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Register a new userA strongly magnetized pulsar within grasp of the Milky Ways supermassive black hole
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The center of our Galaxy hosts a supermassive black hole, Sagittarius (Sgr) A*. Young, massive stars within 0.5 pc of SgrA* are evidence of an episode of intense star formation near the black hole a few Myr ago, which might have left behind a young neutron star traveling deep into SgrA*s gravitational potential. On 2013 April 25, a short X-ray burst was observed from the direction of the Galactic center. Thanks to a series of observations with the Chandra and the Swift satellites, we pinpoint the associated magnetar at an angular distance of 2.4+/-0.3 arcsec from SgrA*, and refine the source spin period and its derivative (P=3.7635537(2) s and dot{P} = 6.61(4)x10^{-12} s/s), confirmed by quasi simultaneous radio observations performed with the Green Bank (GBT) and Parkes antennas, which also constrain a Dispersion Measure of DM=1750+/-50 pc cm^{-3}, the highest ever observed for a radio pulsar. We have found that this X-ray source is a young magnetar at ~0.07-2 pc from SgrA*. Simulations of its possible motion around SgrA* show that it is likely (~90% probability) in a bound orbit around the black hole. The radiation front produced by the past activity from the magnetar passing through the molecular clouds surrounding the Galactic center region, might be responsible for a large fraction of the light echoes observed in the Fe fluorescence features.
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Supermassive black hole binaries (SMBHBs) should form frequently in galactic nuclei as a result of galaxy mergers. At sub-parsec separations, binaries become strong sources of low-frequency gravitational waves (GWs), targeted by Pulsar Timing Arrays (PTAs). We used recent upper limits on continuous GWs from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11yr dataset to place constraints on putative SMBHBs in nearby massive galaxies. We compiled a comprehensive catalog of ~44,000 galaxies in the local universe (up to redshift ~0.05) and populated them with hypothetical binaries, assuming that the total mass of the binary is equal to the SMBH mass derived from global scaling relations. Assuming circular equal-mass binaries emitting at NANOGravs most sensitive frequency of 8nHz, we found that 216 galaxies are within NANOGravs sensitivity volume. We ranked the potential SMBHBs based on GW detectability by calculating the total signal-to-noise ratio (S/N) such binaries would induce within the NANOGrav array. We placed constraints on the chirp mass and mass ratio of the 216 hypothetical binaries. For 19 galaxies, only very unequal-mass binaries are allowed, with the mass of the secondary less than 10 percent that of the primary, roughly comparable to constraints on a SMBHB in the Milky Way. Additionally, we were able to exclude binaries delivered by major mergers (mass ratio of at least 1/4) for several of these galaxies. We also derived the first limit on the density of binaries delivered by major mergers purely based on GW data.
We present a new 300 ks Chandra observation of M87 that limits pileup to only a few per cent of photon events and maps the hot gas properties closer to the nucleus than has previously been possible. Within the supermassive black holes gravitational sphere of influence, the hot gas is multiphase and spans temperatures from 0.2 to 1 keV. The radiative cooling time of the lowest temperature gas drops to only 0.1-0.5 Myr, which is comparable to its free fall time. Whilst the temperature structure is remarkably symmetric about the nucleus, the density gradient is steep in sectors to the N and S, with $rho{propto}r^{-1.5pm0.1}$, and significantly shallower along the jet axis to the E, where $rho{propto}r^{-0.93pm0.07}$. The density structure within the Bondi radius is therefore consistent with steady inflows perpendicular to the jet axis and an outflow directed E along the jet axis. By putting limits on the radial flow speed, we rule out Bondi accretion on the scale resolved at the Bondi radius. We show that deprojected spectra extracted within the Bondi radius can be equivalently fit with only a single cooling flow model, where gas cools from 1.5 keV down below 0.1 keV at a rate of 0.03 M$_{odot}$/yr. For the alternative multi-temperature spectral fits, the emission measures for each temperature component are also consistent with a cooling flow model. The lowest temperature and most rapidly cooling gas in M87 is therefore located at the smallest radii at ~100 pc and may form a mini cooling flow. If this cooling gas has some angular momentum, it will feed into the cold gas disk around the nucleus, which has a radius of ~80 pc and therefore lies just inside the observed transition in the hot gas structure.
Pulsar timing arrays (PTAs) are expected to detect gravitational waves (GWs) from individual low-redshift (z<1.5) compact supermassive (M>10^9 Msun) black hole (SMBH) binaries with orbital periods of approx. 0.1 - 10 yrs. Identifying the electromagnetic (EM) counterparts of these sources would provide confirmation of putative direct detections of GWs, present a rare opportunity to study the environments of compact SMBH binaries, and could enable the use of these sources as standard sirens for cosmology. Here we consider the feasibility of such an EM identification. We show that because the host galaxies of resolved PTA sources are expected to be exceptionally massive and rare, it should be possible to find unique hosts of resolved sources out to redshift z=0.2. At higher redshifts, the PTA error boxes are larger, and may contain as many as 100 massive-galaxy interlopers. The number of candidates, however, remains tractable for follow-up searches in upcoming wide-field EM surveys. We develop a toy model to characterize the dynamics and the thermal emission from a geometrically thin, gaseous disc accreting onto a PTA-source SMBH binary. Our model predicts that at optical and infrared frequencies, the source should appear similar to a typical luminous active galactic nucleus (AGN). However, owing to the evacuation of the accretion flow by the binarys tidal torques, the source might have an unusually low soft X-ray luminosity and weak UV and broad optical emission lines, as compared to an AGN powered by a single SMBH with the same total mass. For sources near z=1, the decrement in the rest-frame UV should be observable as an extremely red optical color. These properties would make the PTA sources stand out among optically luminous AGN, and could allow their unique identification.
We consider black hole - galaxy coevolution using simple analytic arguments. We focus on the fact that several supermassive black holes are known with masses significantly larger than suggested by the $M - {sigma}$ relation, sometimes also with rather small stellar masses. We show that these are likely to have descended from extremely compact `blue nugget galaxies born at high redshift, whose very high velocity dispersions allowed the black holes to reach unusually large masses. Subsequent interactions reduce the velocity dispersion, so the black holes lie above the usual $M - {sigma}$ relation and expel a large fraction of the bulge gas (as in WISE J104222.11+164115.3) that would otherwise make stars, before ending at low redshift as very massive holes in galaxies with relatively low stellar masses, such as NGC 4889 and NGC 1600. We further suggest the possible existence of two new types of galaxy: low-mass dwarfs whose central black holes lie below the $M - {sigma}$ relation at low redshift, and galaxies consisting of very massive ($gtrsim 10^{11}$M$_{odot}$) black holes with extremely small stellar masses. This second group would be very difficult to detect electromagnetically, but potentially offer targets of considerable interest for LISA.
The compact and, with 4.3+-0.3 million solar masses, very massive object located at the center of the Milky Way is currently the very best candidate for a supermassive black hole (SMBH) in our immediate vicinity. The strongest evidence for this is provided by measurements of stellar orbits, variable X-ray emission, and strongly variable polarized near-infrared emission from the location of the radio source Sagittarius~A* (SgrA*) in the middle of the central stellar cluster. If SgrA* is indeed a SMBH it will, in projection onto the sky, have the largest event horizon and will certainly be the first and most important target of the Event Horizon Telescope (EHT) Very Long Baseline Interferometry (VLBI) observations currently being prepared. These observations in combination with the infrared interferometry experiment GRAVITY at the Very Large Telescope Interferometer (VLTI) and other experiments across the electromagnetic spectrum might yield proof for the presence of a black hole at the center of the Milky Way. It is, however, unclear when the ever mounting evidence for SgrA* being associated with a SMBH will suffice as a convincing proof. Additional compelling evidence may come from future gravitational wave observatories. This manuscript reviews the observational facts, theoretical grounds and conceptual aspects for the case of SgrA* being a black hole. We treat theory and observations in the framework of the philosophical discussions about (Anti)Realism and Underdetermination, as this line of arguments allows us to describe the situation in observational astrophysics with respect to supermassive black holes. Questions concerning the existence of supermassive black holes and in particular SgrA* are discussed using causation as an indispensable element. We show that the results of our investigation are convincingly mapped out by this combination of concepts.
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