TypeLMXB/XNClick for lightcurveLMXB are Low Mass X-ray Binaries, systems where one of the components is either a black hole or a neutron star. The other, donor, component usually fills its Roche lobe and therefore transfers mass to the compact object. The donor can be a normal dwarf, a white dwarf, or an evolved star (red giant). X-rays are emitted as the mass falls onto the compact object or onto an accretion disk that's surrounding it. The X-ray emission is incident upon the atmosphere of the cooler companion of the compact object and is reradiated in the form of optical high-temperature radiation (reflection effect), thus making that area of the cooler companion's surface an earlier spectral type. These effects lead to quite a peculiar complex character of optical variability in such systems. In X-ray binaries, the subtypes indicate which kind of behavior the binary displays, like X-ray bursts (XB), large amplitude outbursts also in the visual (XN), reflection effect (XR) or it may also inform about the object's nature, e.g.: if the compact object is a pulsar (XP).XN are X-ray systems that occasionally rapidly increase in brightness by 1-9 mag. in V simultaneously with the X-ray range.AAVSO Alert Notice 575 - 28 April 2017Dr. Gregory Sivakoff (University of Alberta) has requested AAVSO observers' assistance in monitoring Swift J1357.2-0933 (CRTS J135716.8-093238) during its current outburst.Dr. Sivakoff writes: "The black hole X-ray binary Swift J1357.2-0933 / CRTS J135716.8-093238 is in outburst. This is a rare black hole X-ray binary that is at high Galactic latitude. This means that extinction is relatively small and the bright blue nature of the outburst can be observed readily by citizen astronomers as the source fades into quiescence on a timescale of a few months. AAVSO observations will be critical in complementing multiple multi-wavelength campaigns observing this outburst. The combination of X-ray, UV, and optical light curves will be used to constrain the physics that explains why these sources go into outburst (the irradiated disk instability model). In addition, astronomers will be seeking to establish the flux of this source across the entire electromagnetic spectrum on a few occasions (for which we will request special coordination with AAVSO). This too will constrain theoretical models of accretion disks. In addition, this source is known to undergo recurring rapid dips. These dips can last for 10s of seconds, and recur on timescales of a few minutes. This is a great source for AAVSO observers, particularly CCD observers to follow."From now until the object is no longer observable with your equipment (quiescence is r/i~20, V~22.3), UBV photometry is requested in the form of nightly snapshot observations (once to a few times per night). However, more frequent observations are also welcome. Dr. Sivakoff writes: "[The recurring rapid dips]...might be interesting for some observers to go after. These dips can be on the order of a minute long. Observers wishing to probe that should go for as fast observations as the telescope allows for them to get a SNR of ~10-20 in their telescope. To capture the longer term evolution, an hourly cadence would be wonderful, where people can get it."Regarding priority of filters, he writes that "U B V places a priority on the blue filters, which are typically harder to get at higher Galactic extinction (lower Galactic latitude). That being said, I would definitely request that some observers get V data to connect with the daily SMARTS campaign (V I J K). As for other filters, "if people can get down to I ~ 17 - 18, then...Ic is good for connecting with the SMARTS campaign and LCOGT work on this source." The source may be too faint for Rc or unfiltered observations.Precision of the photometry is requested to be at least 0.05 to 0.1 magnitude if possible, with allowance for it to go down to 0.2 mag as the source becomes fainter. A S/N of 10-20, and 5 at a minimum, is requested.Please submit observations to the AAVSO International Database using the name SWIFT J1357.2-0933.