Steven Silverberg

Mildly surprised that the low point isn't Rocky V.

Traditional July 4 Programming Note

TCM’s annual airing of 1776 will be a late one this evening

And 3) the Republicans held the House from January 2011 on.

...Second-threepeat Jordan? (Space Jam notwithstanding)

For my money the Saint Louis Zoo is the best zoo in the country. Also Ted Drewes. Science Center is good. And if you enjoy theater and don't mind outdoors see if the Muny has anything of interest on that week--one of the best theater companies in the country.

The Oven of Akhnai! en.m.wikipedia.org/wiki/The_Ove...

Which has many many layers of interpretation beyond the story itself, as well.

Highly recommend the Spokane Indians, who worked with the Spokane tribal leaders to create a uniform in the variant of Salish spoken by the Spokane

It me, the other person equipped to appreciate that.

Berkman and Pierzynski, though.

(But you're probably right and this team would probably infuriate me more if I wasn't primarily radio rn.)

Even the 2013 team?

To be fair, the most impactful bad calls literally evened out--one strike called a ball, one ball called a strike, in the same plate appearance. One could argue that this was simply cleaning up an earlier mistake. (One could also reasonably argue that neither mistake should have been made.)

My vague recollection (having not watched The Search in a few years) was that it was less that it had to be Romulan, more that because of some treaty (Khitomer, maybe?) the Federation wasn't allowed to have cloaking devices so the way to get around *that* was to have a Romulan supervise it?

Read more about it on arXiv and in AJ! ui.adsabs.harvard.edu/abs/2025AJ..... 14/14

Another possibility is repeated high-resolution spectroscopy of HL Tau to observe how the 6.4 keV iron fluorescence changes over time with the 6.7 keV iron line. If the 6.4 keV line didn't change as the 6.7 keV line does, that could indicate changing geometry rather than changing emission. 13/14

BUT to say anything for certain about the nature of the variability would require more data, of course. The observing window for HL Tau in 2020 was six observations over 40 days--a longer baseline (comparable to the ~81 days from K2 Campaign 13) would be beneficial. 12/14

A 21 day period would be much larger than the rotation period of the star. BUT it would match the period of an orbiting body in the disk at a circular orbit of ~0.15 AU(!!!). We discuss possibilities for what that could be (accretion funnels, induced hot spots on the star, disk currents). 11/14

So, what to make of the variability? The simplest answer is that it's random--we might be seeing parts of "slow-rise, flat-top" flares. But that's not the most interesting possibility, so we decided to consider possible explanations for if it *is* periodic at 21 days. 10/14

Figure 9 from Silverberg et al. (2025). Comparison of model fits to HL Tau observations from 2000 to 2020. All models use one collisionally-excited plasma component with variable temperature and a fixed abundance for Fe (and elements with Fe-like first ionization potential) set at the best-fit value from a joint fit to the faint XMM-Newton observations from 2020 and a variable absorption component. Top row: flux (in units of pico-ergs per second per square centimeter) is shown as both absorbed (as observed; blue for XMM-Newton, green for Chandra) and unabsorbed (corrected for absorption; brown). Second row: the plasma temperature for the single plasma component in keV (red) in comparison to 4 keV (dashed gray line). Third row: the emission measure (in units of 10^52 per cubic centimeter) for the single plasma component (red). Bottom row: the hydrogen column density NH (in units of 10^22 square centimeters) of the absorbing material (black). Observing windows are labeled above each column. The observations during the flare identified by G. Giardino et al. (2006) in 2004 are highlighted in gray. The data show that HL Tau is variable, that the by-eye period in 2020 does not necessarily extend to earlier epochs (but might be there?), that the variation in 2020 is due to changing emission rather than changing absorption, and that this pattern does not necessarily hold at earlier epochs.

We were able to reprocess and analyze archival Chandra & XMM-Newton observations of HL Tau from 2000, 2004, 2017, and 2018, and look at how it changes (or doesn't) over time. The temperatures stay consistent around 4 keV. The bright 2020 observations are not as bright as a 2004 flare. 9/14

Figure 8 from Silverberg et al. (2025). Temperature vs. unabsorbed X-ray luminosity for single-temperature fits to HL Tau (green crosses), in comparison to temperature and unabsorbed luminosities from two-temperature fits (blue and orange circles) to the COUP pre-main-sequence stars (T. Preibisch et al. 2005). The plot shows temperature (measured in keV) in log space on the y axis, as a function of log(X-ray luminosity) on the x axis. The plot depicts blue, orange, and green scatter plots. Blue points represent the temperatures and luminosities associated with cool plasma from COUP, while orange points represent temperatures and luminosities associated with hot plasma. The blue scatter points are flat in temperature as a function of log(luminosity), while the orange points increase linearly as a function of log(luminosity). HL Tau is plotted as green "x" shapes, which overlap with the high-luminosity end of the COUP data. The right y axis shows temperature in millions of Kelvin.

So, what all does this tell us? Our best fit models and the consistent presence of 6.7 keV emission indicate that the spectrum of HL Tau is hot (consistently ~4 keV), on the hot end and bright end of YSOs compared to the COUP observations, but generally consistent with being a Class I YSO. 8/14

Iron fluorescence at 6.4 keV is emission from cold iron in the disk, stimulated by the 6.7 keV emission from the star. The detection here is marginal, but plausible. We also see the Si XIII r-i-f triplet, with f stronger than r, indicating the presence of both hot and cool plasma. 7/14

Figure 7. Combined grating data from all 11 Chandra/ACIS/HETG observations of HL Tau, with spectral lines of interest labeled. The best-fit model fit jointly to the individual zeroth-order and combined grating spectra is shown in orange. Left inset: a zoom-in on the region around the Fe xxv feature at 1.85 Å (6.7 keV) shows a clear detection of the Fe xxv line and a marginal detection of the 6.4 keV neutral Fe fluorescence line in ∼300 ks. Right inset: a zoom-in on the 6–7 Å region shows a strong detection of the f line of the Si xiii He-like triplet and a fainter but still present detection of the r line. Inset units are same as in the larger figure. The plot itself is of counts per kilosecond per angstrom as a function of wavelength in angstroms (on the lower x axis) and energy in keV (on the upper x axis). The observed data are blue, while the best fit model is in orange. A left inset shows a strong peak in Fe XXV emission at 1.85 angstroms, and marginal emission at 1.94 angstroms. The right inset shows strong Si XIV emission, and clear detections of the Si XIII r and f lines. The f line is stronger than the r line.

We also got a high-resolution spectrum using the High-Energy Transmission Grating on Chandra! These data show clear emission lines that help constrain the temperature and density of the plasma. We also see indications of cooler plasma hidden by the absorption, and a hint at iron fluorescence! 6/14

Figure 6 from Silverberg et al. (2025). Left: the zeroth-order light curve from the 2018 Chandra observations. Right: the Lomb–Scargle periodogram for these data (solid) and its window function (dotted). The strong peak in the window function near 2.5 days is consistent with the orbital period of Chandra. The left plot shows count rate (in counts per second) as a function of time (in MJD) for Chandra---the count rates are plotted as blue scatter plots. There are brief gaps between the first nine observations, then a longer duration gap before the final two. The right plot shows power as a function of period in days. There are no clear peaks in this data.

We did similar with the zeroth-order data from our Chandra gratings observations in 2018. The data were taken over 18 days, so we don't have sensitivity to a 21-day signal. There is some rise and fall in the data, but not nearly as clear as in the 2020 data. 5/14

The high-energy (>2 keV) XMM/PN light curve of HL Tau from 2020, in chronological order (left) and phased to a ∼21-day period (right). Colors distinguish data from each observation and are the same for each observation in both panels. Count rates are derived from binning by 1 ks. Two plots, side by side. The first one plots a series of points as a function of time--the y axis is counts per second, while the x axis is time in modified Julian days. Data are color-coded chronologically as black, blue, orange, red, purple, green. On the right, the data are phase folded to a period of ~21 days. Y axis is counts per second, while the x axis is phase (in days). The data are at a low point in green and orange observations at phase -10, rise to their highest in the red data around phase -4.5, decrease slightly to the black data at phase 0 days, and then diminish to nearly the level of the green and orange points in the purple and blue points at phase +4.5 days. Each data set is a large clump of scatter plot in the appropriate color, with gaps of a few days to a week between observations.

Figure 4 from Silverberg et al. (2025): Lomb–Scargle periodograms of the high-energy light curves for HL Tau from the XMM-Newton MOS1 (solid blue), MOS2 (solid orange), and PN (solid green) detectors and all three jointly (black). The Lomb–Scargle periodograms of the window functions (i.e., flat data with identical timing and uncertainties to the observed data) for the three detectors are depicted as fainter dotted lines with the same colors as those derived from data. The strongest peak of the data is at a minimum of the periodogram of the window function, indicating that this is not due solely to the observing window. The strongest peak of the window function, at ∼2 days, corresponds to XMM-Newton's orbital period. A plot showing Power (from 0 to 1) on the y axis, as a function of period (in days) on the x axis. Blue solid lines represent the periodogram of data from the MOS1 detector on XMM-Newton, while orange represent data from the MOS2 detector and green represent data from the PN detector. A black solid line is a joint fit to data from all three detectors. Thin dotted lines in the colors for each detector show the periodogram of the window function for each detector. The observed data show a peak at ~21 days, while the window function shows a local minimum near 21 days. This suggests that there is a signal of variability on a 21-day timescale.

We looked at the variation over time in 2020, and saw something that looked like it might be a period. A Lomb-Scargle periodogram of the data shows a peak signal at 21 days (albeit with other peaks at lower levels at other periods). So, variability on a 21-day timescale! 4/14

A plot from Silverberg et al. (2025) shows XMM-Newton EPIC-PN spectra from each of the six ∼40 ks observations (represented by different colors as listed in the legend) in the the 2020 monitoring campaign. Spectra are binned by 15 counts per bin. The y axis is counts per second per kilo-electron volt (keV) on a logarithmic scale, while the x axis is energy (in keV) on a linear scale. Observations are plotted at scatter plots showing all data points for each spectrum. Spectra are color coded chronologically as black, blue, orange, green, purple, brown. All spectra show a clear drop-off of emission below ~2 keV, indicating very high absorption. The other spectra show clear decreasing emission as a function of energy at energies above 2 keV, with the exception of a broad emission line that appears in all spectra at 6.7 keV. While all spectra show the same decreasing pattern, some of them are brighter than others--the continuum for the black and green spectra are above the other four. Each point has associated uncertainties in plotted error bars.

In our paper, we began with monitoring data of HL Tau from the XMM-Newton satellite in 2020, and high-resolution spectroscopy from Chandra in 2018. The monitoring data show that the spectrum is consistently hot, and heavily absorbed below 2 keV. 3/14

A 2014 ALMA image of the Class I Young Stellar Object HL Tau. Image is an red-orange-yellow disk on a black field. The disk has alternating light and dark (black) rings, indicating gaps in the disk. The light gets brighter (more orange-yellow) as you look towards the center. It is canted at an angle. Image credit: NRAO/ESO/NAOJ

You may recall HL Tau from such hits as "a notable infrared source" and "subject of absolutely beautiful ALMA disk images." It's also an excellent subject for observation in X-rays, which can get at the stellar surface in ways other wavelengths can't as easily (because of absorption). 2/14

Been a while since I did one of these, but my recent paper on long-term X-ray variability on HL Tau was published by AJ! Accessible from here: ui.adsabs.harvard.edu/abs/2025AJ..... ☄️🔭🧵1/14

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Extragalactic Astronomy and Cosmology

Currently teaching 300-level Galaxies and Cosmology using link.springer.com/book/10.1007... Going quite well so far.