Context. The environment of Sagittarius A* (Sgr A*), the central black hole of the Milky Way, is the only place in the Universe where we can currently study the interaction between a nuclear star cluster and a massive black hole and infer the properties of a nuclear cluster from observations of individual stars. Aims. This work aims to explore the star formation history of the nuclear cluster and the structure of the innermost stellar cusp around Sgr A*. Methods. We combined and analysed multi epoch high quality AO observations. For the region close to Sgr A* we apply the speckle holography technique to the AO data and obtain images that are ≥50% complete down to Ks ≈ 19 within a projected radius of 5″ around Sgr A*. We used H-band images to derive extinction maps. Results. We provide Ks photometry for roughly 39 000 stars and H-band photometry for ∼11 000 stars within a field of about 40″ × 40″, centred on Sgr A*. In addition, we provide Ks photometry of ∼3000 stars in a very deep central field of 10″ × 10″, centred on Sgr A*. We find that the Ks luminosity function (KLF) is rather homogeneous within the studied field and does not show any significant changes as a function of distance from the central black hole on scales of a few 0.1 pc. By fitting theoretical luminosity functions to the KLF, we derive the star formation history of the nuclear star cluster. We find that about 80% of the original star formation took place 10 Gyr ago or longer, followed by a largely quiescent phase that lasted for more than 5 Gyr. We clearly detect the presence of intermediate-age stars of about 3 Gyr in age. This event makes up about 15% of the originally formed stellar mass of the cluster. A few percent of the stellar mass formed in the past few 100 Myr. Our results appear to be inconsistent with a quasi-continuous star formation history. The mean metallicity of the stars is consistent with being slightly super solar. The stellar density increases exponentially towards Sgr A* at all magnitudes between Ks = 15−19. We also show that the precise properties of the stellar cusp around Sgr A* are hard to determine because the star formation history suggests that the star counts can be significantly contaminated, at all magnitudes, by stars that are too young to be dynamically relaxed. We find that the probability of observing any young (non-millisecond) pulsar in a tight orbit around Sgr A* and beamed towards Earth is very low. We argue that typical globular clusters, such as they are observed in and around the Milky Way today, have probably not contributed to the nuclear cluster’s mass in any significant way. The nuclear cluster may have formed following major merger events in the early history of the Milky Way.

Multi-messenger astronomy received a great boost following the discovery of kilonova AT2017gfo, the optical counterpart of the gravitational wave source GW170817 associated with the short Gamma Ray Burst GRB 170817A. AT2017gfo was the first kilonova that could be extensively monitored in time both photometrically and spectroscopically. Previously, only few candidates have been observed against the glare of short GRB afterglows. In this work, we aim to search the fingerprints of AT2017gfo-like kilonova emissions in the optical/NIR light curves of 39 short GRBs with known redshift. For the first time, our results allow us to study separately the range of luminosities of the blue and red components of AT2017gfo-like kilonovae in short GRBs. In particular, the red component can be up to 3 times brighter than AT2017gfo, while the blue kilonova can be more than 10 times and possibly 100 times brighter. We find further evidence to support all the claimed kilonova detections and also find four new bluekilonova candidates, namely GRBs 061210, 051221A, 080905A, and 090515 although the first two are suspiciously bright and the redshift of last two is not well defined. Finally, we exclude an AT2017gfo-like kilonova in GRBs 050509B, 061201, and 100206A.

Abstract. We compare simulations from three high-top (with upper lid above 120 km) and five medium-top (with upper lid around 80 km) atmospheric models with observations of odd nitrogen (NO x = NO + NO 2 ), temperature, and carbon monoxide from seven satellite instruments (ACE-FTS on SciSat, GOMOS, MIPAS, and SCIAMACHY on Envisat, MLS on Aura, SABER on TIMED, and SMR on Odin) dur- Larger discrepancies of a few model simulations could be traced back either to the impact of the models' gravity wave drag scheme on the polar wintertime meridional circulation or to a combination of prescribed NO x mixing ratio at the uppermost model layer and low vertical resolution. In March-April, after the ES event, however, modelled mesospheric and stratospheric NO x distributions deviate significantly from the observations. The too-fast and early downward propagation of the NO x tongue, encountered in most simulations, coincides with a temperature high bias in the lower mesosphere (0.2-0.05 hPa), likely caused by an overestimation of descent velocities. In contrast, uppermesospheric temperatures (at 0.05-0.001 hPa) are generally underestimated by the high-top models after the onset of the ES event, being indicative for too-slow descent and hence too-low NO x fluxes. As a consequence, the magnitude of the simulated NO x tongue is generally underestimated by these models. Descending NO x amounts simulated with mediumtop models are on average closer to the observations but show a large spread of up to several hundred percent. This is primarily attributed to the different vertical model domains in which the NO x upper boundary condition is applied. In general, the intercomparison demonstrates the ability of stateof-the-art atmospheric models to reproduce the EPP indirect effect in dynamically and geomagnetically quiescent NH winter conditions. The encountered differences between observed and simulated NO x , CO, and temperature distributions during the perturbed phase of the 2009 NH winter, however, emphasize the need for model improvements in the dynamical representation of elevated stratopause events in order to allow for a better description of the EPP indirect effect under these particular conditions.

Abstract. We present vM21 MIPAS temperatures from the lower stratosphere to the lower thermosphere, which cover all optimized resolution measurements performed by MIPAS in the middle-atmosphere, upper-atmosphere and noctilucent-cloud modes during its lifetime, i.e., from January 2005 to April 2012. The main upgrades with respect to the previous version of MIPAS temperatures (vM11) are the update of the spectroscopic database, the use of a different climatology of atomic oxygen and carbon dioxide, and the improvement in important technical aspects of the retrieval setup (temperature gradient along the line of sight and offset regularizations, apodization accuracy). Additionally, an updated version of ESA-calibrated L1b spectra (5.02/5.06) is used. The vM21 temperatures correct the main systematic errors of the previous version because they provide on average a 1–2 K warmer stratopause and middle mesosphere, and a 6–10 K colder mesopause (except in high-latitude summers) and lower thermosphere. These lead to a remarkable improvement in MIPAS comparisons with ACE-FTS, MLS, OSIRIS, SABER, SOFIE and the two Rayleigh lidars at Mauna Loa and Table Mountain, which, with a few specific exceptions, typically exhibit differences smaller than 1 K below 50 km and than 2 K at 50–80 km in spring, autumn and winter at all latitudes, and summer at low to midlatitudes. Differences in the high-latitude summers are typically smaller than 1 K below 50 km, smaller than 2 K at 50–65 km and 5 K at 65–80 km. Differences between MIPAS and the other instruments in the mid-mesosphere are generally negative. MIPAS mesopause is within 4 K of the other instruments measurements, except in the high-latitude summers, when it is within 5–10 K, being warmer there than SABER, MLS and OSIRIS and colder than ACE-FTS and SOFIE. The agreement in the lower thermosphere is typically better than 5 K, except for high latitudes during spring and summer, when MIPAS usually exhibits larger vertical gradients.

In early 2012, a strong sudden stratospheric warming (SSW) took place, accompanied by several medium-scale solar proton events (SPEs). Here we use a chemistry transport model (CTM) in order to assess the relative contributions of (1) intensified downward transport of odd nitrogen (NO x ) and (2) in situ production of NO x by protons, on stratospheric NO x and ozone during January-March 2012. The CTM is constrained by an upper boundary condition for reactive nitrogen (NO y ) species, based on satellite observations from Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board Envisat, and includes a new parameterization of the SPE-caused effects on NO y and odd hydrogen (HO x ) species. We found that the amount of NO x increases due to both transport and in situ production effects, the intensified descent of NO x dominating the middle and upper stratospheric impact. The model results indicate NO x enhancements of 120-3300% (5-48 ppbv) between 38 and 50 km, caused by the transport of mesosphere/lower thermosphere NO x down to the stratosphere following the SSW. The SPEs increase NO x by up to 820-1200% (14-21 ppbv) at 33 to 50 km. The effect on the stratospheric ozone is larger following the downward transport of NO x than during and after the SPEs. The model predicts ozone losses of up to 17% and 9% at around 40 km due to transport and SPE effects, respectively.