First ultraviolet outburst detected from ASASSN-18eh strengthens its interpretation as a cataclysmic variable

As part of the Transient UV Objects project, we have discovered a new outburst (at the beginning of October 2020) of the candidate cataclysmic variable (CV) ASASSN-18eh using the UV/Optical Telescope aboard the Neil Gehrels Swift Observatory. During the outburst its brightness increased by about 6 mag in UV compared to its brightness in the quiescent state. The properties of this outburst are consistent with it being a dwarf nova, strongly supporting the CV nature of ASASSN-18eh

Radial velocities from Gaia BP/RP spectra

The Gaia mission has provided us full astrometric solutions for over $1.5$B sources. However, only the brightest 34M of those have radial velocity measurements. As a proof of concept, this paper aims to close that gap, by obtaining radial velocity estimates from the low-resolution BP/RP spectra that Gaia now provides. These spectra are currently published for about 220M sources, with this number increasing to the full $\sim 2$B Gaia sources with Gaia Data Release 4. To obtain the radial velocity measurements, we fit Gaia BP/RP spectra with models based on a grid of synthetic spectra, with which we obtain the posterior probability on the radial velocity for each object. Our measured velocities show systematic biases that depend mainly on colours and magnitudes of stars. We correct for these effects by using external catalogues of radial velocity measurements. We present in this work a catalogue of about $6.4$M sources with our most reliable radial velocity measurements and uncertainties $<300$ km s$^{-1}$ obtained from the BP/RP spectra. About 23% of these have no previous radial velocity measurement in Gaia RVS. Furthermore, we provide an extended catalogue containing all 125M sources for which we were able to obtain radial velocity measurements. The latter catalogue, however, also contains a fraction of measurements for which the reported radial velocities and uncertainties are inaccurate. Although typical uncertainties in the catalogue are significantly higher compared to those obtained with precision spectroscopy instruments, the number of potential sources for which this method can be applied is orders of magnitude higher than any previous radial velocity catalogue. Further development of the analysis could therefore prove extremely valuable in our understanding of Galactic dynamics.Comment: 14 pages, 17 figures, submitted to A&A, comments welcom

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.Comment: This White Paper will be updated when required (e.g. new authors joining, editing of content). Most recent update: 24 Oct 202

Gaia XP radial velocity catalogues

&lt;p&gt;Here we provide the radial velocities we measured from the Gaia XP spectra.&lt;/p&gt

: A pipeline to search for UV transients with

Despite the prevalence of transient-searching facilities operating across most wavelengths, the ultraviolet (UV) transient sky remains to be systematically studied. Therefore, we recently initiated the Transient Ultraviolet Objects (TUVO) project, with which we search for serendipitous UV transients in data obtained using currently available UV instruments with a strong focus on the UV and Optical (UVOT) telescope aboard the Neil Gehrels Swift Observatory (an overview of the project is described in a companion paper). Here, we describe the pipeline (named TUVOpip

TUVO-21acq: A new cataclysmic variable discovered through a UV outburst

Outbursts from cataclysmic variables, such as dwarf novae (DNe), are prevalent throughout the galaxy and are known to emit strongly in the ultraviolet (UV). However, the UV emission of DN outbursts has not been studied extensively compared with the optical. Detailed characterisation of the physical processes responsible for outburst behaviour requires further UV data, because the UV probes the inner regions of the accretion disc. Here we report, as part of our recently launched Transient UV Objects (TUVO) project, the discovery of TUVO-21acq, a new transient which we detected in the UV using data from the Ultraviolet Optical Telescope (UVOT) aboard Swift. We detected two separate outbursts and used the UVOT data to constrain source properties, focusing on the amplitudes and timescales of the outbursts. We found that during the first outburst the source increased in brightness by at least 4.1, 2.4, and 3.5 mag and during the second outburst by 4.4, 3.4, and 3.6 mag in the UVW1, UVM2, and UVW2 bands, respectively. The outburst durations were in the range of 6–21 days and 11–46 days, and we determined an upper limit for the recurrence time of 316 days. To further characterise the source, we obtained an optical spectrum during quiescence with the Southern African Large Telescope (SALT). The spectrum exhibited hydrogen Balmer series and helium emission lines, and a flat overall spectral shape. We confirm the nature of the source as an accreting white dwarf which underwent DN outburst based on photometric and spectroscopic properties. This study serves as a proof of concept for the TUVO project strategy, demonstrating that it has the capability of discovering and classifying new, interesting UV transients. We also discuss the implications of our findings for our understanding of the physics underlying DN outbursts, in particular with respect to the UV emission. We examine the need for simultaneous UV and optical observations during the onset of DN outbursts in order to help answer remaining questions as to the characteristics and implications of the UV delay, for example