Swift monitoring of M51: A 38-day super-orbital period for the pulsar ULX7 and a new transient ULX

TL;DR

This study used Swift observations over 1.5 years to discover a 38-day super-orbital period in ULX7 and identify a new transient ULX in M51, revealing key variability behaviors of neutron star ULXs.

Contribution

The paper reports the first detection of a super-orbital period in ULX7 and the discovery of a new transient ULX, providing new insights into ULX variability and super-Eddington phenomena.

Findings

ULX7 shows a 38-day super-orbital flux modulation.

A new transient ULX, M51 XT-1, was identified with a peak luminosity of ~10^40 erg/s.

The transient faded over approximately 200 days, indicating a super-Eddington event.

Abstract

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 May 2018, and we report here on $\sim1.5$ years of observations. The campaign, which consists of 105 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 star powered 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 super-orbital, which is a near-ubiquitous property of ULX pulsars. Secondly we identify a new transient ULX, M51 XT-1, the onset of which occurred during our campaign, reaching a peak luminosity of $\sim10^{40}$ erg s$^{-1}$, before gradually fading over the next $\sim200$ days until it slipped below the detection limit of our observations. Combined with the high-quality Swift/XRT 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.