M51 ULX-7: superorbital periodicity and constraints on the neutron star magnetic field

Vasilopoulos, Georgios; Lander, S. K.; Koliopanos, Filippos; Bailyn, Charles D.

doi:10.1093/mnras/stz3298

Cited by 55 publications

(83 citation statements)

References 94 publications

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“…We assume a distance of 8.58 Mpc to M51 throughout this study (McQuinn et al 2016). During the preparation of this manuscript, Vasilopoulos et al (2020) was published presenting results similar to ours regarding ULX7 based on the publicly available Swift data. Our analyses differ, however, since they use the online tool described in Evans et al (2009), to calculate the X-ray Telescope (XRT) count rates, whereas we use our own tailored analysis method.…”

Section: Introductionmentioning

confidence: 81%

“…This leads to the possibility that identifying periodic flux modulations in ULXs where the accretor is unknown can be used as strong evidence that it is powered by a NS. During the preparation of this manuscript, Vasilopoulos et al (2020) also presented the identification of the 38 day superorbital period of M51 ULX7 from the same Swift data we have presented here.…”

Section: A 38 Day Superorbital Flux Modulation From Ulx7mentioning

confidence: 92%

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Swift Monitoring of M51: A 38 day Superorbital Period for the Pulsar ULX7 and a New Transient Ultraluminous X-Ray Source

Brightman

Earnshaw

Fürst

et al. 2020

ApJ

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We present the results from a monitoring campaign made with the Neil Gehrels Swift Observatory of the M51 galaxies, which contain several variable ultraluminous X-ray sources (ULXs). The ongoing campaign started in 2018 May, and we report here on ∼1.5 yr of observations. The campaign, which consists of 106 observations, has a typical cadence of 3-6 days, and has the goal of determining the long-term X-ray variability of the ULXs. Two of the most variable sources were ULX7 and ULX8, both of which are known to be powered by neutron stars that are exceeding their isotropic Eddington luminosities by factors of up to 100. This is further evidence that neutron-starpowered ULXs are the most variable. Our two main results are, first, that ULX7 exhibits a periodic flux modulation with a period of 38 days varying over a magnitude and a half in flux from peak to trough. Since the orbital period of the system is known to be 2 days, the modulation is superorbital, which is a near-ubiquitous property of ULX pulsars. Second, we identify a new transient ULX, M51 XT-1, the onset of which occurred during our campaign, reaching a peak luminosity of ∼10 40 erg s −1 , before gradually fading over the next ∼200 days until it slipped below the detection limit of our observations. Combined with the high-quality Swift/X-ray Telescope lightcurve of the transient, serendipitous observations made with Chandra and XMM-Newton provide insights into the onset and evolution of a likely super-Eddington event. Unified Astronomy Thesaurus concepts: X-ray point sources (1270); X-ray transient sources (1852); X-ray photometry (1820); X-ray bright point (1812); X-ray active galactic nuclei (2035); Transient sources (1851); Light curves (918); Galaxy nuclei (609); Active galactic nuclei (16)

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Section: Introductionmentioning

confidence: 81%

Section: A 38 Day Superorbital Flux Modulation From Ulx7mentioning

confidence: 92%

Swift Monitoring of M51: A 38 day Superorbital Period for the Pulsar ULX7 and a New Transient Ultraluminous X-Ray Source

Brightman

Earnshaw

Fürst

et al. 2020

ApJ

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“…While such an explanation would potentially need ULX3 to have an extremely long X-ray period, given the apparent stability of the low-flux state across many years prior to our recent observations, it is also interesting to note that the long-time-scale Xray periods in ULXs are themselves most robustly seen in the known ULX pulsar systems (e.g. Walton et al 2016a;Hu et al 2017;Fürst et al 2018;Brightman et al 2019Brightman et al , 2020Vasilopoulos et al 2020a). Although we do not have any significant detection of X-ray pulsations from ULX3, the current limits are only mildly constraining, and the pulsations are seen to be transient in a number of the known ULX pulsars (e.g.…”

Section: Discussion a N D C O N C L U S I O N Smentioning

confidence: 89%

“…Most notably, obscuration of the inner accretion flow by its outer regions/winds has also been invoked to explain high-amplitude variability events in some cases (e.g. Vasilopoulos et al 2019), particularly where long-time-scale ( 10s of days) super-orbital Xray periods with large variability amplitudes are present (Brightman et al 2019(Brightman et al , 2020Vasilopoulos et al 2020a; see also Middleton et al 2015Middleton et al , 2018. While such an explanation would potentially need ULX3 to have an extremely long X-ray period, given the apparent stability of the low-flux state across many years prior to our recent observations, it is also interesting to note that the long-time-scale Xray periods in ULXs are themselves most robustly seen in the known ULX pulsar systems (e.g.…”

Section: Discussion a N D C O N C L U S I O N Smentioning

confidence: 99%

A new transient ultraluminous X-ray source in NGC 7090

Walton

Heida

Bachetti

et al. 2020

Monthly Notices of the Royal Astronomical Society

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We report on the discovery of a new, transient ultraluminous X-ray source (ULX) in the galaxy NGC 7090. This new ULX, which we refer to as NGC 7090 ULX3, was discovered via monitoring with Swift during 2019–20, and to date has exhibited a peak luminosity of LX ∼ 6 × 1039 erg s−1. Archival searches show that, prior to its recent transition into the ULX regime, ULX3 appeared to exhibit a fairly stable luminosity of LX ∼ 1038 erg s−1. Such strong long-timescale variability may be reminiscent of the small population of known ULX pulsars, although deep follow-up observations with XMM-Newton and NuSTAR do not reveal any robust X-ray pulsation signals. Pulsations similar to those seen from known ULX pulsars cannot be completely excluded, however, as the limit on the pulsed fraction of any signal that remains undetected in these data is ≲20%. The broadband spectrum from these observations is well modelled with a simple thin disc model, consistent with sub-Eddington accretion, which may instead imply a moderately large black hole accretor (MBH ∼ 40 M⊙). Similarly, though, more complex models consistent with the super-Eddington spectra seen in other ULXs (and the known ULX pulsars) cannot be excluded given the limited signal-to-noise of the available broadband data. The nature of the accretor powering this new ULX therefore remains uncertain.

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“…Motivated by their work and using the products of their analysis, we studied the variability of M51 ULX-7, the only ULXP with an orbit that can be continuously monitored by X-ray observatories (Townsend & Charles 2020). M51 ULX-7 is a ULXP (Rodríguez Castillo et al 2020) hosting an NS rotating with a spin period of ∼2.8 s. The binary period is 1.9969 days, while Swift/X-ray Telescope (XRT) monitoring revealed the presence of a superorbital modulation with a period of ∼38-39 days (Brightman et al 2020;Vasilopoulos et al 2020a). Here we report on the discovery of an irregular off-state within the superorbital cycle of M51 ULX-7 and the discovery of periodic dips in the X-ray light curve computed from archival Chandra data.…”

Section: Introductionmentioning

confidence: 99%

Chandra Probes the X-Ray Variability of M51 ULX-7: Evidence of Propeller Transition and X-Ray Dips on Orbital Periods

Vasilopoulos

Koliopanos

Haberl

et al. 2021

ApJ

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We report on the temporal properties of the ultraluminous X-ray (ULX) pulsar M51 ULX-7 inferred from the analysis of the 2018-2020 Swift/X-ray Telescope monitoring data and archival Chandra data obtained over a period of 33 days in 2012. We find an extended low flux state, which might be indicative of propeller transition, lending further support to the interpretation that the neutron star is rotating near equilibrium. Alternatively, this offstate could be related to a variable superorbital period. Moreover, we report the discovery of periodic dips in the X-ray light curve that are associated with the binary orbital period. The presence of the dips implies a configuration where the orbital plane of the binary is closer to an edge-on orientation, and thus demonstrates that favorable geometries are not necessary in order to observe ULX pulsars. These characteristics are similar to those seen in prototypical X-ray pulsars such as Her X-1 and SMC X-1 or other ULX pulsars such as NGC 5907 ULX1.

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M51 ULX-7: superorbital periodicity and constraints on the neutron star magnetic field

Cited by 55 publications

References 94 publications

Swift Monitoring of M51: A 38 day Superorbital Period for the Pulsar ULX7 and a New Transient Ultraluminous X-Ray Source

Swift Monitoring of M51: A 38 day Superorbital Period for the Pulsar ULX7 and a New Transient Ultraluminous X-Ray Source

A new transient ultraluminous X-ray source in NGC 7090

Chandra Probes the X-Ray Variability of M51 ULX-7: Evidence of Propeller Transition and X-Ray Dips on Orbital Periods

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