text: 129,62,1204826730734571520,308361234,Murray Brightman,"New paper on arXiv today! What we found when we asked @NASASwift to look at the M51 galaxies repeatedly over the last year and a half - <LINK> M51 is a pair of merging galaxies, each with an accreting supermassive black hole (SMBH) at its center, plus several ultraluminous X-ray sources (ULXs). We know 2 of these ULXs are powered by neutron stars, a million times less massive then the SMBHs! <LINK> We got great long-term X-ray lightcurves of all these sources <LINK> The X-ray lightcurve of one of these neutron star ULXs shows huge swings in brightness, on a period of 38 days. The orbit of the neutron star and its companion is known to be only 2 days, so this is a super-orbital modulation, possibly caused by an extreme disk precession. <LINK> We also found a new ULX, that appeared and disappeared during the year and a half. Maybe also a neutron star, but maybe something more exotic (red line shows the typical decline of a tidal disruption event...). Now looking for more of these in other Swift observations! <LINK> @AstroGnomie @NASASwift That wasn’t me :-) It is weird what’s happening with that source though!",https://arxiv.org/abs/1912.04431,"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. ","Swift monitoring of M51: A 38-day super-orbital period for the pulsar