Ferreira R.S.,Federal University of Rio de Janeiro |
Borges C.L.T.,Federal University of Rio de Janeiro |
IEEE Transactions on Power Systems | Year: 2014
In this paper, we propose a flexible mixed-integer linear programming formulation of the AC OPF problem for distribution systems, using convexification and linearization techniques. The proposed formulation allows the representation of discrete decisions via integer decision variables, captures the nonlinear behavior of the electrical network via approximations of controllable accuracy, and can be solved to global optimality with commercial optimization solvers. The formulation is based on conventional variables that describe network behavior, which ensures its flexibility and the possibility of application to various distribution system problems, as we indicate with case studies. © 2014 IEEE.
News Article | November 14, 2015
Exciting news for space lovers and exploration seekers: researchers have discovered the first gamma-ray pulsar outside the Milky Way – and it sets the record of being the most luminous known gamma-ray pulsar to date. Imaged by NASA’s Fermi gamma-ray Space Telescope, the pulsar lies in the Tarantula Nebula’s outskirts in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way found 163,000 light years away. The Tarantula Nebula is the biggest, most active, and most intricate star-formation area in the galactic community, identified as a bright gamma-ray source. Lead scientist and astrophysicist Pierrick Martin said PSR J0540-6919 is responsible for about half of the gamma-ray brightness originally believed to hail from the nebula. "That is a genuine surprise," he said. The new findings were announced Nov. 13 in the journal Science. The highest-energy light form, gamma-rays are deemed borne out of subatomic particles that collided in the wake of supernova explosions. Extremely condensed having collapsed in on itself, a supernova rotates quickly; during spinning, electromagnetic field shoots out energy pulses in the form of gamma rays, X-rays, radio waves, and visible light. The Tarantula Nebula is known for another pulsar, PSR J0537−6910 (J0537), discovered with the help of the Rossi X-ray Timing Explorer (RXTE) satellite and spins at nearly 62 times a second, the fastest-known rotation time for a young pulsar. J0540, on the other hand, whirls at just under 20 times per second. Co-author Lucas Guillemot said J0540’s gamma-ray pulses have 20 times the intensity of the pulsar in the Crab Nebula, the previous record-holder. “[Y]et they have roughly similar levels of radio, optical and X-ray emission,” he explained. J0540 also has an age of about 1,700 years, twice of the Crab Nebular pulsar’s and in contrast with most of over 2,500 known pulsars ages 10,000 to hundreds of millions of years. It took over six years for the telescope and for a reanalysis of telescope data to detect the pulsations. Before Fermi was launched in 2008, there were only seven gamma-ray pulsars detected, unlike today when the mission has found over 160 within the Milky Way. Few gamma rays reach the telescope to detect the pulsations without knowing the period ahead of time. Discoveries such as this one, according to Guillemot, offer “a better understanding of the extreme physics at work in young pulsars.”
News Article | January 7, 2016
After decades of studies and research, scientists have estimated the age of the observable universe to be roughly 13.8 billion years old. The connection between distance and the speed of light -- explained by Albert Einstein's theory of relativity -- has allowed scientists to look at different regions of the vast outer space which lie 13.8 billion light-years away. The age and distance of the universe -- are these small hints to the possible existence of alien life? Scientists have yet to form a firm conclusion, but in late November last year, some experts were able to detect five mysterious radio bursts which may have all come from outside the Milky Way galaxy. These radio signals were discovered after an "alien megastructure" was reported to be orbiting around a distant star known as KIC 8462852. "It almost doesn't matter where you point your telescope, because there are planets everywhere. If there's somebody out there, there are going to be so many of them out there that I do think there's a chance," explained astronomer Seth Shostak of the Search for Extraterrestrial Intelligence (SETI) Institute in California. Now, a new study presented at the annual meeting of the American Astronomical Society in Florida suggests that an old, densely-packed and isolated group of stars located within the Milky Way may possibly sustain extraterrestrial life. These stars, collectively called globular clusters, may be a cradle of advanced civilizations, experts said. The Possibility Of Alien Life In Globular Star Clusters Scientists from the Harvard-Smithsonian Center for Astrophysics (CfA) and the Tata Institute of Fundamental Research in Mumbai believe that globular star clusters may be the first place in our galaxy to contain intelligent life beyond Earth. What exactly are globular star clusters? These are densely-packed and tight groups that contain thousands or millions of stars. These balls of star clusters may be about 100 light-years across each other on average, and are as old as the Milky Way galaxy itself. Our galaxy is home to about 150 globular star clusters, where most of them orbit the galactic outskirts. On average, these star clusters may be 10 to 12 billion years old, just a couple billion years younger than the observable universe. But Houston, We Have A Problem The stars within globular clusters have fewer of the essential elements considered as "building blocks" of planets, such as silicon (Si) and iron (Fe), because these elements must be formed in earlier generations of stars. This lack in heavy elements has led other scientists to argue that globular star clusters are less likely to contain planets. In fact, only one planet has been found within globular clusters: the oldest known exoplanet called PSR B1620-26 b or Methuselah. Still, astronomers Rosanne DiStefano and Alak Ray said these views are "too pessimistic." "It's premature to say there are no planets in globular clusters," said Ray. The duo explained that a lot of exoplanets have been discovered around host stars that are only one-tenth as rich with metals as our Sun. While planets that are Jupiter-sized are found more around stars that contained higher levels of Fe and Si, planets that are Earth-sized show no such bias. Another main problem: because globular clusters are too close-knit, this specific environment could threaten the possible formation and existence of planets within it. Scientists said a neighboring star could wander too close to a planetary system, consequently disrupting the gravitational forces and resulting to the unfortunate hurling of worlds into interstellar space. What Could Be the Right Clue? DiStefano and Ray explained that the habitable zone or the "Goldilocks" zone of a star varies greatly. The Goldilocks zone is the right distance at which planets would be not too warm or not too cold to have liquid water. Brighter stars have more distant Goldilocks zones, and have shorter life spans. Because globular clusters are old, these extremely bright stars have died out. In contrast, planets that orbit around dimmer stars huddle closer to each other. These dimmer stars are faint and closer, but they also live long enough to become red dwarfs. Potentially habitable planets that these faint stars host would orbit nearby and be relatively safe from stellar interactions. "Once planets form, they can survive for long periods of time, even longer than the current age of the universe," said DiStefano. What If Planets Within Globular Clusters Evolve? If livable planets could form within globular star clusters and survive for billions of years, extraterrestrial life in said planets would have enough time to become complex and even develop intelligence. The alien civilization would truly be different from our own. In our solar system, the nearest star is about four light-years (24 trillion miles) away. In a globular cluster, the nearest star may be 20 times closer or only one trillion miles apart. Interstellar exploration and communication, as well as space travel, would definitely be easier. DiStefano and Ray call this potential theory the "Globular Cluster Opportunity." "Sending a broadcast between the stars wouldn't take any longer than a letter from the U.S. to Europe in the 18th century," said DiStefano. Space missions would definitely take less time. NASA's Voyager probes are 100 billion miles away from our planet. In terms of globular cluster distance, this is one-tenth as far as it would take to reach the nearest star. A civilization at Earth's current technological level could easily send interstellar probes within the realm of a globular star cluster. DiStefano said the nearest globular cluster to our planet is thousand light-years away. This is why it is difficult for us to find planets, particularly in a space environment with a crowded core. However, it is possible to detect globular cluster planets on galactic outskirts. Through gravitational lensing, scientists might even spot free-floating planets or planets whose gravity magnifies light from a star. Lastly, scientists say that using SETI search methods to target globular clusters is an intriguing idea. SETI uses arrays of radio telescopes called Allen Telescope Array (ATA) to look for laser or radio broadcasts. Astronomer Frank Drake used the Arecibo radio telescope to broadcast the first deliberate message from our planet to outer space, a message directed to globular cluster Messier 13 (M13) or the Hercules Globular Cluster.
News Article | April 13, 2016
This story has been updated throughout with further information from the sentencing. On Wednesday, the former Reuters journalist Matthew Keys was sentenced to two years in prison for computer hacking. Keys, who once worked for Tribune Company-owned Sacramento television station Fox 40, left that job in 2010 and went on to copy and paste login credentials for the Tribune Company’s content management system (CMS) into a chatroom where members of the hacking collective Anonymous planned out their operations. (Keys still denies all allegations.) An unknown person under the username “sharpie” then went on to log into the CMS and deface a Los Angeles Times article. The article’s headline and dek (the subtitle beneath the headline) remained defaced for about forty minutes before an editor noticed and changed it back. In October 2015, a jury found Keys guilty of three counts of the Computer Fraud and Abuse Act (CFAA), a conviction that carried with it a maximum sentence of 25 years. A pre-sentence report prepared by the probation office recommended 87 months. In its sentencing memorandum, the US Attorney’s Office ended up seeking a lower sentence of five years. Keys’s attorneys asked for probation instead, claiming that the defacement did not result in enough loss to the Tribune Company to warrant any prison time. Their contention goes to one of the most controversial aspects of the case. In order to be convicted of felony under the particular provisions of the Computer Fraud and Abuse Act which prosecutors used to charge Keys, the conduct must exceed a threshold of $5,000. But the evidence brought at trial with respect to the total loss—and which was likely cited in the sealed presentence report prepared by the probation office—is tied to the Tribune Company’s reaction in the wake of the hack, including an extensive assessment of the entire CMS, as well as emails, phone calls, and meetings made by both journalists and highly-paid executives. In the still-sealed pre-sentence report (PSR), loss to the Tribune Company was placed at about $249,000. At trial, prosecutors presented evidence of loss ranging between $10,206 and $13,147. All of these numbers are lowballs compared to the numbers that were tossed out at various stages before trial. One document calculated loss at $929,977. In an unexpected twist, while going over the defense’s objections to the PSR, Judge Kimberly Mueller limited the amount of loss (for purposes of sentencing) to whatever had been presented at trial, thus drastically reducing the amount of prison time recommended by the sentencing guidelines. In the end, by the judge’s own determination, the appropriate range for sentencing was between 37 and 46 months. Prosecutors said in their sentencing recommendation that a “sentence of five years imprisonment reflects Keys’s culpability and places his case appropriately among those of other white collar criminals who do not accept responsibility for their crimes.” But when pressed to provide a recommendation within the judge’s guidelines, Assistant US Attorney Matt Segal asked for 42 months. “We have not gone overboard on this case, and a midrange recommendation doesn’t go overboard either,” he said to the judge. Keys, third from the left, after his sentencing on April 13, 2016. Photo: Sarah Jeong Although Keys’s lawyers said that the defacement was a prank borne out of the “spirit of the time,” AUSA Segal said that Keys’s actions weren’t motivated by mischief (or as one would have it, the lulz), but rather a vindictive desire to harm his former employer. “This is not just a prank. We might be talking about a prank if this were sharpie’s sentencing,” Segal said, adding that “This was rage driven by profound narcissism.” Segal pointed to chat transcripts that showed that sharpie had planned to deface the entire front page of the LA Times website the next day, a plan that had come to nothing because the Tribune Company was on high alert after the first defacement, and had taken steps that cut off Keys’s access to the CMS. “The only thing that limited his crime was that sharpie didn’t want to hurt the LA Times as much as Keys did,” said Segal in court. The story that the prosecution told at trial was not of a one-off, regretful copy/paste into a chatroom, but rather of a weeks-long harassment campaign launched at a former employer. In a taped confession, the validity of which Keys contests, Keys admitted to sending a series of harassing emails under various pseudonyms taken from the TV show The X-Files. Pseudonymous emails were also were sent to viewers who had signed up for emails from the television station—some of whom were elderly, and reacted very poorly. Only later did he contact Anonymous. In court, Segal read out a victim statement written by Brandon Mercer, Keys’s former supervisor at Fox 40. Mercer spoke on behalf of Fox 40. Mercer said that as a journalist who had broken “the sacred trust of his employment,” Keys should be held to a higher standard. Dan Gaines, formerly of the LA Times, also gave a victim statement on behalf of that paper. Gaines said that Keys’s actions posed a dire threat to the LA Times, due to how difficult it was to differentiate between fake and real news on the web today. “We are one of the few points of stability on the web. The risk is real, and for our industry, it’s crucial that we be a beacon in a confusing world," he said. The sentencing recommendation memo suggests that if anything, both the prosecution and the ultimate recommended sentence were at least partly spurred on by how little Keys endeared himself to law enforcement. “Keys’s characteristics include narcissism and an arrogant indifference to the suffering of innocent and vulnerable people,” prosecutors wrote in the memorandum. Keys has said that he was targeted for his work as a journalist, and that the prosecution was politically motivated. In the sentencing recommendation, prosecutors denied this, stating outright, “Keys was not targeted because he was a journalist.” Prosecutors said that these statements amounting to lying and promoting “cynicism about the justice system,” pointing out numerous statements that Keys had made on Twitter and to news outlets after the verdict. Defense attorney Jay Leiderman defended Keys’s statements, saying that even if the record did not show that Keys had been targeted for being a journalist (something the judge acknowledged very clearly), their client was still entitled to believe that he had been targeted for that reason. Leiderman also said that Keys’s unapologetic stance could hardly be held against him, since their view was that Keys had been charged and convicted under an unjust and overbroad law, and they were planning on appealing. “Congress is regressive not progressive,” said Leiderman. “And in fact they are completely inactive nowadays. Although there have been proposals for remediation, no action has been taken.” He said that the CFAA was a “buggy law” that did not fit the present day, and “with that, the punishments do not fit the crime.” Judge Mueller showed a keen interest in contextualizing the Keys case within the larger universe that it inhabits. She asked both sides to compare the Keys case to other cases involving members of Anonymous (including the case against the Paypal 14 and the case against Jeremy Hammond), and to justify variances from the sentences that those people had received. “Is Keys himself a hacker?” she asked the prosecutor at one point. “Or is he an instigator with modest computer skills?” “I think the parties agree, it’s not the crime of the century,” said Judge Mueller. In the end, she went below the guidelines she had calculated out, considering mitigating factors such as Keys’s otherwise clean record, his history of complying with law enforcement, and the unlikeliness that he would reoffend. She sentenced him to 24 months in prison, with 24 months supervised release following. In sentencing Keys, Judge Mueller said that the effect of the defacement was “relatively modest and did not do much to actually damage the reputation of that publication,” but that she could not ignore that his “intent was to wreak further damage which could have had further consequences.” She said that his actions had harmed his former supervisor, coworkers, successors, as well as viewers of the TV station. “The mask that Mr. Keys put on appeared to allow a heartless character to utter lines that are unbecoming of the professional journalist that he holds himself to be, and is, in most respects, in fact.” Outside the courtroom, Leiderman expressed disappointment with the decision, saying, “We walked in today facing an 87 month sentence, we walked out with 24. I suppose we should be heartened by that. But I’m not.” Keys is to surrender to a facility—likely a prison in Lompoc, California—on June 15, 2016. His defense team plans to appeal, and to stay his sentence pending that appeal.
(Phys.org)—NASA's Fermi Gamma-ray Space Telescope has once again proven that it is an excellent tool to search for rotating neutron stars emitting beams of electromagnetic radiation, known as pulsars. A team of astronomers, led by H. Thankful Cromartie of the University of Virginia, has recently used the 305-meter Arecibo radio telescope in Puerto Rico to observe unidentified sources of gamma rays detected by the Large Area Telescope (LAT) onboard the Fermi spacecraft. As it turns out, six of these objects indicated by LAT are rapidly rotating neutron stars, with periods of a few thousandths of a second, called millisecond pulsars (MSPs). The scientists published their results online on Jan. 20 on the arXiv pre-print server. The objects of the study were chosen from the LAT's 4-year point source catalog. The astronomers chose 34 from over 1,000 unidentified sources of gamma rays to observe them in detail with the Arecibo telescope. The catalog provided crucial spectral data that helped distinguish possible MSPs from other gamma-ray-emitting objects, like active galactic nuclei (AGNs). "Overall, the search for MSPs in the galactic disk has been made extremely efficient by employing Fermi-LAT data in selecting radio search targets," the researchers noted in their paper posted on arXiv. Arecibo observations were conducted from June to September 2013. The telescope's raw sensitivity and its large gain makes it a very efficient tool for finding millisecond pulsars. Thanks to Arecibo, the researchers were able to detect six MSPs with rotation periods ranging between 1.99 and 4.66 ms. One of the newly detected pulsars is a typical neutron star, a white dwarf binary with an 83-day orbital period. According to the research, the other MSPs are in interacting compact binaries wit orbital period less than eight hours. Three of the discovered pulsars were categorized as "black widows" and two as "redbacks," while one is a more classical neutron star-white dwarf binary. "Black widows" are neutron stars in which much of the mass has been stripped away or accreted by the pulsar, leaving a companion with a mass less than 0.1 the mass of the sun. In contrast, "redback" is a term coined to describe binaries in which the pulsar is frequently eclipsed by outflows from a companion weighing over 0.1 solar masses. The fastest MSP discovered by Cromartie's team is a "black widow" designated PSR J2052+1218. Located about 7,400 light years from the Earth, it has a pulse period of 1.99 ms. The pulsar is also intriguing for the scientists due to its short binary period, only 2.6 hours. The "slowest" one is a "redback" that received designation J1048+2339. It has a spin rate of 4.66 ms, orbital period of six hours and is approximately 2,160 light years away. The odd neutron star-white dwarf with an orbital period of 1980 hours was named J1824+10. It is located about 8,160 light years from our planet and has a pulse period of 4.07 ms. Other MSPs include two "black widows": J1805+06 and J0251+26, having spin periods of 2.13 and 2.54 ms respectively. The second "redback" spins at a rate of 2.56 ms. According to the scientists, 17 other still unidentified sources indicated by LAT are strong MSP candidates and need follow-up observations to eliminate uncertainties. Overall, 30 percent of known millisecond pulsars in the galactic disk have been detected in previously unidentified sources of gamma rays pointed out by Fermi. These auspicious results herald more future findings regarding rapidly rotating neutron stars. More information: Six New Millisecond Pulsars from Arecibo Searches of Fermi Gamma-Ray Sources, arXiv:1601.05343 [astro-ph.HE] arxiv.org/abs/1601.05343 Abstract We have discovered six radio millisecond pulsars (MSPs) in a search with the Arecibo telescope of 34 unidentified gamma-ray sources from the Fermi Large Area Telescope (LAT) 4-year point source catalog. Among the 34 sources, we also detected two MSPs previously discovered elsewhere. Each source was observed at a center frequency of 327 MHz, typically at three epochs with individual integration times of 15 minutes. The new MSP spin periods range from 1.99 to 4.66 ms. Five of the six pulsars are in interacting compact binaries (period