25 research outputs found
Wind-Reprocessed Transients from Stellar-mass Black Hole Tidal Disruption Events
Tidal disruptions of stars by stellar-mass black holes are expected to occur
frequently in dense star clusters. Building upon previous studies that
performed hydrodynamic simulations of these encounters, we explore the
formation and long-term evolution of the thick, super-Eddington accretion disks
formed. We build a disk model that includes fallback of material from the tidal
disruption, accretion onto the black hole, and disk mass losses through winds
launched in association with the super-Eddington flow. We demonstrate that
bright transients are expected when radiation from the central engine powered
by accretion onto the black hole is reprocessed at large radii by the
optically-thick disk wind. By combining hydrodynamic simulations of these
disruption events with our disk+wind model, we compute light curves of these
wind-reprocessed transients for a wide range of stellar masses and encounter
penetration depths. We find typical peak bolometric luminosities of roughly
erg/s (depending mostly on accretion physics parameters) and
temperatures of roughly K, suggesting peak emission in the
ultraviolet/blue bands. We predict all-sky surveys such as the Vera Rubin
Observatory and ULTRASAT will detect up to thousands of these events per year
in dense star clusters out to distances of several Gpc.Comment: 16 Pages, 13 figures, 2 tables. Accepted for publication in MNRA
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Strong armies, slow adaptation: civil-military relations and diffusion of military power
Why are some states more willing to adopt military innovations than others? Why, for example, were the great powers of Europe able to successfully reform their military practices to better adapt to and participate in the so-called military revolution of the sixteenth and seventeenth centuries while their most important extra-European competitor, the Ottoman Empire, failed to do so? This puzzle is best explained by two factors: civil-military relations and historical timing. In the Ottoman Empire, the emergence of an institutionally strong and internally cohesive army during the early stages of state formation—in the late fourteenth century—equipped the military with substantial bargaining powers. In contrast, the great powers of Europe drew heavily on private providers of military power during the military revolution and developed similar armies only by the second half of the seventeenth century, limiting the bargaining leverage of European militaries over their rulers. In essence, the Ottoman standing army was able to block reform efforts that it believed challenged its parochial interests. Absent a similar institutional challenge, European rulers initiated military reforms and motivated officers and military entrepreneurs to participate in the ongoing military revolution
Double-Peaked Balmer Emission Indicating Prompt Accretion Disk Formation in an X-Ray Faint Tidal Disruption Event
We present the multi-wavelength analysis of the tidal disruption event (TDE)
AT~2018hyz (ASASSN-18zj). From follow-up optical spectroscopy, we detect the
first unambiguous case of resolved double-peaked Balmer emission in a TDE. The
distinct line profile can be well-modelled by a low eccentricity
() accretion disk extending out to 100 and a
Gaussian component originating from non-disk clouds, though a bipolar outflow
origin cannot be completely ruled out. Our analysis indicates that in
AT~2018hyz, disk formation took place promptly after the most-bound debris
returned to pericenter, which we estimate to be roughly tens of days before the
first detection. Redistribution of angular momentum and mass transport,
possibly through shocks, must occur on the observed timescale of about a month
to create the large \Ha-emitting disk that comprises 5\% of the
initial stellar mass. With these new insights from AT~2018hyz, we infer that
circularization is efficient in at least some, if not all optically-bright,
X-ray faint TDEs. In these efficiently circularized TDEs, the detection of
double-peaked emission depends on the disk inclination angle and the relative
strength of the disk contribution to the non-disk component, possibly
explaining the diversity seen in the current sample.Comment: 24 pages, 8 figures, 6 tables. Accepted for publication in Ap
Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT2019qiz
We report the results of UV and optical photometric and spectroscopic
analysis on the tidal disruption event (TDE) AT2019qiz. Our follow-up
observations started shortly after (10 days) its optical light begin
to rise and lasted for a period of six months. Our late-time host-dominated
spectrum indicates that the host galaxy likely harbours a weak AGN. The initial
HST spectrum of AT2019qiz exhibits a iron and low-ionization broad absorption
line (FeLoBAL) system that is seen for the first time in a TDE. This spectrum
also bears a striking resemblance to that of Gaia16apd, a superluminous
supernova. Our observations provide insights into the outflow properties in
TDEs and show evidence for a connection between TDEs and engine-powered
supernovae at early phase, as originally suggested in Metzger & Stone (2016).
In a time frame of 50 days, the UV spectra of AT2019qiz started to resemble
previous TDEs with only high-ionization BALs. The change in UV spectral
signatures is accompanied by a decrease in the outflow velocity, which began at
km s and decelerated to km s. A similar
evolution in the H emission line width further supports the speculation
that the broad Balmer emission lines are formed in TDE outflows. In addition,
we detect narrow absorption features on top of the FeLoBAL signatures in the
early HST UV spectrum of AT2019qiz. The measured HI column density corresponds
to a Lyman-limit system whereas the metal absorption lines, such as NV, CIV,
FeII, and MgII, are likely probing the circumnuclear gas and interstellar
medium in the host galaxy.Comment: 24 pages, 13 figures, 3 tables. Submitted to ApJ on Nov 03 202
Transcriptome annotation using tandem SAGE tags
Analysis of several million expressed gene signatures (tags) revealed an increasing number of different sequences, largely exceeding that of annotated genes in mammalian genomes. Serial analysis of gene expression (SAGE) can reveal new Poly(A) RNAs transcribed from previously unrecognized chromosomal regions. However, conventional SAGE tags are too short to identify unambiguously unique sites in large genomes. Here, we design a novel strategy with tags anchored on two different restrictions sites of cDNAs. New transcripts are then tentatively defined by the two SAGE tags in tandem and by the spanning sequence read on the genome between these tagged sites. Having developed a new algorithm to locate these tag-delimited genomic sequences (TDGS), we first validated its capacity to recognize known genes and its ability to reveal new transcripts with two SAGE libraries built in parallel from a single RNA sample. Our algorithm proves fast enough to experiment this strategy at a large scale. We then collected and processed the complete sets of human SAGE tags to predict yet unknown transcripts. A cross-validation with tiling arrays data shows that 47% of these TDGS overlap transcriptional active regions. Our method provides a new and complementary approach for complex transcriptome annotation
The Impact of Mercenaries and Private Military and Security Companies on Civil War Severity between 1946 and 2002
Research has long abandoned the view that only states wage war. On the contrary, civil war research has produced an impressive body of literature on violent non-state actors. Still, a particular group of actors—mercenaries—has been widely neglected so far, although they have participated in numerous conflicts in the second half of the twentieth century. Whether their presence aggravated or improved the situation is a matter of dispute. Some believe that the additional military capabilities provided by mercenaries help to end civil wars quickly without increased bloodshed, while others deem mercenaries greedy and bloodthirsty combatants who contribute to making civil wars more brutal, while a third opinion differentiates between different types of mercenaries. This article tests the impact of mercenaries on civil war severity. The evidence indicates that the presence of both mercenaries and private military and security contractors increases its severity