as well. During the radio pulsar state, the MUV orbital variations of PSR J1023+0038 detected with GALEX, suggests the presence of an asymmetric intrabinary shock.

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arXiv2018

New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae

We present 22 new (+3 confirmed) cataclysmic variables (CVs) in the non core-collapsed globular cluster 47 Tucanae (47 Tuc). The total number of CVs in the cluster is now 43, the largest sample in any globular cluster so far. For the identifications we used near-ultraviolet (NUV) and optical images from the Hubble Space Telescope, in combination with X-ray results from the Chandra X-ray Observatory. This allowed us to build the deepest NUV CV luminosity function of the cluster to date. We found that the CVs in 47 Tuc are more concentrated towards the cluster center than the main sequence turnoff stars. We compared our results to the CV populations of the core-collapsed globular clusters NGC 6397 and NGC 6752. We found that 47 Tuc has fewer bright CVs per unit mass than those two other clusters. That suggests that dynamical interactions in core-collapsed clusters play a major role creating new CVs. In 47 Tuc, the CV population is probably dominated by primordial and old dynamically formed systems. We estimated that the CVs in 47 Tuc have total masses of approx. 1.4 M_sun. We also found that the X-ray luminosity function of the CVs in the three clusters is bimodal. Additionally, we discuss a possible double degenerate system and an intriguing/unclassified object. Finally, we present four systems that could be millisecond pulsar companions given their X-ray and NUV/optical colors. For one of them we present very strong evidence for being an ablated companion. The other three could be CO- or He-WDs.

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