Development of a pulsar-based timescale

Using observations of pulsars from the Parkes Pulsar Timing Array (PPTA) project we develop the first pulsar-based timescale that has a precision comparable to the uncertainties in international atomic timescales. Our ensemble of pulsars provides an Ensemble Pulsar Scale (EPS) analogous to the free atomic timescale Echelle Atomique Libre (EAL). The EPS can be used to detect fluctuations in atomic timescales and therefore can lead to a new realisation of Terrestrial Time, TT(PPTA11). We successfully follow features known to affect the frequency of the International Atomic Timescale (TAI) and we find marginally significant differences between TT(PPTA11) and TT(BIPM11). We discuss the various phenomena that lead to a correlated signal in the pulsar timing residuals and therefore limit the stability of the pulsar timescale.Comment: Accepted for publication in MNRA

The Grizzly, September 23, 2004

Get Down in the Lounge • USGA Amendments Cause Controversy • Wismer Rumors Exposed • Family Day is Just Around the Corner • You got SERVed! • Medulla: Soul for Your Brain • Lead the Way: UC Leadership Studies Program • Care to Dance? • Opinions: Should More Public Places Move Towards a Complete No-smoking Policy?; Tattoo or not to Tattoo? • My Summer Vacation Camping at Death Pond • The Kobe Bryant Sagahttps://digitalcommons.ursinus.edu/grizzlynews/1565/thumbnail.jp

The Grizzly, November 18, 2004

Student Reactions to Sigma Pi • President Strassburger\u27s Letter to the Collegeville Community • Poe on Poe Recommended for Theater Festival Nomination • Ursinus College Dance Company Concert Debuts this Week • Men and Women for Feminism: Review of From the Belly • Review: What did the Moon See? • Major Highlight: Business and Economics • Opinions: Welcome to Dubya\u27s Fun World; The Wismer Incident: Issues of Food and Money; What\u27s Real About Reality TV • Success Does not Equal Playoff Contention for the Men\u27s Rugby Team • Wrestling Team Puts Yet Another Beating on its Oppositionhttps://digitalcommons.ursinus.edu/grizzlynews/1572/thumbnail.jp

Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves

We present an analysis of high-precision pulsar timing data taken as part of the North American Nanohertz Observatory for Gravitational waves (NANOGrav) project. We have observed 17 pulsars for a span of roughly five years using the Green Bank and Arecibo radio telescopes. We analyze these data using standard pulsar timing models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Sub-microsecond timing residuals are obtained in nearly all cases, and the best root-mean-square timing residuals in this set are ~30-50 ns. We present methods for analyzing post-fit timing residuals for the presence of a gravitational wave signal with a specified spectral shape. These optimally take into account the timing fluctuation power removed by the model fit, and can be applied to either data from a single pulsar, or to a set of pulsars to detect a correlated signal. We apply these methods to our dataset to set an upper limit on the strength of the nHz-frequency stochastic supermassive black hole gravitational wave background of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of the two best pulsars in the set, PSRs J1713+0747 and J1909-3744.Comment: To be submitted to Ap

The Grizzly, October 14, 2004

Walk for AIDS Awareness • Tenure: How to Make Your Vote Count • Powderpuff Football Raises Money for Cancer Research • Ex Voto Artist Speaks on Campus • Princeton Review Wants Your Opinion • Ursinus Faith • Learning Versus Earning in the College Years • From Humor to Horror: Poe on Poe Preview • Opinions: Who is to Blame for the Vioxx Withdrawal?; Segregation in Wismer; Phone Phollies; Blurring the Lines in American Media • Women\u27s Soccer Team Fools the Oddsmakers • Conduct in Pro Sports Becoming an Issuehttps://digitalcommons.ursinus.edu/grizzlynews/1568/thumbnail.jp

The Grizzly, September 16, 2004

Ursinus Goes Wireless • New Bookstore, New Look, Same Sentiments • Project Pericles Seeks to Remedy Political Apathy • Election 101: Who do you Side With? • Environmental Studies Professor gets with the Program • Ursinus Student Awarded $40,000 Grant • Are Ursinus Students Apathetic? • Ursinus Professors Dance the Night Away in Russia • Opinions: Should the Miss America Pageant Qualify as one of the Great American pastimes?; Why America Needs Four More Years of George W. Bush; Surviving \u27R-ships\u27 in College • Field Hockey Team Continues Where They Left Off • Men\u27s Soccer Shows Promisehttps://digitalcommons.ursinus.edu/grizzlynews/1564/thumbnail.jp

The Grizzly, November 11, 2004

USGA Passes Sigma Pi • Two Students Wear Questionable Costumes • Lonnie Graham is the Spark • Ursinus Proposes Possible Plans for Honor Code • The Benefits for a Professor on Sabbatical • Effects of Election Still Resonate in Ursinus Community • Do Ursinus Students Make use of Proximity to Philadelphia? • Opinions: Is Online Dating a Safe Alternative for Meeting People or a Risky Plea of Desperation?; All is not Lost for Liberals • Field Hockey Team Wins Centennial Conference Title • It\u27s All Over for Three Women Soccer Players • The Collegeville Cursehttps://digitalcommons.ursinus.edu/grizzlynews/1571/thumbnail.jp

The Grizzly, October 28, 2004

Don\u27t Let Wismer Food Scare You; DSAC is Here to Help! • Senior Halloween Party Coming to a Barn Near You • RHA Halloween Decorating Contest Returns • Halloween not for Everyone • Omwake, a Haunted House for the Community • Cheap, Last-minute Costumes Down the Road • It\u27s a Witch! • Hobson Ghosts: Past and Present • Eastern State Penitentiary: Does it Frighten You? • Opinions: Are Halloween Costumes Just for Fun, or Can They be Offensive?; How Old is Too Old to Trick or Treat?; Halloween Hijinks • The Thompson-Gay Era and the Gym Named in Their Honor • Nightmares on Broad Streethttps://digitalcommons.ursinus.edu/grizzlynews/1569/thumbnail.jp

The Grizzly, September 30, 2004

USGA has Second Thoughts • Campus Philly Kick-off Week • Lights Out at UC • Where Will You go? UC Study Abroad Programs • Andrew Sullivan to Speak at Ursinus • Easy Access to Social Security Numbers Causes Some Concerns • The Aid Game • Why Study Abroad? • Opinions: Appealing to a Younger Demographic; Experiencing the Miss America Pageant Live; MTV Generation of Choosers and Losers; Ready Maim Fire • Baseball Preview • Women\u27s Soccer Defeats Immaculata • Women\u27s Rugby Claws way to Victoryhttps://digitalcommons.ursinus.edu/grizzlynews/1566/thumbnail.jp

Single-cell transcriptomic reconstruction reveals cell cycle and multi-lineage differentiation defects in Bcl11a-deficient hematopoietic stem cells.

BACKGROUND: Hematopoietic stem cells (HSCs) are a rare cell type with the ability of long-term self-renewal and multipotency to reconstitute all blood lineages. HSCs are typically purified from the bone marrow using cell surface markers. Recent studies have identified significant cellular heterogeneities in the HSC compartment with subsets of HSCs displaying lineage bias. We previously discovered that the transcription factor Bcl11a has critical functions in the lymphoid development of the HSC compartment. RESULTS: In this report, we employ single-cell transcriptomic analysis to dissect the molecular heterogeneities in HSCs. We profile the transcriptomes of 180 highly purified HSCs (Bcl11a (+/+) and Bcl11a (-/-)). Detailed analysis of the RNA-seq data identifies cell cycle activity as the major source of transcriptomic variation in the HSC compartment, which allows reconstruction of HSC cell cycle progression in silico. Single-cell RNA-seq profiling of Bcl11a (-/-) HSCs reveals abnormal proliferative phenotypes. Analysis of lineage gene expression suggests that the Bcl11a (-/-) HSCs are constituted of two distinct myeloerythroid-restricted subpopulations. Remarkably, similar myeloid-restricted cells could also be detected in the wild-type HSC compartment, suggesting selective elimination of lymphoid-competent HSCs after Bcl11a deletion. These defects are experimentally validated in serial transplantation experiments where Bcl11a (-/-) HSCs are myeloerythroid-restricted and defective in self-renewal. CONCLUSIONS: Our study demonstrates the power of single-cell transcriptomics in dissecting cellular process and lineage heterogeneities in stem cell compartments, and further reveals the molecular and cellular defects in the Bcl11a-deficient HSC compartment