arXiv subjects

Sean N. Pike

Publications and source records attributed to Sean N. Pike.

25 records · Page 2Linked to original sources

arXiv2022

Discovery of a new supergiant fast X-ray transient MAXI J0709-159 associated with the Be star LY CMa

We report on the discovery of a new supergiant fast X-ray transient (SFXT), MAXI J0709$-$159, and its identification with LY CMa (also known as HD 54786). On 2022 January 25, a new flaring X-ray object named MAXI J0709$-$159, was detected by Monitor of All-sky X-ray Image (MAXI). Two flaring activities were observed in the two scans of $\sim 3$ hours apart, where the 2-10 keV flux reached $5\times 10^{-9}$ erg cm$^{-2}$ s$^{-1}$. During the period, the source exhibited a large spectral change suggesting that the absorption column density $N_\mathrm{H}$ increased from $10^{22}$ cm$^{-2}$ to $10^{23}$ cm$^{-2}$. NuSTAR follow-up observation on January 29 identified a new X-ray source with a flux of $6\times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$ at the position consistent with LY CMa, which has been identified as B supergiant as well as Be star, located at the 3 kpc distance. The observed X-ray activity characterized by the short ($\lesssim$ several hours) duration, the rapid ($\lesssim$ a few seconds) variabilities accompanied with spectral changes, and the large luminosity swing ($10^{32}$-$10^{37}$ erg s$^{-1}$) agree with those of SFXT. On the other hand, optical spectroscopic observations of LY CMa revealed a broad $Hα$ emission line, which may indicate the existence of a Be circumstellar disk. These obtained results suggest that the optical companion, LY CMa, certainly has a complex circumstellar medium including dense clumps.

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arXiv2022

MAXI and NuSTAR observations of the faint X-ray transient MAXI J1848-015 in the GLIMPSE-C01 Cluster

We present the results of MAXI monitoring and two NuSTAR observations of the recently discovered faint X-ray transient MAXI J1848-015. Analysis of the MAXI light-curve shows that the source underwent a rapid flux increase beginning on 2020 December 20, followed by a rapid decrease in flux after only $\sim5$ days. NuSTAR observations reveal that the source transitioned from a bright soft state with unabsorbed, bolometric ($0.1$-$100$ keV) flux $F=6.9 \pm 0.1 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$, to a low hard state with flux $F=2.85 \pm 0.04 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. Given a distance of $3.3$ kpc, inferred via association of the source with the GLIMPSE-C01 cluster, these fluxes correspond to an Eddington fraction of order $10^{-3}$ for an accreting neutron star of mass $M=1.4M_\odot$, or even lower for a more massive accretor. However, the source spectra exhibit strong relativistic reflection features, indicating the presence of an accretion disk which extends close to the accretor, for which we measure a high spin, $a=0.967\pm0.013$. In addition to a change in flux and spectral shape, we find evidence for other changes between the soft and hard states, including moderate disk truncation with the inner disk radius increasing from $R_\mathrm{in}\approx3\,R_\mathrm{g}$ to $R_\mathrm{in}\approx8\,R_\mathrm{g}$, narrow Fe emission whose centroid decreases from $6.8\pm0.1$ keV to $6.3 \pm 0.1$ keV, and an increase in low-frequency ($10^{-3}$-$10^{-1}$ Hz) variability. Due to the high spin we conclude that the source is likely to be a black hole rather than a neutron star, and we discuss physical interpretations of the low apparent luminosity as well as the narrow Fe emission.

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arXiv2021

Spectral and Timing Analysis of NuSTAR and Swift/XRT Observations of the X-Ray Transient MAXI J0637-430

We present results for the first observed outburst from the transient X-ray binary source MAXI J0637-430. This study is based on eight observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and six observations from the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT) collected from 2019 November 19 to 2020 April 26 as the 3-79 keV source flux declined from 8.2e-10 to 1.4e-12 erg/cm^2/s. We see the source transition from a soft state with a strong disk-blackbody component to a hard state dominated by a power-law or thermal Comptonization component. NuSTAR provides the first reported coverage of MAXI J0637-430 above 10 keV, and these broadband spectra show that a two-component model does not provide an adequate description of the soft state spectrum. As such, we test whether blackbody emission from the plunging region could explain the excess emission. As an alternative, we test a reflection model that includes a physical Comptonization continuum. Finally, we also test a spectral component based on reflection of a blackbody illumination spectrum, which can be interpreted as a simple approximation to the reflection produced by returning disk radiation due to the bending of light by the strong gravity of the black hole. We discuss the physical implications of each scenario and demonstrate the value of constraining the source distance.

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arXiv2021

A Comprehensive X-ray Report on AT2019wey

Here, we present MAXI, SWIFT, NICER, NuSTAR and Chandra observations of the X-ray transient AT2019wey (SRGA J043520.9+552226, SRGE J043523.3+552234). From spectral and timing analyses we classify it as a Galactic low-mass X-ray binary (LMXB) with a black hole (BH) or neutron star (NS) accretor. AT2019wey stayed in the low/hard state (LHS) from 2019 December to 2020 August 21, and the hard-intermediate state (HIMS) from 2020 August 21 to 2020 November. For the first six months of the LHS, AT2019wey had a flux of $\sim 1$ mCrab, and displayed a power-law X-ray spectrum with photon index $Γ= 1.8$. From 2020 June to August, it brightened to $\sim 20$ mCrab. Spectral features characteristic of relativistic reflection became prominent. On 2020 August 21, the source left the "hard line" on the rms--intensity diagram, and transitioned from LHS to HIMS. The thermal disk component became comparable to the power-law component. A low-frequency quasi-periodic oscillation (QPO) was observed. The QPO central frequency increased as the spectrum softened. No evidence of pulsation was detected. We are not able to decisively determine the nature of the accretor (BH or NS). However, the BH option is favored by the position of this source on the $Γ$--$L_{\rm X}$, $L_{\rm radio}$--$L_{\rm X}$, and $L_{\rm opt}$--$L_{\rm X}$ diagrams. We find the BH candidate XTE J1752-223 to be an analog of AT2019wey. Both systems display outbursts with long plateau phases in the hard states. We conclude by noting the potential of SRG in finding new members of this emerging class of low luminosity and long-duration LMXB outbursts.

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arXiv2021

Photospheric Radius Expansion and a double-peaked type-I X-ray burst from GRS 1741.9-2853

We present analysis of two type-I X-ray bursts observed by NuSTAR originating from the very faint transient neutron star low-mass X-ray binary GRS 1741.9-2853 during a period of outburst in May 2020. We show that the persistent emission can be modeled as an absorbed, Comptonized blackbody in addition to Fe K$α$ emission which can be attributed to relativistic disk reflection. We measure a persistent bolometric, unabsorbed luminosity of $L_{\mathrm{bol}}=7.03^{+0.04}_{-0.05}\times10^{36}\,\mathrm{erg\,s^{-1}}$, assuming a distance of 7 kpc, corresponding to an Eddington ratio of $4.5\%$. This persistent luminosity combined with light curve analysis leads us to infer that the bursts were the result of pure He burning rather than mixed H/He burning. Time-resolved spectroscopy reveals that the bolometric flux of the first burst exhibits a double-peaked structure, placing the source within a small population of accreting neutron stars which exhibit multiple-peaked type-I X-ray bursts. We find that the second, brighter burst shows evidence for photospheric radius expansion (PRE) and that at its peak, this PRE event had an unabsorbed bolometric flux of $F_{\mathrm{peak}}=2.94^{+0.28}_{-0.26}\times10^{-8}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. This yields a new distance estimate of $d=9.0\pm0.5$ kpc, assuming that this corresponds to the Eddington limit for pure He burning on the surface of a canonical neutron star. Additionally, we performed a detailed timing analysis which failed to find evidence for quasiperiodic oscillations or burst oscillations, and we place an upper limit of $16\%$ on the rms variability around 589 Hz, the frequency at which oscillations have previously been reported.

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arXiv2019

A broadband look at the old and new ULXs of NGC 6946

Two recent observations of the nearby galaxy NGC 6946 with NuSTAR, one simultaneous with an XMM-Newton observation, provide an opportunity to examine its population of bright accreting sources from a broadband perspective. We study the three known ultraluminous X-ray sources (ULXs) in the galaxy, and find that ULX-1 and ULX-2 have very steep power-law spectra with $Γ=3.6^{+0.4}_{-0.3}$ in both cases. Their properties are consistent with being super-Eddington accreting sources with the majority of their hard emission obscured and down-scattered. ULX-3 (NGC 6946 X-1) is significantly detected by both XMM-Newton and NuSTAR at $L_{\rm X}=(6.5\pm0.1)\times10^{39}$ erg s$^{-1}$, and has a power-law spectrum with $Γ=2.51\pm0.05$. We are unable to identify a high-energy break in its spectrum like that found in other ULXs, but the soft spectrum likely hinders our ability to detect one. We also characterise the new source, ULX-4, which is only detected in the joint XMM-Newton and NuSTAR observation, at $L_{\rm X}=(2.27\pm0.07)\times10^{39}$ erg s$^{-1}$, and is absent in a Chandra observation ten days later. It has a very hard cut-off power-law spectrum with $Γ=0.7\pm0.1$ and $E_{\rm cut}=11^{+9}_{-4}$ keV. We do not detect pulsations from ULX-4, but its transient nature can be explained either as a neutron star ULX briefly leaving the propeller regime or as a micro-tidal disruption event induced by a stellar-mass compact object.

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arXiv2019

Observing the transient pulsations of SMC X-1 with NuSTAR

We report on NuSTAR observations of transient pulsations in the neutron star X-ray binary SMC X-1. The transition from non-pulsing to pulsing states was not accompanied by a large change in flux. Instead, both pulsing and non-pulsing states were observed in a single observation during the low-flux super-orbital state. During the high-state, we measure a pulse period of $P = 0.70117(9)\,\mathrm{s}$ at $T_{ref} = 56145\,\mathrm{MJD}$. Spectral analysis during non-pulsing and pulsing states reveals that the observations can be consistently modeled by an absorbed power law with a phenomenological cutoff resembling a Fermi-Dirac distribution, or by a partially obscured cutoff power law. The shapes of the underlying continua show little variability between epochs, while the covering fraction and column density vary between super-orbital states. The strength of pulsations also varies, leading us to infer that the absence and reemergence of pulsations are related to changing obscuration, such as by a warped accretion disk. SMC X-1 is accreting near or above its Eddington limit, reaching an unabsorbed X-ray luminosity of $L_{\rm X}({\rm 2-10~keV}) \approx 5 \times 10^{38}\, {\rm erg}\, {\rm s}^{-1}$. This suggests that SMC X-1 may be a useful local analog to ultraluminous X-ray pulsars (ULXPs), which likewise exhibit strong variability in their pulsed fractions, as well as flux variability on similar timescales. In particular, the gradual pulse turn-on which has been observed in M82 X-2 is similar to the behavior we observe in SMC X-1. Thus we propose that pulse fraction variability of ULXPs may also be due to variable obscuration.

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